ISTRAŽIVANJE U NACIONALNOJ VAZDUHOPLOVNOJ...

Russell Wanhill

ISTRAŽIVANJE U NACIONALNOJ VAZDUHOPLOVNOJ LABORATORIJI (NLR) ZNAČAJNIH EKSPLOATACIJSKIH OTKAZA VAZDUHOPLOVA

SOME NOTABLE AIRCRAFT SERVICE FAILURES INVESTIGATED BY THE NATIONAL AEROSPACE LABORATORY (NLR)

Originalni naučni rad / Original scientific paper UDK /UDC: 629.735.018.4 Rad primljen / Paper received: 10.03.2009.

Adresa autora / Author's address: National Aerospace Laboratory NLR, Amsterdam, the Netherlands, [email protected]

Ključne reči • vazduhoplov • otkaz u eksploataciji • zamor • otkaz

Izvod

Dat je pregled više značajnih eksploatacijskih otkaza vazduhoplova i njihovih motora koji su istraživani u NLR od sredine 70-tih godina. Ovi su otkazi izabrani delom zbog toga da bi se istaklo potrebno znanje stručnjaka za utvrđi-vanje fizičkih uzroka otkaza, ali je pažnja usmerena i na širi aspekt sigurnosti konstrukcija i sprečavanje novih otkaza. Izabrani otkazi su: lopatice rotora helikoptera Sikorski S-61N (1974); lopatice turbine Dženeral Elektrik CF6-50 G40 (1977-9); lopatice propelera Aero Traktor AT-301 (1987); uške repa helikoptera Aerospasial Aluet III (1990); osovinica poluge Dženeral Dajnamiks F-16 / Prat & Vitni F100-PW-220 RCVV (1992); Boeing 747-258F pad motora zbog otkaza pilona krila (1992); glavčine rotora helikop-tera Vestