SA- 3% C I V I L A E R O N A U T I C S B O A R D

File No. 1-0003-

REkEASEIBt Apri l 28, 1961

NORTHWEST AIRLINES LOCKHEEX) EUCTRA, N 121US, NEAR CANNELTON, INDIANA, MARCH 17, 1960

SYNOPSIS

A t 3525 c.s .~. , on March 17, 1960, a Lockhced Electra , model L-188c, N 121US, owned and operated by Northwest Ai r l ines , Inc., crashed approximately six miles from Cannelton, Indiana, a f t e r f a i l u r e of t he r i g h t wing. persons on board were k i l led .

AU. 63

Fl igh t 710 departed Chicago, I l l i n o i s , a t 1438 c . s . t r , on an intended. nonstop f l i g h t t o M i a m i , Florida. All report ing poin ts were made on time and the f l i g h t was progressing according $0 plan; no messages were received which ind ica ted t h a t t h e crew of F l igh t 710 was experiencing any d i f f i c u l t i e s .

The fl ight was t o c r u i s e a t 18,000 f e e t .

It is the conclusion of t he Board t h a t f l u t t e r was induced by o s c i l l a t i o n s of the outboard nace l les and t h a t t h i s reached a magnitude s u f f i c i e n t t o f a i l the r i g h t wing, Reduced s t i f f n e s s of the s t r u c t u r e and the en t ry of the air- c r a f t i n t o an area of severe c l e a r air turbulence were contr ibut ing f ac to r s ,

Invest igat ion

Northwest A i r l i nes F l igh t 710 of March 17, 1960, a Lockheed E lec t r a air- craft, N 121US, was scheduled between Minneapolis, Minnesota, and Miami., Florida, with an intermediate s top a t Midway Aix-port, Chicago, 111L?ois. The crew con- s i s t e d of Captain Edgar E. LaParle, F i r s t Officer Joseph C. Mills, F l ight b g i - neer Arnold W. Kowal, Stewardesses Constance M. Nutter and Barbara A . Sci-reibcr, and F l igh t Attendant Mitchell D. Foster.

Fr ior t o departure the crew was br ie fed by the company meteorologist xi the present and expected weather conditions along the route. This b r i e f ing consisted of a general discussion of the synoptic s i t ua t ion , a review of t h e en route and terminal forecas ts , together with a l l sequence and p i l o t repoT%so The meteorologist said t h a t thunderstorms which were located i n F lcr ida were discussed as was the i n t e n s i t y of t h e j e t stream over the southeastern s t a t m , the l a t t e r because it appeared t o be growing i n in t ens i ty . however, of any c l ea r a i r turbulence being present along the route.

No mention was made,

A f l i g h t plan was prepared which indicated a f l i g h t i n accordance with instrument f l i g h t ru l e s (IFR), v i a Victor Airways 2, 97, and 6, t o Midway, a cruis ing a l t i t u d e of 15,000 f e e t , a t r u e airspecd of 340 knots, cmd an e s t i - mated time en route of 53 minutes.

- The f l i g l i t departed Minneapolis

t h e t r i p t o Chicago was routine. It sa id t h e landing a t Chicago was very ing i n every respect .

2 -

a t 1253.,&/ and a r r ived a t Midway a t 1355; should be noted t h a t some of the passengers hard; o thers s a i d t h a t it was a normal la&

During the s h o r t time t h e aircraft was on t h e ground a t Chicago, approxi- mately 30 minutes, it was re fue led and prepared f o r cont inuat ion of t h e f l i g h t t o M i a m i . While t h i s was being done Captain LaParle went t o t h e company oper- a t i o n s o f f i c e where he reviewed the la tes t hreather j.nfornation per ta in ing Lo t h e f l i g h t . A s a. flatter of company pol icy t h i s same weather information was l a t e r a t tached t o the f l i g h t ' s c learance papers and given t h e crew p r i o r t o departure. t h e cockpi t throughout t he f l i g h t .

There i s no company meteorologist s ta t ioned i n Chicago.

These papers m e kol;t in

The f l i g h t plan, prepared by t h e crew and f i l e d with c o q a n y operations, i n d i c a t e d ' a f l ight from Midway Airport v i a Victor Airway 53 t o Peotone, I l l i n o i s , Victor 1 7 1 t o Scotland, Indiana, Victor 243 t o Chattanooga, Tennessee, Victor 5lW t o Atlanta, Georgia, Victor 97 t o Albany, Georgia, Victor 159W t o Cross City, Florida, Victor 7 t o For t Myers, Florida, and Victor 35 t o M i a m i , Florid;; a cruis- ing a l t i t u d e of 18,000 feet, a t rue airspeed of 337 h o t s , and an estima5ed t h o en route of 3 hours and 37 minutes. The clearance given t h e f l i g h t by A i r Rozte Traffic Control (ARTC) was, "W 710 c leared t o t h e Miami Airport , Peotone', Victor 1 7 1 t o Scotland, F l igh t planned route, maintain 18,000."

The f l i g h t departed Chicago a t 1438 with 57 passengers including one infant. The gross weight of t h e aircraft a t t h e time o f ' t akeof f was 107,661 pounds; t h i s was within t h e maxirmun allowable gross takeoff weight l i m i t a t i o n of 110,590 pouida, which l i m i t a t i o n was inrposed i n order t o comply with t h e maJrimum allowable gross landing weight a t Miami. According Lo company records t h e center of grav i ty was within prescribed limits.

At ius, t h e f l i g h t reported t o t h e Indianapolis, Indiana, ARTC Center over A t Milford, I l l i n o i s , a t 18,000 fee t and est imat ing Scotland, Indiana, a t 1512.

1513, F l igh t 710 reported over Scotland maintaining 18,000 f e e t and estimating Bowling Green, Kentucky, at 1535. A t this time the f l i g h t was advised by ARTC t o contac t t h e Memphis, Tennessee, ARTC Center on 12b.6 mcs., a t 1530. contact was t h e l a s t known r a d i o communication With t h e f l i g h t .

The 1513

A t approximately 1640, r e p o r t s were received by Northwest Air l ines a t Minneapolis t h a t F l i g h t 710 had crashed near Cannelton, Indiana. t h e c rash was es tab l i shed as approxknately 1525.

The time of

A radar operator a t t h e Indianapol is A i r Survei l lance S ta t ion reported that he monitored t h e f l i g h t by r a d a r t o within seven miles of Scotland, Indiana, and t h a t t h e f l i g h t appeared t o be normal.

Weather

Surface s t a t i o n s neares t t o t h e route reported a broken t o overcast cloud deck from t h e Chicago area t o Terre Haute, Indiana, with an average base altitude

1/ All times here in are central staiidard based on t h e 24-hour c lock . -

- 3 -

of 1,500 f e e t above the ground. layer which was broken t o overcast with i t s base a t approximately 4,000 f e e t . This upper layer extended from Chicago t o the scene of t h e accident and its tops were reported by p i l o t s a s being 7,000 t o 7,500 f e e t near Chicago and becoming 5,000 t o 6,000 f e e t between Evansville, Indiana, and Louisvi l le , Kentucky. wi th i n f l i g h t v i s i b i l i t y unres t r ic ted i n cen t r a l United States .

Above t h i s cloud deck was a second cloud

Generally c l e a r conditions preyai led above these cloud formations

Considering upper wind reports available from ground based observations as well as p i l o t reports , the following da ta have been extracted r e l a t i v e t o the magnitude of horizontal and v e r t i c a l shears as well as wind s h i f t &cross t he troughline (along

1800 c.s.t. Data Horizontal Shear

the I l l ino is - Indiana border) . a t 16,000 f e e t a t 18,000 f e e t

39 kts./lSO N.M. 52 kts./lSO N.M.

Vert ica l Shear Nashville, 16,000-18,000 f e e t P 10 k t s ./1,000 f e e t . Dayton, 16,000-18,000 f e e t = 2.5 kts,/l,OOO feet .

Wind s h i f t across trough A t lt1,OOO fee t , 50 degrees between Peoria

60 degrees between Peoria A t 16,000 f e e t , 40 degrees between Peoria

30 degrees between Peoria

0600 c.s.t. Data Horizontal Shear a t 16,000 f e e t I 48

a t 17.000 f e e t 80 .. Vert ica l Shear

and Dayton and Nashville and Dayton and Nashville

kts./lsO N.M. k t s ./150 N.M.

Nashville, 16,000-17,000 f e e t Dayton, 16,000-18,000 f e e t 0

26 kts./l,OOO f e e t

18,000-20,000 f e e t I 5.5 kts,/l,000 f e e t

Wind s h i f t across trough A t lt1,OOO f e e t , 60 degrees between Peoria and Dayton A t 16,000 fee t , 50 degrees between Peoria and Dayton

40 degrees between Peoria and Nashville

P i l o t Reports (resume of f e w of most s i g n i f i c a n t )

1. 1310 c.s.t., 50 miles south Dayton, moderate t o severe c l e a r a i r turbulence 20,000 f e e t - 500 f e e t th ick - F l O l .

2. 400 c.s.~., Cincinnati, moderate t o heavy

a30 c.s.~., New Hope, Kentucky, choppy a t

25,000 fee t , T-33.

3. centigrade temperature drop a t edge of j e t stream. airspeed f luc tua t ion and turbulence - Electra .

turbulence 20,000 t o

21,000, 10-15 degrees Descended and showed severe

4- 4. 1515 c.s.t., Columbus, Ohio, t o York, Kentucky, moderate t o severe

turbulence 11,000 - 17,000 f ee t - alrcraft type unknown.

5. 1500 c.s.t. , Louisvi l le t o Cincinnati, noderate c l e a r air turbulence 17,000 t o 25,500 f e e t - B-57.

6. 1530 c.s.t., 50 miles north Louisvil le, decreased speed and descended from between 20,000 and 25,000 t o 15,000 f e e t because of heavy t o severe c l ea r a i r turbulence - B-57.

7. 1640 c.s.~., Louisv i l le t o Nabb, Indiana, severe c l e a r air turbulence 19,000 feet - DC-7.

8, 1830 c.s.t., Cincinnati, moderate t o severe c l e a r a i r turbulence 11,000 t o 17,000 f e e t - Constellation.

The synoptic s i t u a t i o n a s reported on t h e sur face weather char t f o r t he midafternoon of March 17, 1960, was as follows: over the northern por t ion of the lower peninsula of Michigan. a rea extended t o ,high l e v e l s (above 30,000 f e e t ) , A marked troughline afi a l l a l t i t u d e s extended southward from t h i s low along the I l l inois-Indiana border, A r idge of high pressure extended from the southern plains northeastward across Arkansas, western Tennessee, c e n t r a l Kentucky, and i n t o southern Ohio.

A low pressure area was centered T h i s low pressure

The wr i t t en fo recas t s published by both Northwest A i r l i nes a d t h e United S ta t e s Weather Bureau and ava i l ab le t o the crew of F l igh t 710 a t Minneapolis wAci Chicago r e f l ec t ed the wind f i e l d as shown by the observed data. It i s important t o note, however, t h a t ne i ther of these sources of weather information iazfiticned the p o s s i b i l i t y of c l e a r air turbulence along the route.

Witnesses I

Approximately 75 groundwitnesses who were near the accident scene and a number of airmen who were flying i n the area..at t h e time were interviewed.

The laywitnesses were loca ted within an area t h a t included the most distantd p laces from which the a i r c r a f t could be seen or t h e a e r i a l explosion heard by a person having normal v is ion and hearing,

A composite descr ip t ion of what these witnesses saw and heard follows: The t h e was 15x5. The weather was c l e a r except f o r s ca t t e r ed cumulus clouds d t h bases a t about 4,000 fee t ; v i s i b i l i t y was good. The a i r c r a f t w a s flying in an approximate nor th t o south d i rec t ion , i n l e v e l f l i g h t , and a t a high a l t i t ude .

by a l a r g e cloud of dark smoke. ob jec t was seen t o emerge from the smoke cloud and fall near ly ve r t i ca l ly , t ra i l - ing smoke and flame. Ttle fuselage con- t inued I n l e v e l f l igh t f o r a f e w seconds and then f e l l t o the ground describing a l a r g e t r a j e c t o r y a rc as it did SO. force t h a t debr i s w a s thrown nearly 250 f e e t i n t o the air.

31,000 t o 32,000 f e e t .

/Suddenly two puffs of white smoke were seen, Seconds l a t e r these were followed Two loud explosions were then heard and a l a r & e

Smaller ob jec ts were l a t e r seen t o fall .

I t s t ruck the ground with such t e r r i f i c

Six USAF a i r c r a f t were on a re fue l ing mission i n the a rea a t an a l t i t u d e of Three of these a i r c r a f t were KC-1.3S's and three were

l3-521~. t r a i l of this accident a t 1532. dark i n co lor as if produced by burning some product with a petroleum base. The bottom of t h i s smoke 'disappeared i n t o s c a t t e r e d clouds. streamer of dark smoke which began a considerable d is tance nor th terminated a t t h e smoke cloud.

Afrmcn manning these a i r c r a f t s a id t h a t they f irst saw t h e smoke The cloud was the shape of a c h i l d t s top,

A hor izonta l

The smoke cloud was first s ighted when they were 26 n a u t i c a l miles north- Their bear ing of approximately 170 degrees near ly pa ra l l e l ed northwest of it.,

t h e course of Fl.1ght 710. time they were a b o u t 1 2 nau t i ca l miles west of it.

They passed abeam of t h e sinske cloud at 1539, a t which

During the seven-minute period, from first s ight ing t h e smoke of t h e abeam check, t he smoke cloud and streamer re ta ined i t s o r i g i n a l form with l i t t l e or no ind ica t ion of d i s s ipa t ing o r breaking up. cloud t o be a t an a l t i t u d e of 25,000 feet,

The USAF airmen estimated t h e smoke

Wreckage Pi:> t r ibu t io i i - --..-I

The mjor port ion of t h e a i r c r a f b s t ruck t h e ground in a near ly v e r t i c a l a t t i t u d e i n a f i e l d where t h e ground sloped t o t h e south. The s o i l a t t h e point of impact was s o f t and contained no rocks. Small trees near t h e poin t of impact were not s t ruck by t h e a i r c r a f t . 30 f e e t across i t s top from east t o west and 40 f ee t from nor th t o south; it was 1 2 f e e t deep. ing t h i s c r a t e r d i s in t eg ra t ed and was buried wi th in it. Only a f e w fragments of wreckage were visible i n t h e bottom of' t he crater. deep and 11 f e e t wide extended southward from t h e crater f o r a d is tance of 16 f e e t . Fragments of t h e l e f t wing were v i s i b l e i n t h i s depression. t i c a l t a i l were imbedded i n t h e west r i m of t h e c r a t e r with t h e crumpled upper end of the rudder protruding from the' ground. imbedded i n t h e north r i m of the c r a t e r . face of the c r a t e r bottom f o r several days u n t i l extinguished during excavation. The impact explosion hurled s m a l l p ieces of wreckage i n a l l d i r ec t ions from t h e crater, t he g rea t e s t d i s tance being approxiinately 1,500 feet t o t h e e a s t and south- eas t . The heavies t concentrat ion of wreckage s c a t t e r e d by t h e impact, explosion was i n the southeast quadrant within a radius of 100 feet from t h e c r a t e r . west quadrant contained a moderate t o heavy concentrat ion of pieces, and only R l i g h t s c a t t e r i n g of p ieces was found i n the northwest and northeast quadrants. The geographical l oca t ion of t he c r a t e r was 370SLt'381N. Lat. and 86038tfW. Long., o r approximately s i x miles due east of Cannelton.

Impact forces formed a crater which measured

Most of t h a t por t ion of t h e aircraft which s t ruck t h e ground form-

A shallow depression 2 Sect

Port ions of t h e vsr-

Pieces of hor izonta l t a i l s t r u c t u r e were A smoldering f i r e burned below t h e sur-

The south-

The south end of the c r a t e r contained t h e No. 2 engine and propel le r , p a r t s of t he l e f i main gear, and wing s t ruc ture , including f l a p pieces, a i le ron , and trim tab sectiono. cockpit cont ro l sys tem, e l e c t r i c a l panel b i t s , various system ccmponents, nose gear pieces, e leva tor torque tubes and ruddcr post, b i t s of tail. s t ruc tu re , servos, e tc . Upon rwnoval of t h e wreckage from t h e c r a t e r it was apparent that t h e fuse- l age with i t s t a i l , most of t h e l e f t wing, and t h e No. 2 powerplant had contacted t h e ground i n an almost v e r t i c a l nosc-down pos i t ion . t he c r a t e r W;LS found t o be severely f~agmcnted from ground impact.

The north end of the c r a t e r contained t h e fuselage s t ruc ture ,

All s t r u c t u r e removed from

- 6 -

A prel iminary survcy of t h e accident a w a by he? i w p t e r disclaset-: thtrt,, in addl t i o n t o severely fratgmcnted meekage at‘ t ! i~ crzt t;Lb9 t h e m i n po r t ion of ehe r i g h t wing, the outboanid powerplants, and many a:i!b;% pieces o f wreckage Kere widely s c a t t e r e d In a FprAcr.al d i r e c t i o n ricrLh carid northeast f t’or :.?,e cr~ite;,

Approxiiazitoly 3% a o l d i e r s from neai%y fort Krlox were 11: cii i n a grounnci search of a 2T-square mile area. a l ine abreast . Inves t iga t ing persormel followed t h @ line of S G ’ L ~ ~ ~ C ~ F S , marking, ident i fy ing , and p l o t t i n g t h e locut ion of each m a t t e r e d piece of wreckage as it. was found.

The soldiers coveTed t h i s a r e s by w i i k i ~ r g iyL

The p a r t s which separated froin t h e a 9 r c r a f t i n f l i g h t consisted of t h e con- p l o t e right wing with powerplants, the o u t m end of‘ the l e f t wi!,g arid aileron, the No. l Qj3C and nace l le , and t h e outboard po r t ion of t h e left e leva to r .

The r i g h t wing was found on a bearing of 10°301t301 at a d is tance of 11,291. feet from t h e crater’. p o l l e r s with reduction gears wer8 found as f o u r separate p ieces all within a rad ius of less than 2,000 f e e t of t h e r i g h t wing. parts of t h e r i g h t and l e f t i d ing~ aid t h e outboard nacel les were d i s t r i b u t e d along a pa th approximately one m i l e ui.de arid nea r ly seven Eil.es l o r g , Ths mg.- n e t i c heading of t h i s s c a t t e r path ms approximately 70 degrees c.r a t fiear:l.jr ri$?, angles t o t h e f l i gh tpa th , with t h e hwvier of t he sca t te red pieces f o ~ i n d at, tl!i2 west end and t h e least dense a t t h e e a s t end.

The Nos. 1 and IC power sections and t h e b respect ive pro-

Nearly 611 of t h e sc::t,tcred r-

I The outlying components of the aircraft, t h e powerplants I a d prope l l e r s W Z Y ~

t ransported by Army he l i cop te r t o an area near t h e c ra t e r . From tha t l o c a t i o n ~2’: of the . sa lvaged aircraft s t ruc tu re was t ransported i n CAB sea led vans t o t he I,c.‘:::-. heed Aircraft Corporation a t Burbank, Cal i fornia , f o r further exmdnalicn, Th: powerplaits were similarly t ransported t o t h e Al l i son factory a t Irici3.cwqoXs3 Indiana.

Aircraf t Struc tures

Study of t h e wreckage arid wrezkage d i s t z ibu t i cn a t t he scene c:.f t h c h ixc: ,!XI i disclosed t h a t t he outboard powerplants and engine :;u-?por.t, ~ t r u c t u r e ~ , t h e c,.,’, . p l e t e r i g h t wing, and the ou te r por t ions of t he left wing arid ailerori S C + F J U ~ > ~ ? ~

f rom t h e rest of t h e a.irplane i n f l i g h t during such a shor t t h e in t4er ra l tt1a.t ’ 9 : ‘

sequence of these separat ions was riot appilI*erit. In addition, t h e main ~ r e c k ? < ~ . : , t h e outboard cnginc support structnres, t h e ciutoi- end of 2 1.ef.L king, asti - . ! I .

por t ion of thc r igh t . wing betwoen thi: fuseloge and t h e i ~y 3 n:ic, e]. 1 e L ~

s eve re:l. y d i :J i n t. eg r n t ed t h iz .i; de t ai I ed s t~iily w a s ne c e s s axy. p ieces of wrcckage and. t o determine t h e nature of the f a i l u r e s .

i d.enti Cy’

- 7 -

a t t h u i,cxkliced factory. support s t ruc tu res and on t h e inboard por+Aon of t he i*ight wing. As a resdt,, these p o n i o n s of t h e s t r u c t u r e were mocked-up i n t h r e e dimensions. Simpler llreconajtructionsll were used i n corw.xtion with the r e s t of the wing, t he lloidi- sont.al t a i l , and t h e vcr t icr t l t a i l , F l igh t control system parts and fuse!.nge wreckagcA were examined without tlrccons-l,ruletion.lt

Primary intc?rr:st centcrsq n:i t h e outboard engine

Nurwous f rac tured pieces of wreckage were subjected t o me t a l l ~ i r g i c a l ex- anlinati ons and ana1y:;;s i n t h e T.ockhecd 1,aborntories. These examiriati ons dis- closec! no evidence of' fa t igue cracking, nonconforming mater ia l , overtorquing of nuts, o r rri-issinc attachments. loads p~*o&~c i r lg failures.

They did y i c l d evidence as t o the d i r a c t i m of

Exminat ion of t h e fusidage, t he v t r L : i ~ a l t a i l , the landing gear, ,and t h e f l i g h t cr;ritrol sys tems disclosed no indi catLon of malfunction o r fal.l.ure p r i o r t o i m p c t with t h e ground, other than con. i . ro l system f a i l u r e s r e s d t i n g frorr, the ri&t I.' i ng separat ion i n fl ight, .

Study of the d.a.magc? t o t h e r i g M wing bet,ween the fuselage a.nd t h e inboard. nace l le disclosed a frac-Lure i n t h e f r o i i t spar a t witlg s t a t i o n 78 and one Sn thc re,w spar a t wing s t a t i o n 101, These were t h e inboard ends of the spar Secti.o!-is which remained wtth t h e separated rlggkt, wing. The upper and lower. wing covet's aid r i b s between t,he s p ~ ~ . ~ ~ s i n t h i s a rez were sha t te red and may pl.eces f e l l t o t h e ground separa te ly when the w l x ~ g sepwat ion occurred. between the fuselage and the r f g h t inboard riacelle a l s o fell. t o t h e grolmd scpa- I

rately i n a, number of pieces .

.

The hinged l ead ing i'df;e

T h i s hinged leading edge separated from t h e wing by t h e upper s k i n pu1l-iO.g off t h e r i v e t heads i n a genera l ly forward d i r e c t i o n a t t h e attachrwnt .to thi! terminal. extrusion which connects tLo t h e upper cap of t h e f r o n t spar , f a i l u r e tha leading cdge rotnt,ed. dotmward t o t h e lirriit of t r a v e l of t h e p i m o hinge wli?.ch a t taches t o the lower cap of t h e f r o n t spar, and the rear luzs of t he piano hinge f r ac tu red i n bending. were darxiged by in t e r f e rence with t h e ends of t he adjoining f ixed leading edge sect ions. t he gap scal back-up angleso Superimposed on t h i s were scratches arid r i b def<>i*ir.-. a t i ons ras i l l t ing from abnormally l a r g e up-and-down movement of the hingeti :;egwn.t r e l a t i v e t o the f ixed segments and from abnormally l a r g e up-and-dowri spaln,i ;.'z

bending def lec t ions .

Af'ter ~ i : i : ~

The end ribs of t h i s hinged l ex l ing edge

Some of t h i s dma.,:e was normal s e r v i c e wear from in te r fe rence r*.i;th

P r io r t o separa t ion of t,he inboard hinged leading edge from the win?.$ as ind ica ted by an abrupt d i scon t inu i ty i n t h e d i r ec t ion of t h e leading r:;?gi? u p p ~ r sk in p u l l i n g o f f t h e heads of t h e r i v e t s i n t h e seam pLnal le l ing the upper. spx' cap, a sec t ion of t h e r i g h t wing front, spar upper cap flange buckled. i i p ~ a d . , separat ing from t h e v e r t i c a l leg of t h e spar cap with outward progr<?Ssii?:1 of t!le

crack between s t a t i o n s 78 and. 8 9 , of the f r o n t spar web buclclcd and took an "Slt-shaped deformation af.tc;.. tiic i?.

mentioned f lange separat ion. t h e f r o n t spar web, which preceded a ho r i zon ta l f r a c t u r e extending oulboard f r m s t a t i o n 78. ?'cars oi.' t h e spar cap flarigea from t h e v e r t i c a l l e g s of t h e spar caps progressad 0 ~ 2 ~ 6 i a d considerable d is tances from t h e spar f r a c t u r e s .

A t and ad jacent t o s t a t i o n 78 t n c q p r ::ut,

A t s t a t i o n 78 the re was a l so a v e r t i c d . . fract-Lire :).?

The r e a r spar f a i l e d a t s t a t i o n 101 I n rearward bsnding.

The upper and lower covers o f the E1.ectx.i wine; cons is t of 1o.m; 7178 a-id 7075 aluminum a l l o y extruded planks machined t o form spanwise stiffeiie:t-:: int::g:sal with t h e sk in . t o failure results in f r a c t u r e s s i n g d - a r l y lack ing i n ind ica t ions of the producing f a i l u r e and of failure progression. However, t h e f a c t t h a t ir saw-toothed diagonnl. f r a c t u r e l i n e s i n the bott,on cover of the r i g , h t int. extended from t h e f r o n t spar a t s t a t i o n 83 t o t h e a f t edge of plan!! 4 at; s t a t i o n 155, and from t h e a f t edge of plank 5 a t s t a t i o n 155 t o t h e rear spar a t s-iatim 95, i n d i c a t e s repeated r eve r sa l s of. 1oaNr-g during the breakup, I_z.borii-tr;rJ' fx inat iors of t h e f r a c t u r e s a l s o d isc losed some indica t ions of both tensicr? ;c.nrf compression due t o a l t e r n a t e upwni~l and downward :Loading of the v ine , combi :!t-td with corresponding r eve r sa l s of she3.r due t o torsion. two l a r g e separated pieces of t h e lower cover i n t h i s srea i s cc!nsi:;t,e:?~ x i t h permanent s e t due t o long column buckling a t a t h e whc?n t h e r i b s at :;t,at:;or:s I:.: I

101, and 119 were providing l i t t l e o r no la ternl rest,ra.i.nt.

The low energy absorpt ion and yieldj-ng o f t h e w mstnrialis prid.-

Sl ight , s p a ~ ~ ~ i :;c b ~ i ~ j < ~ : . ~ at'

The f r a c t u r e s of t h e r i g h t King uppes. cover betyeen t h e fusel 'ipe and U A P inboard nace l le had some c h a r a c t e r i s t i c s o f loc,?l conipression bucl i i n ? of' Lhi? upper surface between s t a t i o n s 83 and 101 and soze cha rac t c r i s t l c s of t.l\ic: wLrAr<

- 9 -

hinging about a chordwise l i n e through the upper surface between s t a t i o n s 83 and 101 a f t e r t h e s t r u c t u r a l i n t e g r i t y of the f r o n t spar and lower surface w a s de- stroyed. upper cover f r a c t u r e s occurred i n conjunction with the spar cap f lange separat- ing from t h e v e r t i c a l l e g and buckling upward,

Di rec t ly a f t of t h e f r o n t spar, a t and adjacent t o s t a t i o n 83, t h e

Reconstruction and study of t h e r i g h t wing r i b s a t s t a t i o n s 83, 101, 119, -137, lh3, and 15s disc losed n3 uniform p a t t e r n of failure i n t h e diagonals, p a r t l y due t o t h e l ack of s u f f i c i e n t evidence t o make a p o s i t i v e determination of t he exact manner i n which most of the diagonals failed. However, t h e r e were a f e w pos i t i ve ins tances of f a i l u r e cons i s t en t with nosedown t o r s i o n a l loading, Several of t h e r i b diagonal f a i l u r e s were q u i t e similar I n appearance t o corre- sponding ones found i n an American Ai r l ines E l e c t r a a f t e r t h e main landing gear wheels s t ruck a snow bank during takeoff. A por t ion of t h e lower cap of the rib a t s t a t i o n 83, which remained a t tached t o one p iece of lower cover by Huck bo l t s a t t h e plank 1-2 and 3-4 sp l ices , had a l l r i v e t s sheared i n a random p a t t e r n a t f i v e of t h e six "HIt c l i p s between t h e above-mentioned po in t s , On t h i s p iece of r i b cap t h e r e were abrasions due t o rubbing of separated diagonals aga ins t the cap and of one r i b t a b on t h e mating IrHtt c l i p ,

I n t h i s area of t h e wing most attachments of r i b caps t o diagonals and t o t h e wing cover f a i l ed . on mating p a r t s a r e ind ica t ive of dynamic condi t ions with high and changing stress- es i n t h e wing covers during and a f te r t h e r i b failures. r i b and r i b attachment failures i n t h e No, 3 tank area i s e n t i r e l y incons i s t en t with known failures produced by overpressur iza t ion of an E lec t r a f u e l tank,

Both t h e random p a t t e r n of t h e failures and the abrasiclils

This random pa t t e rn of

Study of t h e damage t o t h e l e f t outboard engine support s t r u c t u r e and nace l le disclosed t h a t t h e lower r i g h t longeron f i t t i n g a f t of the firetral.l f a i l e d i n a clean tens ion break. After t h i s t h e f r o n t end of t he engine s q p o r t moved upward and t o t h e l e f t with progression of damage unti l separat ion frcm t h e a i rp l ane occurred. Laboratory examinations o f t h e Lord mounts which support the propel le r reduction gear box d isc losed evidence of repeated bottoming due t o ab- normal loading i n var ious d i rec t ions . Numerous cu rv i l i nea r scratches, approxi- mately p a r t i a l e l l i p ses , on t h e outer surface of t h e s-irl s t r a igh tene r extension and severely scratched areas on t h e inner sur face of t he t a i l pipe b e l l mouth ind ica t e repeated in t e r f e rence due t o l a r g e c y c l i c motions o f t h e engine r e l a t i v e t o the wing p r i o r t o gross over-al l displacement of t h e engine support s t ruc tu re from i t s normal posi t ion. Init ial failure of t h e r i g h t outboard cng:i.ne support s t r u c t u r e consis ted of a tens ion separa t ion of t h e upper l e f t longeron from the a t t a c h f i t t i n g a t the firewall. The f r o n t end of t h e engine support then moved downward and t o t he r i g h t and ro t a t ed with t h e p rope l l e r with progression of damage un t i l separat ion from the wing occurred. I n the process of des t ruc t ion the r i g h t lower longeron t o r e off t h e r e a r a t t a c h f i t t i n g with repeated i n t e r - ference between the longeron and f i t t i n g , p e l l e r reduction gear box a l s o were damaged due t o abnormal loading i n var ious d i rec t ions .

The Lord mounts which support the pro-

Numerous f r ac tu red f i t t i n g s and p a r t s of t h e l e f t and r i g h t outboard engine These support and nace l le s t p x t u r e s were subjected t o l a b o r a t o r j exaninations.

examinations disclosed no evidence of f a t i g u e cracking, nonconforming mater ia l , overtorquing of nuts, o r missing attachments. pos i t i ve evidence ident i fy ing any of t h e noted damage t o t h e outboard engine sup- p o r t and nace l l e s t r u c t u r e s as having ex is ted p r i o r t o the l a s t f l i g h t .

All e f f o r t s f a i l e d t o disc; ose

- 10 - All recovered wreckage was examined f o r evidence of f i r e having caused t h e

d i s i n t e g r a t i o n i n f l i g h t . Only one probable area of f i r e p r i o r t o the r i g h t wing separat ion was found. T h i s was i n t h e aft p a t o f the r i g h t outboard nacelle and t h e port ion of t h e f l q afb thereof: arid wr,s o - a y of sufficient durchion t o melt t h i n alum_in?un alloy sheei yr io i . t o iuipxt, with the ground. weakening of t h e s t r u c t u r e in t h e areas where the i n f l i g h t separat ions of the r i g h t wing and outboard engine support s t r u c t w e occurreds

it ptwduced iio

Powerplants

This accident was character ized by separat ion in t h e air of port ions of' Nos. 1 and 4 powerplants as we11 as t h e r i g h t wing. were found along t h e wreckage path in t h e following order:

Plajor powerplant co~~;x):'~cr~t.:.::

1. No. 4 propeller, reduct ion gear assembly, and torqueineter, 2. No, 1 propel le r and major port ion of t he reduction gear assembly. 3. No, 4 power sect ion. 4. No. 3 powerplant En i t s e n t i r e t y with %he r i g h t wing. 5. No, 1 power s e c t i m , t,orquemeter aid a por t ion of t h e reduc.tlon

gear assembly. 6. No. 2 powerplant i n c r a t e r a t t h e fuselage s i te .

A l l components lay a t the l o c a t i o n a t which they first contasted the g ~ o m d except f o r a minor displacenierrL downslope of t h e No. 1 power sect ione

I

There was no fire damage on " ~ n y of the powerplants. The blades o f all. Ijso- ..

pel le rs , except, those of No. 2, were fouid i n t h e i r entir*ety w i t h or ir, Lhc ime- d i a t e v i c i n i t y of t h e i r respcctive h.i:bs. remainder of t h e powerplant, tdas dug Erom t h e fu.selagc, cra.ter s i t e a i d oCLy :\den- t i f i a b l e port ions of all blades were recovered.

The No, 2 propelle?, along w5-t.h the

Powerplant control. systems were daiunaGc?d by separat ion an?, impsct so 'chal, they revealed no r e l i a b l e information with respect, t o power conFigwa.t,i.on a t a r ~ y time p r i o r t o j q a c t .

1. O i l systems f o r s ignif icant , coniamhation, 2.

3 4. 5. 6 .

7 e

8.

Propel ler reduct ion gear aid accessory d r i v e systems f o r gear and/or bearing f a i l u r e Torquerneters f o r rotation.4. 2.nter.fer.suice e

Pouer sec-Lion r o t o r s f o r ovestc;npesature, bearhg distress, o r r o t o r failures. Safe ty couplings f o r typical. operational decoupling ma1 kinzs Fuel pumps and f u e l contr.ol.s for s igni f icant contamination o r evidence of malfimction o r f a i lu re e

Propel ler pitch--change rcechanisms fojn s i g n i f i c a n t contramhation o r evidence of mczlSurrlction o r fai.lure. Propell.ers and engines for evidence o f ovc::rnpeeds

This examination was negative trith rcspcc t Lo evidence of operat,ion:il f a i l u r e s . remained with thr power sectlion of both f:rit: i PCS.

B a n i n a t i o n of t h e Nos. 1 and 1, powsrylants showed t h a t t h e ~ L r i i t s The torqueirletcrs remai I,cd with

- 11 - the No, 1 engine power sec t ion and with the No. b propel le r reduct ion gear assembly. a t approximately t h e main diaphragm s p l i t l i n e . separation were necked down and bent i n a p a t t e r n showing counterclockwise rota- t i o n of the forward p a r t as separat ion occurred. IJos. 1 and ings which indicated load r e v e r s a l s ,

Separation of No. 1 engine occurred through t h e reduction gear case Stud f a i l u r e s t h a t occurred a t

The separat ion f r a c t u r e s of engines d i d not show evidence of f a t i g u e o r pos t Separation mark-

The inner surface of t h e a i r i n l e t housings and compressor eases of Nos, 1 and 4 engines were rubbed by the compressor blades. on the thermocouples and turbine nozzles of t h e same engines. pinion of No. 1 engine remained with t h e torquemeter asserably. evident on the di-ive s i d e of the pinion gear t e e t h . p a r t s of !lo. 11 engine occurred a t t h e forward end of the compressor extension s h a f t where i t engages t h e torquemeter shaf t . connect showing the power sec t ion end was r o t a t i n g f a s t e r than t h e torquemeter end. Fzids of some of the sp l ines of t h e extension s h a f t were upset rearwayd; a l s o , 1ii;ht axial scrape marks from f r o n t t o r e a r showed on some of t h e exten- s ion s h a f t spl ines .

There were aluminum deposits

Spiral. narks .irere The high s p e d

Separation of t h e r o t a t i n g

Spiral marks were made a f t e r dis-

Ai rcraf t Systems

Damage t o the a i r c r a f t was s o extensive t h a t no a i rcraf t system, as such, remained i n t a c t . i n many cases f i r e damage was evident . closed by examination of the recovered components.

Systems components received l i g h t t o extreme impact dmage and The following are t h e salient; f a c t s dis-

Many items of t h e hydraulic system were badly damaged by impact f o r c e s and. i n some cases f i r e had been present. a t i o n a l checks of such uni t s . Hy- d r a u l i c system items associated with tho r i g h t main gear remained intact O I ~ t he gear but were f o r t h e most p a r t covered i&th a moderate coat ing of smoke depcsit, The r i g h t f l u i d heat exchanger was recovered separa te ly with only a small dent, i n the housing, and undamaged by f i r e . Both serv ice center packages received moder- a t e impact and f i r e damage.

Damage t o valves and pumps precluded oper- No operat ional damage was noted on ar~y units.

Most of t h e recovered p a r t s of t h e c o n t r o l surface boosters received. severe impact damage. and e leva tor boosters were i n t h e I1ON1l pos i t ion and the rudder was almost com- p l e t e l y i n t h e llPIanual'l posi t ion. However, a l l cables t o the cont ro ls m r c ? broken and the i n f l i g h t pos i t ions of these valves may have been a l t e r e d at, impact.

Based on t h e p o s i t i o n of shutof f and bypass valves, t h e a i le ron

Loss of e l e c t r i c a l o r hydraulic power permits the a u t o p i l o t engaging valve t o move t o t h e disengaged posi t ion. rudder valves; however, the solenoid core of t h e a i le ron valve was found trapped i n the body i n t h e l1ONl1 posi t ion, t h e probable result of j q a c t forces.

This was t h e pos i t ion of the e l e v a t o r arid

The four check valves of each hydraulic manifold were tes ted . A l l except These three permitted a v e r y th ree from the a i le ron manifold passed the t e s t .

s l i g h t i n t e r n a l lccikage i n t h e closed pos i t ion and s igns of operatiorial d i s t r e s s were noted during t h e i r inspection.

The load cont ro l u n i t s and sensors were considerably damaged. The only operable p a r t s were the p i s t o n s t h a t a c t u a t e the sensors. Tho main spool cont ro l

- 1.2 -

One of t h e two r e t a i n i n g p ins of t h e divider sec t ion of t h e rudds r cylin- der was missing. The hole showed no s i g n of scoring nor W L L S t h e birrel danaged i n t h e i .mediate arc33 of t h e hole. malfunction p r i o r t o t he acci.dcnt

There was no ind ica t ion of f l a r i n g on the remaining pin.

Nothing w i i s observed dur ing t h a exanli~iation t o suggest booster

Because of extensive danage exarniric;tion o f t,he e l .ec t r ica l system yielded l i t t l e information. There .was ind ica t ion of r o t a t i o n of t h e No. 2 AC ger,wator a t t h e time of impact and no ind ica t ion of r o t a t i o n f o r those on engines Nos. I., 3, and I r a t h e time of impact. any of t h e u n i t s o r wire bundles.

Impact rnarlrs on the i n v e r t e r i n d i c a t e t h a t . it was n o t operating at, Them was no ind ica t ion of e l e c t r i c a l ffrbe o r overload m.

The r ad io equipment was completely demolished by impact,; t h e r t were no ind ica t ions of electrical. overload o r burning of any of these units, t i o n of t h e badly damaged com?onents of t h e var ious instivnients arid the auto- p i l o t yielded no use fu l information.

Examina-

Impact damage prevented t e s t i n g o f f u e l i n g valves No. 1, No, 2, and Eoe 3. F i r e damage t o seals, switches, etc., prevented the t e s t i n g of valve No. 4. Ilisasseinbly of t hese valves showed no s i g n s of opera t i m a l d i s t r e s s o r ~ t b n o n x ~ l wear. All f o u r dump valves were i n Lhe closed pos i t i on . The r i g h t rliunp check and swivel valve was r e l a t i v e l y undamaged a i d closed. severe inrpact damage and was p a - t i a l l y open.

The l e f t assembly yeceived ,

The r e l i e f valves of t h e vent v d v e s were bench checked. Rlthoc.& the valves f r C J D 1 No. 1 and No. 3 tanks h a d been burned and were s t i c k i n g 011 t.he Tirst tes t , a l l four valves c o n s i s t e n t l y opened at t he spec i f i ed pressure on repezted tes t s *

Only the r i g h t wing f u e l shutoff valves were recovered, The No, 3 was approximately 2 5 percent open and t h e Noa 11 was open,, off was riot i d e n t i f i e d mong the recovered items. The No. I w a s me-half open, t h e No. 3 was approximately f; percent i:.pen9 and t h e No.

The No, 2 erne-rgency shut-

m:; closed,

A l l crossfeed valves were ac@ounted.for , The No, 1, No. 3> rind ?loi h were closed, t h i r d open.

The No. 2 and thi. lef t - to-r ighi ; -win2 valves were approxin!a.t.ely one-

0

The engine--dri.ven compressor. from t h e No, 3 engine was I n t a c t , The compressor ~ I * C J ~ t h e No. 2 erigirie vas

It, had becn damaged by impact bu t was unburned. demolished by impact bu t was unburned. items disclosed no s igr i i f ican t inform- c~ t ' i o n v

Exminat ion o f oxygen and a i r -s tar t systeir,

Inspection of t h e an t i - i c ing system ducting disclosed no evidence of i n h e r n a l duct f'im. The two f i r e ext inguisher containers of t h e No. 2 nace l l e 'were de- s t royed by impact. It had n o t been f i r e d . Roth contni.ners frorri t h e No. 3 rlacelle were recovered i n t a c t arid f~i l :L; i r charged. One por tab le C O P cylinder, ?.c!c:iitd'ied as being from t he flight, :jte>J.tJio:12 ,

was dauriay,ed by iin~xict. r e t a i n e d a charge.

One of t h e discharge hea.ds xas recovered.

Its handle and di:;c:!ia?ge horn werc missing b u t i t sLi.11 ',,

- 1.3 - Maintenance Records

Review of t h e mainteriance records of N 121US disclosed only one i tem t h a t appeared of possible s ignif icance t o t h e accident invest igat ion. . This per ta ined t o a re fue l ing inc ident i n which Fie. 3 f u e l tank developed a leak a t a nacel le wing f a i r i n g attachment crew l o c a t i o n . The l e a k was a t t r i b u t e d by Northwest Ai r l ines personnel t o rupture of t h e tank s e a l a n t by an excessively long screw. Although subsequent inves t iga t ion disclosed t h a t the tank had been over f i l l ed , de ta i led study of t h e r i g h t wing wreckage, as previously mentioned, disclosed no s t r u c t u r a l damage o r deformations due t o over pressur iza t ion of t h e tank. review of all other Northwest Air l ines records p e r t i n e n t t o t h e airworthiness of t h i s a i rp lane disclosed nothing of s ignif icance.

A

Original C e r t i f i c a t i o n of t h e L-188

To obtain a type certif icate f o r a fixed-wing airplane, which has a weight

A s a general ru l e t’ie provis ions of t h a t

I n t h e case of t h e Lockheed A i r c r a f t

in excess of l 2 , 5 O O pounds, compliance must be shown with the provis ions of Part 4b of the C i v i l A i r Regulations. P a r t which are i n e f f e c t on t h e da te of appl ica t ion f o r a type C e r t i f i c a t e are the regulat ions appl icable t o t h e type, L-188 appl ica t ion f o r type c e r t i f i c a t i o n was made on November 11, 1955, d t h the r e s u l t t h a t t h e appl icable airworthiness regula t ions were contained i n Part 4b ef fec t ive December 31, 1953, and Amendments 4b-1 and 4b-2 of t h a t P a r t ,

I n addi t ion t o t h e s p e c i f i c requirements contained i n P a r t 4b, Sect ion 4b.lO of t h a t Par t s t a t e s t h a t an a i rp lane s h a l l be e l i g i b l e f o r type c e r t i f i c a t i o n if it complies with t h e airworthiness provisions establ ished by t h e Par t o r if the

‘Administrator f inds t h a t t h e provisions not complied with a r e compensated f o r by other fac tors which provide an equivalent l e v e l of safety. requi res t h e Administrator t o make a finding t h a t no f e a t u r e o r c h a r a c t e r i s t i c of the airplane would render it unsafe f o r t h e t r a n s p o r t category,

This sec t ion a l so

Since t h e turbine-powered a i rp lanes , a t t h e time of t h i s applicati.on, were s t i l l i n the design s tages , t h e C i v i l A i r Xegulations did n o t encoinpass airworthi- ness requirements s p e c i f i c a l l y appl icable t o t h e unique design of these airplanes. Accordingly, the C i v i l Aeronautics Administration developed. a se t of s p e c i a l con- d i t i o n s t o be applicable t o t h i s a i rp lane type. veloped through the a c t i v i t i e s of a Turbine-Powered Transport Evaluation Team composed of employees of t h e C i v i l Aeronautics Administration. i c a t i o n process numerous amendments were made t o P a r t bb of the C i v i l A i r Regula- t i o n s which included many of the appl icable s p e c i a l conditions t o the L-188. July 23, 1957, the C i v i l Aeronautics Board adopted Special C i v i l A i r Regulation Number SR-422 which became ef fec t ive on August 27, 1957. This spec ia l regulat ion contained a revised set of performance requirements for turbine-powered a i rp lanes and made applicable the provisions of P a r t kb of the Civ i l A i r Regulations effec- t i v e on the d a t e of appl ica t ion f o r type c e r t i f i c a t i o n together wi th such provi- s ions of all subsequent amendments t o Par t 4b, i n e f f e c t p r i o r t o August 27, 1957, as t h e Ahministrator of C i v i l Aeronautics f i n d s necessary t o insure . t h a t t h e l eve l of sa fe ty of such a i rp lanes i s equivalent t o t h a t general ly intended by P a r t kb.

The s p e c i a l conditions were de-

Ihrring t h e c e r t i f -

On

I n view of Special Civil A i r Regulation Number SR-422, t h e C i v i l Aeronautics Administration amended the s e t of specin1 conditions appl icable t o t h e Locklicsd 6 1 8 8 t o incorporate those provisions of P a r t IrG i n Amendments 4b-3, /kb-4, and

4b-6 which were comparable t o those spec i f i c spec ia l condi t ions previously establ ished by the Adninis t ra tor , as well as the performance requirements con- ta ined i n SR-422. Those spec ia l conditions which were not incorporated i n t h e aforementioned amendments were re ta ined ,

I n order t o monitor and approve t h e type c e r t i f i c a t i o n of a i rcraf t the C i v i l Aeronautics Administration es tab l i shed Regional Off ices throughout the United S ta tes . I n the case of t h e Loclchead I,-188, Region I V was respansib2.e f o r determining t h a t t h e a i rp lane type complied with t h e C i v i l A i r Regulations and t h e appl icable s p e c i a l condi t ions, For many years t h e C i v i l Aeronautics Administration has u t i l i z e d a designee system t o assure compliance with the C i v i l A i r Regulations. number of personnel ava i l ab le i n the CM's f i e l d o f f i c e s , IJnnder this system designated employees of the appl icant are delegated t o approve c e r t a i n data, drawings, e t c , t a ined by t h e C i v i l Aeronautics Administration, but t h e ac tua l ana lys i s of t h e d a t a was approved by the designees and reviewed by t he Administrator. a r ea of t h e c e r t i f i c a t i o n process where designees are used qu i t e spa r ing ly is in t he f l i g h t t e s t area, C i v i l Aeronautics Administration employees.

The establishment of t h i s system was due t o t h e limited

The approval of t h e bas i c d a t a and methad of ana lys i s was re-

The o&g

In almost all cases t h e f l i g h t tests were conducted by

On August 22, 1958, t h e C i v i l Aeronautics Administration issued type certif- icate No. 4A22 approving t h e Lockheed L-188A-08, and L188C type a i rp lanes .

Major S t r u c t u r a l D i f f i c u l t i e s Encountered a f t e r C e r t i f i c a t i o n / -

Subsequent t o t h e de l ive ry of t h e first f e w a i rp lanes , Lockheed Ai rc ra f t conducted a f l i g h t t e s t t o de te rn ine t h e c h a r a c t e r i s t i c s of a mechanical dis- connect f c r t h e f l i g h t con t ro l boost system at the design dive speed of !rOS knots. This f l i g h t t e s t was conducted on October 31, 1959, and consis ted of d:i.v:irig "che a i rp l ane from c r u i s e a l t i t u d e with var ious boos te rs disconnected. dive, with the speed maintained a t o r s l i g h t l y below t h e speed f o r l h d C mch number, turbulence was encountered; t h e speed was dropped .off 6 t o 8 Icnots, After passing through t h e turbulence a f u e l leak was observed from under the r i g h t - i g . Ground inspect ion disclosed t h a t t he main damage was halfway between Nos, 3 xici h engines. leaking; i n addi t ion, t he re was a shallow buckle near t h e rear beam j1.iS.t inboard of t h e No. 4 nacel le . measurements ind ica ted t h a t t h e failure was due t o high wing to r s ions .

On t h e second

This consis ted of some r i v e t s with missing heads from which f u e l was

The nature of t h e wing damage and subsequont, i n f l i g h t

A s a r e s u l t of t h i s d i f f i c u l t y , t h e aimplanes already de l ivered were speed The r e s u l t i n g fix con- r e s t r i c t e d u n t i l a f ix could be designed and i n s t a l l e d .

s i s t e d of re inforc ing t h e wing between t h e inboard and outboard nace l les .

During t h e o r i g i n a l c e r t i f i c a t i o n of t h e E l e c t m t h e a i rg l ane was equipped with Al l i son engines and Aeroproducts propel le rs , C e r t i f i c a t i o n included Q vibra- t o r y stress survey of the propel le rs . t h a t t h e inboard p rope l l e r s were the more c r i t i c a l and only t h e inboard propel le rs were instrumented. La ter a Hamiltor? Standard p rope l l e r was i n s t a l l e d on t h e air- plane and a new c e r t i f i c a t i o n was sought. A t t h i s time it was decided t o conduct t he v ib ra to ry s t r e s s survey on one outboard and one inboard propel.ler. of t h i s t e s t it was found t h a t t h e outboard propel le rs were more h ighly stmssetl than t h e inboard p rope l l e r s and t h a t these s t r e s s e s exceeded acceptable Invc1.s. This condi t ion was caused by a higher t h a n ant:lcipated i n f l o w angle due t o a

It was determined, based on p a s t experience,

fis a r e s u l '

. i * .

" 15 - downward torsional. bending of t h e wlng with increasing speed, Outboard propeller blade stresses were r edwed s a t i s f a c t o r i l y bj rekorking the nace l les t o provide a. .- 3 O u p t i l t of t h e propel le r plane, reduce cabin noises cgnd v ibra t ions ,

during landing which caused cracks i n t h e rLlJ-ed wing skin both outboard m d in- board of the inboard nace l l e s and loosening of the f a s t ene r s a t tach ing the uppar and lowor wing panels t o t h e main l a d i n g gear ribs, Service Bu l l e t in s 306 and 337 which requi red tho i n a t a l l a t i o n of a doubler outhoKx?. and inboard of each inboard nacelle on tho upper w h g surface respectf.iely.

In addi t ion, d i f f i c u l t y has r e s u l t e d from overpressurizat ion of t h e fuo l

Inboard nace l l e s were similarly modified t o

D i f f i c u l t y was encountered on the E lec t r a oisglanes with impact, s t r e s s e s

AB a re$ul.t, Lockhead issuod

system. In one case where forsip material. wa3 i n t h e fue l manifold system trra f u e l in le t valve was held i n t h e open pos i t i on a f t e r the tank was f i l l ed . Con- sequently, structural deformation of t h e wing r e su l t ed and an inspect ion of all fuel manifolds was conducted, Lockbed be l ieves t h a t i f the correct procedures are followed during t h e r e fue l ing operat ion such failures w i l l no t ocewe

Reevaluation Program

On March 20, 1960, t h e FAA issued, as a temporary measure, an emergency airworthiness regula t ion which reduced t h e Eloc t ra Vno from 324 h o t s CA§ t o 275 knots o r 0.55 Mach. Following a meeting on March 22 with ropresentaLives c?f Lackheed, Al l i son (GMC), E lec t r a operators, NASA, and CAB, t h e FAA took t h e f o l - lowing add i t iona l ac t ion :

1, Because this and a previous Eleetra acc ident were bel ieved t o have occurred a t o r near a c ru i s ing speed of 275 knots CAS, it was considered t~sC:esSslp.,. t o make a f u r t h e r speed reduct ion .to provide an adequate s a f e t y mzrgj.nr quently, a second emergency airworthiness regula t ion was issued on March 25, 1960, l i m i t i n g Vno t o 225 knots CAS or 0.55 Mach and e s t ab l i sh ing a Vn, of 245 k n r O t S CAC or 0.55 Mach.

. f o r h m e d i a t e p rope l l e r f ea the r ing if t h e torquemeter ind ica tor r eg i s t e red zero or f u l l scale; deac t iva t ion of the a u t o p i l o t u n t i l appropriate modifications could be designed and instal led; adherence t o Lockheed prescr ibed procedurits In r e fue l ing operations.

Under emergency a u t h o r i t y spec i f i ed i n Sect ions 40.21., bl.1, and 112.5 of t h e C i v i l Air Regulations, t h e FAA, in an amandnient t o the Operations 2y:::Cif- i c a t ions , ordered a one-time inspect ion on a l l E lec t r a s within 30 days of ?At? order date, March 25, 1960. turbulence inspec t ion spec i f i ed in t h e Lockheed S t r u c t u r a l Repair Manual, an i n t e r n a l examination of t h e e n t i r e wing with emphasis on wing ribs f o r danaged attachment tabs, bucMed r i b braces, loose o r sheared r i v e t s , and damaged or cracked c l ip s . Additionally, a thorough inspec t ion of ' t he elevators, elevator tabs, and related attachments was a lso required during the s m e jO-day per iod . The amended Operations Spec i f ica t ions f u r t h e r ca l l ed f o r d a i l y inspec t ions of powerplant magnetic sump, plugs; inspect ion of f u e l tanks involved i n a reported tank overpressurization.; s t r u c t u r a l inspec t ions following any reported inc idents of f l i g h t through severe turbulence, h a d landings, o r overweight landings,

I

Coma-

Also included i n t h i s second regula t ion were requiremefits c a l l i n g

2,

The inspec t ion included, i n addi t ion t o the severe

3. On March 25, 1960, t h e FAA n o t i f i e d the Chiefs, F l igh t S tmdards Divisions t h a t observance and surve i l lance of 1,188 a i r c r a f t en route operation

-16 - and t r a i n i n g was t o be increased f o r a per iod of 30 days. ied t h a t inspect ions should be concentrated i n the a reas of f l i g h t planning, pre- f l i g h t , placard speeds, operat ing techniques, inadvertent en t ry i n t o turbulerlce, abnormal equipment operation, post, f l i g h t a c t i v i t i e s , and f l i g h t t ra in ing .

The telegram specif-

On March 25, 1960, following severa l meetings i n which the Elec t ra problem was discussed, t he Administrator requested the Luckheed Ai rc ra f t Corporation t o conduct an engineering reevaPuat,lon of the Eiectra. The objec t ive of t h i s pro- gram was t o r evea l any design or opera t iona l c h a r a c t e r i s t i c s of the a f~rp lane caus- ing s t r u c t u r a l e f f ec t3 more c r i t i c a l than those provided f o r and possibly influen- cing d i s in t eg ra t ion i n f l i g h t , , Brief ly , the program encompassed f l i g h t t e s t s , s t ruc tu res inves t iga t ions , aerodynamics inves t iga t ions , design s tudies arid spacial. inves t iga t ions , and t e s t s . Extensive a s s i s t ance vas provided by the NASA, Boeing, Douglas, and other organizat ions i n carrying out t h i s program. appropriate t o the equipment, was a l s o ca r r i ed out with respec t t o t he engines and propel lers .

A like program,

Included i n the f l i g h t t es t program were expanded measuranents of wing and nace l l e loads a i d stresses duririg smooth and abrupt maneuvers, meauuremnt o f t h o dynamic response of t h e wing and nace l les during g u s t s , extension of f l i g h t f l u t - t e r response t e s t s , expanded measurements of i n t e r n a l 103d.9 2nd s t r e s s dis'trlbu- t i o n i n t h e wing and nacel les , and reana lys is of i n f l i g h t loads mdasurerients made p r i o r t o the accident.

Numerous s t i f f n e s s and r i g i d i t y tests were made on Elec t ra serial No, 1077 far use i n f l i g h t dynamics analysis. r i g i d i t i e s frdm tho outboard propel le r plane through t h e engine, nacel le , and wing t o the fuselage center l ine . poifits Fn the outboard engine/nacelle i n s t a l l a t i o n were measured, but not a t any pofnt i n the wing o t ruc ture i t s e l f .

Primary a t t e n t i o n was d i r ec t ed t o component

The e f f e c t s of simulated failures at, various

In reevaluat ion of t h e airplane con t ro l system and au top i lo t cha rac t e r i s t i c s , spec ia l a t t e n t i o n was d i r ec t ed t o t h e inf luence of possible malfurictions, fail- u ~ e s , and induced e f f e c t s on the sudden buildup of des t ruc t ive cont ro l forces . The inves t iga t ion included both ana ly t i ca l methods and t h e use of an e leva tor system func t iona l mockup. A rigorous series of tes ts was conducted on t h e mochvp t o induce o s c i l l a t o r y o r o ther performance f a i l u r e s under extreme sirrulnt,ed f a i l - ures and malfunctions i n t h e system. a hazardous s i t u a t i o n under t h e f l i g h t conditions of the subjec t a i r c r a f t ,

Nothing was found t h a t might have produced

A comprehensive revicw was made of all s t r eng th m a l y s i s procedwes covering methods of determining i n t e r n a l loads, allowable s t rengths , and margins of safety. In addi t ion t o review of t h e o r ig ina l analysis, r e f ined procedures were appl ied t o t h e wing, wing r i b , and wing beam analyses. In addi t ion, the e f f e c t of damaged ribs on othei. r i b loads and on t h e r i g i d i t y and s t rength of the wing was computed. Since t h e QEC s t ruc tu re had previously been s t a t i c - t e s t e d t o u l t imate s t rength , a t t e n t i o n was focused on changes i n t h e design loads imposed,

Reinvestigation of t he s t r u c t u r a l loads was performed i n regard t o the following: wing loads i n maneuvers, wing loads due t o gusts, landing loads, and loads produced by au top i lo t malfunct ions. Loads and stresses detcmiined i n the above-mentioned f l i g h t tests were used extensively in this program.

Heaudit of t h e f l u t t e r c h a r a c t e r i s t i c s was divided i n t o two areas of ana lys i s and tes t . cesses of analog and d i g i t a l . Wind tunnel tests were conducted on throe d i f f e r e n t modols. The first consisted of a nacel le-propel ler model i n t h e Lockheed 8-by-12-foot tunnel in which s t s f - ness i n p i t c h and yaw was var ied over broad r a g e s . The second was R I ~ eighth scale half-span dynamic model of the wing with naca l les and propellers. This was t e s t e d i n the Lockheed tunnel with varying engine-p-cpeller s t i f f n e s s m d v-iations in wing f u e l quaratity. The th i rQ , nn eighth s c a l e model of th6 complete a i rp lane , was tested i n the NASA 19-foot Langley tunnel. More corcplote v a r i a t i o n s of engine-propeller s t i f f n e s s and danping and wing fuel. d is t r ibut- l ?as were covered. gated.

Analyt ical so lu t ions wore obtained by two hdspondent pro- I n tho Zat tor , 59 degrees of froedom were used,

I n addi t ion, t h e e f fec ts of propel le r ovarapesding were i n v s j t i -

The reevaluat ion program disclosed two discrepancies in t h e design of t h e a i rplane. mediate r ib s between t h e fuselage and outboard nacel los by she l l d i s t o r t i o n i - ~ d not been included i n t h e design loads. The other was t ha t t h e dynamic rcsponse of t h e outboard nace l les in turbulence was d i f f e r e n t from t h a t used in t h o or ig- inal design, with t h e result t h a t t h e t o r s i o n a l loading of t h e wing i n b o ~ d there- of was increased. I n addi t ion, t h e reevaluat ion program disclosed tha t Mith t h o

o s c i l l a t i o n s could become des t ruc t ive at t h e o p e r a t b g speed of N 121US e% the time of the accident.

Analysis and Conclusions

One of these was t h a t s i g n i f i c a n t loads imposed on the wing i n t e r -

s t i f f n e s s of a powerplant-nacelle i n s t a l l a t i o n reducod below normal prop’.’ J Ah3r

A study of t h e operat ional aspects of F l i g h t 7l.O l eads t o certa:’Ln defkii’ce conclusions, cedurea, t h e f i l e d f l i g h t plan was baing c lose ly follvded, and u.p t o t h e t h e ci’ breakup all checkpoints had been reported a p p r o x h i t d y as e s t b t e c l , segment of t h e inves t iga t ion nothing was found which would produce a clue as t o t h e cause of the accident.

The f l i g h t was being conducted in accorchcre with c m p u y pz-

I n this

Examination of a l l t h e meteorologioal m d operat ional evidence st h,md re- veals t h a t at 18,000 feet i n t h e v i c in i ty of‘ Camelton, IndSana, t h e eircr.Gt concerned was operating In an a r e a devoid of clouds with t h e f‘oI.Pos$rg signi- f i c a n t meteorologictll c h a r a c t e r i s t i c s :

(1) j u s t t o t h e east of a marked trough line. (2 ) beneath and on the northern edge of a j o t stream with high

veloci ty southwest-northeast flow ( b c r ~ a ~ h g with height) at a l l l e v e l s from the surface t o tho j e t stream.

( 3 ) marked horizontal and v e r t i c a l wind shear. ( 4 ) pronounced horizontal thermal gradient arld p o t e n t i a l l y

Tho abovo sununary is derived from ground-based meteorological obssrvnt ~ o n s

unstable lapse r a t e s .

and a s u b s t a n t i a l number of p i l o t weather reports.

The above f a c t o r s and t h e mgn i tudo of each clearly i n d i c a t e t h a t severe c l e a r air turbulence was highly probable :).t t he time and place of t h e accident ,

- 18 - P i l o t weather r epor t s of a c t u a l c l e a r air twhulence encounterv 011 t h a t date likewise afford valuable information subs t an t i a t ing t h e above conclusion.

Aftor observing and f o r e c a s t h g R wind f i e l d embodying widely recognized metoorological factors u t i l i z e d in the fo recas t ing of clear a i r turbulencs ~ t h e Board bel ieves t h a t t h e responsib1.e of f ices of t h e U , S. Neather Bureau Lnd Northwest A i r l i nes should have mentioned clear air turbulence in t h e i r fo recas t s .

Three separa te and independent, s tud ie s of t h e c1ea.r a i r turbulonce s i t u t i o n as it relates t o t h i s accident have been c a r r i e d out by agencies other t h w tha CAB (Westher Bureau, New York University, and Meteorology Research, Inc.) . ‘:hc conclusions reached i r~ these s t u d i e s are in exceptionally good agrement aria support t h e conclusion of t h e Board’s own study as surMnar.ized above. r eca l l ed t h a t c e r t a i n p i l o t s f l y j n g a t 31,000 f e e t observed B horizontal s t r e m e , ’ of smoke extending southward t o a smoke cloud with corkscrew-shaped base. ‘:cns!J- e r ing t h e c h a r a c t e r i s t i c s of c l e a r a i r turbulence as opposed t o convective turSu- lence, i t i s not d i f f i c u l t t o understand t h e pers is tence of a r e l a t i v e l y wel.1- defined smoke column.

Z t w i l l be

Trajectory st ,udies of pieces of t h e a i r c r a f t wreckage indicated possible d i f f e rences i n sequence of separa t ion , p a r t i c u l a r l y i n regard t o li.ght pigces, depending on assumed var iab les . However, t h e s tud ie s indicated as most probable t h a t t h e a i rcraf t was i n l e v e l f l i g h t at an a1t i tud.e of 18,000 f ee t , Find a t r J e course of 170 degrees a t an indicated airspeed of approximately 260 knots during t h e d i s in t eg ra t ion . This ana lys i s i nd ica t e s t h a t t he first p a r t s t o . s c p a r a t e were pieces of t h e r i g h t wing upper surface j u s t outboard of $he fuselage m d t h a t t h e powerplant and wing d i a h t e g r a t i o n s took place within a period of s i x t o 10 seconds. Disregarding t h e calculated results involving l i g h t pieces md extremely shor t differences i n items of’ separation, t h e t r a j e c t o r y analysis i nd ica t e s a lso t h a t separat ions of’ t h s l e f t outboard powerplane irAstal.laticn m d t h e l e f t outer wing s t ruc ture began almost simultaneously with the r i g h t wing separati,on and t h a t separat ion of tho r i g h t outboard powerplant i m t a l l a t i o n began shor t ly afterward.

A s previously mentioned, impact, aid f i r e darnage to conponents of the v:rrious systems of t h e aircraft precluded func t iona l testir lg of Lie r.lajor:Ltfy ef stich ittew. However, d e t a i l e d inspect ion of a l l recovered systems componer&s, ct?d flmct,icnal checks of those itenis s t i l l capable of being t e s t e d , f a i l e d t o disclosa any evidence of operat ional d i s t r e s s o r indicat ion of malfurlctioning of m y c0rnponei:t or system. was not being attempted and t h e crossfeed valve pos i t i ons vere conslatent u i t h normal tank-to-engine f u e l u t i l i z a t i o n procedures. Examination of t h e con t ro l sur face boosters f a i l e d t o show whether t h e autopil.ot was i n opcratirin o r t o i nd ica t e conclusively whether t he boosters were i n t h e ttmrulual’l or Ilboost” con- f i g u r a t i o n ,

The f u e l dump valve and chute pos i t i ons indicated t h a t f u e l dwi,-):ng

Inves t iga t ion of t h e powerplants revealed no evidence of x d f u n c t i o n o r f a i lu re t h a t contributed t o t h e cause of the accident. O f t he numerous i‘,ems studied i n d e t a i l , no one considered alone p rov ides an iJlswer as t o tile causc of t h e accidorit. However, t h e powerplmts d id provide inforiaation t h a t c ; ~ n be co r re l a t ed with other known facts and circunstances of t he accident.

- 19 - Circumstances of tho separation within Nos. 1 and 4 engines a r e of primary

s ignif icance and t h e r e a r e ind ica t ions of s i m i l a r i t y . separat ions was very short,, as evidenced by the loca t ions whore they f e l l and t h e t r a j e c t o r y s tudies with No. 1 separat ing f i rs t .

The time i n t e r v a l between

Obviously, abnormal loads were required t o br lng about these separat ions s ince there i s a complete lack of evidancs of any progressive f a t i g u e f a i l u r a t o t h e point where separation occurred under noma1 loadings. Likewise, it i s not conceivable t h a t f a t i g u e cracks wou-?d start wid progress p r a c t i c a l l y simultaneously t o f a i l u r e i n two d i f fe ren t loca t ions on t h e two engines. more, there i s no s t r u c t u r a l f a i l u r e h i s t o r y of t h i s model e n g h o t o suggest such an occurrence.

Further-

Aluminum deposi ts on t h e thermocouples and t w b i n a i n l e t guide vanes of Nos. 1 and 4 engines a r e believed t o be s igni f icant . on turbine engines when t h e compressor blades contact and machine away aluminum p a r t i c l e s engines t h a t separated i n t h e a i r cannot be accepted as coincidental . It i s believed r o t a t i ~ - n a l in te r fe rence which resu l ted i n the aluminum deposi ts wks caused by a i r i:\lot and compressor case d i s t o r t i o n due t o a b n o m 1 loads being applied through lorquemeter housing and struts of these engines. Furtharmora, t h e abnormal load:. followed d is rupt ion of t h e engine supporting strtzctura so t h a t loads normally taken out by t h e Lord mounts and QEC s t r u c t w e were imposed on t h e engine s t ruc ture . It follows t h a t t h e bas ic engine s t ruc turo forward of t h e compressor must have been i n t a c t i n order t o transmit propel le r generated case d i s t o r t i n g loads.

Such doposi ts are sxpected

while the engine i s operating. These deposi ts on t h e two outboard

A study of t h e pieces of t h e No. 1 reduction gear housing d id not revecd. any evidence of repeated contacts o r movements of tho p a r t s ; howvex-, tiie:-e V ~ C ;

indicat ions of changes i n d i r e c t i o n and a r e v e r s a l of t h e r e l a t i v e motion of adjacent p a r t s , spec i f ica l ly t h e p a r t which includes t h e l e f t s t r u t eyebol t bee, and t h e adjacent piece which encompasses tho l e f t QEC t o reduction gear n~xar3-t pad, i d e n t i f i e d as pieces one and two, respect ively. There a r e marks that were by t h e forward s i d e of piece two novjng i n tho outboard d i r e c t i o n and screpir,g against two corners of t h e c a s t e l l a t e d 0yoboI.t nut. The loca t ion of t h e marks a l s o ind ica tes t h a t piece two moved a shor t dis tance downwwd and forward. Abrasion marks on t h e edge of t h e f r a c t u r e at t h e lower rear corner of t h e l e f t mount pad i n d i c a t e d - a s l i g h t downward, forward, and twis t ing of piece two w i t h respect t o piece one. t h a t t h e nut was contacted; subsequently, abrasion marks were rnade which h i d i - cated piece two moved upward and s1ightI.y toward the rear. not subs tan t ia te whi r l mods; however, they a r e not Inconsis tent with what d g h t be expected were whir l mode t o be in progress as breakup occurred.

These marks probably were made at about tho s'me t h e

These mrks do

The f r a c t u r e s of t h e s t r u c t u r e of t h e No. 4 engine did not reveal any markings which showed load r e v e r s a l s as separat ion occurred. Th5 only i n d i - ca t ion of load r e v e r s a l s on t h i s engine was at t h e f r o n t end of t h e compressor shaft extension where separat ion from t h e torqusmetor occurred. Loadings un both s ides of t h e sp l ines , rearward upset of some of t h e sp l ine ends, and l igh t , longltuclinalmarkings in a rearward d i r e c t i o n on some of t h e s p l i c a s suggest some movement other than a s t r a i g h t pu3.1 away. Gross misul igment as would :*e-- s u l t from a whirling motion at t h e propel-lcr, coupled with an r,p.rc. d i f f e ren t i a l between the two separat ing p a r t s , i s compatible with t h e murkicgs.

- 20 - Examination pad study of t h e a k c m f t s t r u c t u r a l wreckage narrowed tho

f a i l u r e areas of possible s i g n i f i c m c e t 3 tho i n b o u d portion o f the r i g h t t ~ b g , t h e outboard engine support s t ruc tures , and the left; elevator. eliminated t h e probabi l i ty of s t r u c t u r d f a i l u r e dire t o fa t iguo cracking, missing p a r t s , nonconforming mater ia l s , ruld overtorqu.hg of nuts .

Although t h e outer end of the left el.evator d i s in tegra ted because of f l u t t e r , t h e wreckage d i s t r i b u t i o n proves that t h i s eecu-rred appreciably subsequent t o the r i g h t wing separation a i d shor t ly bsforo thc fu.se3.sge s t ruck t h e gro*u~cl. addi t ion, t h e t r a j e c t o r y ca lcu la t ions i n d i c a t s ?&it a% the t h e of the d e v a t o r f l u t t e r tho airspeed was much in excess of the design dive spasd. A s t i r a s l d t , t h e d is in tegra t ion of t h e l e f t outboard e leva tor was Y co~isequence of t k : ~ w h , g separation and c t m o t be ccnsidorsd an indica t ion af unsirworthy conditiors p r i o r thereto. been involved i n t h e catastrophic disintegration a r e the w h g imd e n g h e support s t ruc tures .

This work also

Xn

The only remaking purta of the a i r c m f t which a;)pear t o have

As developed under inves t iga t ion , a detail.ed s t u d y of t h e dar~a,gs t o t h e r i g h t wing s t r u c t u r e between t h e fuselage and t h e inboard nace l le d i s c l o x d numerous ind ica t ions of damage progression d u s h g rapid r e v e r s d s of 1c~:ding. The separation and upward b u c k l h g of the f r o n t spar csp flange f r o 3 t h o vesti- ca l l e g between s t a t i o n s 78 and 89 i s on0 example. If t h i s had occur-red d ~ i n t : a sustained up-gust or p o s i t i v e mmsuver, it and the associated disrupt ion of the wing box upper cover could result only in t h e wing f o l d h g upward during sqnra- t i o n from t h e fuselage r a t h e r than rearward as it did.

I n t h i s same area of t h e wing tho previoudy discussed damace t o t h e and r i b s of t h e inboard hinged leading edge and t h e ix-regillarly saw-toothed diagonal f r a c t u r e l h e s in t h e bottom cover arc3 frlrthcr evidence of reversing loads , both bending and tors ion . only with catastrophic f l u t t e r .

This type of dmurge progression appears t o bc c o n s i s t m t

The r i b and r i b attachment diiwge found im this sane area of t h e wkAg co!d.Cl possibly bo e n t i r e l y t h e r e s u l t of abnormal reversing s t r e s s e s associtited v i t h t h e f l u t t e r . o ther Elec t ras a f t e r abnormal ground loading could ba i r ,dicat ise of damage pr io i . t o t h e . onset of f l u t t e r .

However, t h e s i m i l a r i t y of some of t h i s damage t o t h a t f o m d on

The d e t a i l e d examination of tho olatbocard powerplant support s t r ixt8aes d i s . closed a d d i t i o n a l evidence 'of cycl ing i n t h e f o m of d m g e due t o repeated bot tomhg of t h e f r o n t Lord mounts, curved scratches on one of t h e swi ; . l s t ra ighteners , and repeated In te r fe rence o f f rac tured surfaces. These, parri- cular ly t h e curved scratches on tho swirl s t ra ightener , a r e ind ica t ive of iP.e propel le rs having o s c i l l a t e d v i o l e n t l y f o r a shor t per iod of time p r i o r t o t h e gross overal.1 displacement which occurred during t h e d is in tegra t ion of the ~ ( J w ~ T -

plant support s t ruc ture . obviously caused rapid progression of damago t c thcs powerplsnt support s t r u c t w e .

Tho enorgy associated with t h i s v i o l e n t o sc l l b t iw

Insofar (is t h i s accident is concerned, one developxent of t h e reeva1:jation program i s most signif 'icant. p rope l le r o s c i l l a t i o n known as "whir l modo1' c m under c e r t a i n c o n d i t i w s ca;:se f l u t t e r and s t r u c t u r a l dis is i tegrat lou,

This 1 3 t h a t 011 t h e 2 l e c t r a tho previously mentill:: d

- 21 - This i s t rue desp i t e t h e fact t h a t all of t h e f l f i t ter tests and analyses

made by Lockheed during t h e o r i g i n a l c e r t i f i c a t i o n process and during reevalua- t i o n showed t h e E l e c t r a t o bo f lu t te r - f ree u t and above normal operating speeds, and f u r t h e r disclosed t h a t t h e wing has a high degree of damping. means t h a t an o s c i l l a t i n g mot-ion of t h e s t r u c t u r e w i l l d i e out rapidly when t h e exci t ing f o r c e i s removed; t h e daraping fo rces are those which tslri: energy awny from t h e o s c i l l a t i o n . t i o n jn t h o s t r u c t u r e and in energy absorbsrs such as engine mounts. The most s ign i f i can t damping, however, i s t h e resul t of zerodynmic f o r c e s ac t ing in opposition, t hus absorbing energy from t h e o s c i l l a t i o n . Conversely, i f ri mjor change occurs t h a t allows t h e aerodynamic f o r c e s t o be add i t ive t o t h e e x c i t h g force, t h e o s c i l l a t i o n grows and t h e resu l t i s f l u t t e r .

The l a t t e r

A small amount; of dnlcping i s from i n t e r n a l energy absorp-

Since t h e Electra wing is b a s i c a l l y f l u t t e r resistant, i n order t o produce The possible f o r c e gensrators f l u t t e r t h e r e must be an e x t e r n a l d r iv ing force.

are t h e con t ro l surfaces and t h e propel lers . t r o l sur faces would not produce wing o s c i l l a t i o n s of s u f f i c i e n t amplitude 60 produce a fa i lure , consequently f u r t h e r ana lys i s was centered arourrc! t h e pro-

Analyses indicated t h a t tho con-

p e i l e r . Since t h e p rope l l e r s are normally s t a b i l i z i n g , it was

s i d e r abnormal p rope l l e r behavior such as overspeeding and and tes t s conducted during t h e reevalucttion program proved board p rope l l e r caused by a weakened nace l l e s t r u c t u r e can t ions.

necessary t o con- wobbling. The s tud ie s t h a t a wobbling 0u.t - induce wing o s c i l l a -

Since a p rope l l e r has gyroscopic c h a r a c t e r i s t i c s , it w i l l tend t,o stay i? i t s plane of r o t a t i o n u n t i l it is displaced by some s t rong ex te rna l f o r c s svtch as turbulence, a n abrupt maneuver, o r power surge. When such a. fo rce o r ~ ~ ? e ? n t i s applied, t h e p rope l l e r reacts in a d i r e c t i o n 90 degrees t o t h e force . example, if t h e p rope l l e r i s displace6 updard, t h e r e s i s t ance of t h e structi-;re app l i e s a nosedown p i t ch ing moment, causing t h e p rope l l e r d i s c t o swing t o t h e l e f t due t o precession. The yaw s t i f f n e s s resists t h i s motion causing precessjon downward, r e s i s t e d by pi tching s t i f f n e s s which produces a processional swing t o t h e r i g h t . This, i n t u r n , i s r e s i s t e d t o cause an upward precession t o cmp%eLe t h e cycle , This e f f e c t is termed "whirl mode" and i t s d i r e c t i o n or' twtatioii i s counter t o t h a t of t ho propel ler .

For

Normally, whir l mode can operate only within t h e f l e x i b i l i t y limits oi' tha engine mounting structure, and i s quickly damped. If, however, t h e stifi'ncs:: 02 t h e supporting system i s reduced through f a i l e d or dmaged powerplant st.r:Jc",:>*o I

mounts, o r nace l le s t ruc tu re , t h e damping of whirl Inode i s reduced t o a degree depending on t h e amount of s t i f f n e s s reduction.

Powerplant s t r u c t u r a l weakness o r damage does not s i g n i f i c a n t l y change t h a conditions under which whir l mode may be i n i t i a t e d , but i n t h r e e ways it i rzkes t h e phenomenon a po ten t i a l danger:

1. The g rea t e r f l e x i b i l i t y of a weakened system c m allow whi r l mode more freedom, hence it can become more v io i en t . undmlaged system t h e s t i f fnesa increases with increasing de f l ec t ions , but t h i s i s not nece;:sarily t r u e i f t h e s t r u c t u r e i s damaged.

I n an

- 22 - 2. I n a weakened i n s t a l l a t i o n , the increas.S.ng violence of whirl

mode can f u r t h o r clamnge t h e supporting s t ruc ture , ;ha turn leading progreaairs ly t o more violence and 0ven f u r t h e r h g e .

3. As t h a s t r u c t u r a l system 1s ciaaged reducing t h e spring- constant, t h e amplitude of whirl ruode lncraaacts and tho f re- quency decreases f rm its natural v d u o t o IOWGT values which approach t h e wing fblrmdaulantal frequencies.

The natural frequency of whirl mode in an UCT sd jlnstalbrttion 53 ap- proximately f i v e cycles per second. and wing bending about two cycles per second, with some slight v a r i a t i o n w i t h f u e l loading.

The w h g t o r s i o n a l frequency Pz aboht 3.5,

As whir l mode progresses in an overly flexible or ckmmged p o w e s p h t i n s t a l l a t i o n , i ts frequency can reduce fram f i v e t o three cycles par second where it w i l l d r i v e the wing h three cycles per second t o r s i o n a l and bending o s c i l l a t i o n s . mode. The three o s c i l l a t i o n s are then coupled at t h e s m ~ frequency of about t h r e e cycles per second, thus Becoming a form of induced f l u t t e r forced by a powerful harmonic o s c i l l a t i o n . Thio phenmenon can exist, as demonatratad In wind tunnel tests and in a n a l y t i c a l methods, at an airspeed far below t h a t at which c l a s s i c a l f l u t t e r can develop.

inch pounds per radian (root-man-square) s t i f f n e s s level , whir l mode cmnot f o r c e wing o s c i l l a t i o n s at riny speed below UC percent of t h e design dive speed of t h e aircraft. If, howevor, t h e st .Ufnsss I.: reduced, forced o s c i l l a t i c n s become more l i k e l y depending on m.0im-L of EtiffnesE reduction Rnd on equivalont airspeed. More s p e c i f i c a l l t h e s h m that if

whir l mode could b e c a e a dr iv ing force on t h e wing in t h e c ru is ing speed r,wga, The tests f u r t h e r showed that whir l mods of catastrophic proportions could develop, reduce i t s frequency, and couple with the wing irr a period of f r m 20 t o 40 secoads.

These wing o s c i l l a t i o n s w i l l r e inforce and perpetuate t h e whirl

6 The s t i f f n e s s f a c t o r f o r an undmagad po.derplant i n s t a l l a t i o n i s 15.9 x 10 The tests indicated tb.t at this t

t h e s t i f f n e s s i s reduced t o 80me value less than 8 x 10 %, inch pounds per mxiim3

I n recapi tu la t ion , t h e reevaluatim of the Electra disclosod that t h a whir l mode can induce f l u t t e r i n a wing highl r r e s i s t a n t t o f l u t t e r , t r ~ j e c t c ~ r y stt.,dier disclosed that t h e indicated airspeed of N 121US was approxbata ly 260 k~icts st t h e time of d ie in tegra t ion , study of the wreckage of N 121US disclosad t h a t the r i g h t wing separation r e s u l t e d from f l u t t e r , t h e outboard pwerpltlrat n w e l l c diu- h t e g r a t i o n s involved o s c i a t i o n s c h a r a c t o r i s t i c of the whir l modlo, m d ~,m.lysiu of t h e weather ret the th0 and place of the accident disclosed the existence of clear a i r turbulence which can e x c i t e t h e whirl mode. f o r e , t h a t t h e whi r l mode provided t h e dr iving force essential . t.o destruction of t h e wing. des t ruc t ive at normal operating speed i s not es tabl ished.

It must be concluded, t h e m - -

However, t h e sequence of events that l e d t o t he whir l modo b 8 C C m h g

One p o s s i b i l i t y i s that in penetrat ing th6 clear a i r turbulence, no s k g l e pulae of which could cause an overload, N UlUS may bnve been subjectsd. t o a r a p i d succession of impulses at t h e proper frequency t o cause dynamic response damaging t h e engine support s t r u c t u r e and enabling the whirl. mocie t o becam self- sustaining. However, uniformly timod impulses with ; suf f ic ien t snargy at the

-- 23 -

necessary frequency are extremely improbable in natural turbulence, which usually has the c h a r a c t e r i s t i c of being random both i n frequency and i n i n t e n s i t y .

-I..

A second p o s s i b i l i t y i s t h a t there was s u f f i c i e n t p r i o r damage in one of the outboard nacell-es alone t o reduce tho s t i f f n e s s t o t h e range where, once exciLp6 by turbulence, t h e whir l mode was self-sustaining and rap id ly becane divergent. This p o s s i b i l i t y hinges on extremely severe p r i o r damage, which does not appear l i k e l y t o have escaped detect ion during the d e t a i l e d examination and study of t h e wreckage.

A t h i r d p o s s i b i l i t y appears t o be p r i o r damage t o t h s wing; f o r exmple, p a r t i a l l y disrupted r i b s , as suggested but not proved by the previously r a m t i o a ~ d evidence of rubbing between mating p a r t s found on separate pieces of urecbgtj.. With such a condition, penetrat ion of severe c l e a r air turbulence i n the a r e a of Cannelton could conceivably r e s u l t i n rapid progression of wing d a m p . could a l s o cause change i n t h e already more c r i t i c a l than expected dynamic re- sponse s u f f i c i e n t t o damage t h e outbozrd powerpltmt support s t r u c t u r e s , thersby causing t h e whir l mode t o become s’elf-sustaining. l a t i o n s by t h e manufacturer tend t o discount t h e p o s s i b i l i t y of l h i t e d pz6or wing damage having any s i g n i f i c a n t e f f e c t in t h i s regard, no d y n a i c t e s t s have been conducted t o support t h e calculat ions. ac t ions under dynamic conditions with damaged rib s t r u c t u r e , it is concluded that only such t e s t s of a f u l l - s c a l e s t r u c t u r e could e i t h e r prove o r disprove t h i s p o s s i b i l i t y .

T h i s

Although extensive c d c u -

Due t o the extremely complex i n t e r -

The landing of N 121US at Chicago on t h e day of t h e accident. may well hove caused damage t o t h e wing s t r u c t u r e even though some of t h e passengers considered it a per fec t ly normal landing. senses only t h e r e s u l t a n t of t h e ac t ing forces and t h a t i n p a r t s of t h e caliin of l a r g e a i r c r a f t very high l i n e a r acce lera t ions due t o ground loads can be ~ r a c k i - c a l l y canceled by very high angular accelerat ions. impacts on t h e landing gear s u f f i c i e n t t o cause s t r u c t u r a l damage a r e mai . ler than damaging v e r t i c a l loads with t h e r e s u l t t h a t they can occur without, d a m . This i s borne out by one E l e c t r a accident t h e r e rearward-acting ground i r q a c t loads on t h e main landing wheels were s u f f i c i e n t t o destroy one wing and. t o col lapse t h e opposite main gear , but t h e occupants i n genoral had no idea of my-. th ing being amiss u n t i l t h e fuselage assumed an extremely abnoma1 a t t i t ! id5 .

This i s due i n p a r t t o t h e f a c t t h a t E poxason

I n addi t ion, drag and s i d e

I n conclusion, t h e inves t iga t ion has disclosed t h a t t h e r i g h t wing f a i l c i tiut. t o f l u t t e r h v o l v i n g whi r l mode o s c i l l a t i o n of t h e outboard nace l les . Allholvh contributory t o t h e i n i t i a t i o n of t h e f l u t t e r , t h e severe c lear a i r turhdLcncw above appears t o have been i n s u f f i c i e n t t o produce t h e nace l le d m g e W C ~ S S Q V

t o make t h e whi r l mode self-sustaining. It appears most probable, thers fore , that t h e r e was unrecognizable p r i o r damage i n t h e wing, o r i n t h e wing and outboard nace l les , making t h e e f f e c t s of t h e turbulence more c r i t i c a l than on airplane.

undamaged

- 2 6 -

Probable Cause

The Board determines t h a t t ho probable cause of t h i s accident was t h e separat ion of t h e r i g h t wing i n f l i g h t due t o f l u t t e r induced by oscillations of t h e outboard nacelles. the s t r u c t u r e and the en t ry of the a i r c r a f t i n t o an area of severe c l e a r air

Contributing factors were a reduced stif'fness of ,-

turbulence.

BY THE CIVIL AE3RONAUTICS BOARD:

/3/

/a/

/s/

/d

/3/

ALAN S. BOYD Chairman

ROEIERT T. MURPHY v i c e chairman

CHAN GURNEY Member

G , JOSEPH M I N F T T I Member

WHITNEY GILLILLAND Member

Inves t iga t ion and Heariga

The C i v i l Aeronautics Board was i io t i f icd of t h i s accident a t approximately 1700 c. s . t . , Harch 17, 1960. An i i ivast igat ian was inmediately i n i t i a t e d i.n accordance with t h e provisions of Title VI1 of t h e Federal Aviation Act of 1953. A pub l i c hearing was ordered by the Board and he ld i n Evansville, Indiana, on May 10 arid 11, 1960, and i n Hollywood, Cal i fornia , July 20, 21, arid 22, 1960.

The Carrier

Northwest A i r l i nes , I n c . , i s a Elinnosota corporation with i t s principal. o f f i c e in Minneapolis, Minnesota. convenience and necessi ty issued by t h e C i v i l Aeroneutics Board and an air carrier operating c e r t i f i c a t e issued by t h e Federal Aviation Agency. c e r t i f i c a t e s authorize t h e c a r r i e r t o engage i n a i r t r anspor t a t ion of persons, cargo, and mail within t h e United S ta t e s , including t h e route Involved.

The corporation holds a c e r t i f i c a t e of p u b l i c

These

F l i g h t Personnel

Captain Edgar E. LaParle, age 57, was employed by t h e company March -2.5, 1937. He was promoted t o captain June 4 , 1940. He held a val id F a a i r l i n e t r anspor t p i l o t c e r t i f i c a t e wi th r a t ings : M-202, and L-188 a i r c r a f t . were i n L-188 a i r c r a f t . December 7, 1959; no l i m i t a t i o n s o r de fec t s were noted. in L-188'equipment was December 16, 1959.

AMEL, C-4.6, B-377, DC-3, D C - 4 , E-6, He had a t o t a l of 27,523 f l y i n g hours, of which 254

His las t FAA f i r s t - c l a s s medical examination was taken His last , check f l i g h t

F i r s t Off icer Joseph C . M i l l s , age 27, was employed by t h e compmy o:, August 7, 1957. AS & MEL, B-25, instrument. were i n L-188 a i r c r a f t . H i s l a s t instriment check was Uecenibcr 6, 1959- €iis last FllA fj-rst-class medical examhiation was taken Ju ly U , 1959; 110 l i l L l i t a t i G l l 5

were noted.

He held a v a l i d FAA comnercial p i l o t c e y t i f i c a t e with r a t ings : He had i7, t o t a l of 2,971, f l y h g hours , of ul;?ch 292

Fl igh t Engineer Arnold W. Kowd, age 40, was employed by t h e compmy 1;nrch 1.'. He held a v a l i d FAA f l i g h t engineer c e r t i f i c a t e and was qua l i f i c " , i n L-.i%, 19.42.

B-377, DC-6, and DC-7 a i r c r a f t . 63:36 were in L-188 a i r c r a f t . taken J~nuary 18, 1960.

He had a t o t a l of 5,230 flying hours , of u h i . ~ ? ~ H i s last second-class FAA medical oxmlizst ion i,a

Stewardesses Constance M. Nuttor arid Baybara A . Schreibor and F l i g h t Attendant Mitchel l D. Foster were properly qua l i f i ed .

The Aircraft

N 121US, a Lockheed Electra, model 188C, was manufactured J u J y 20, 1959. Tht : The aircraft had a t o t a l f l y i n g time s ince mmuf'acture of 1,786 hours .

last major inspection was accomplished on Harch 9 , 1960, 74 hours p r i o r t o t h e accident. propel lurs , model A 64-4 FN-606.

It was equipped with f o u r Allison 501-D 13 engines and Acro Products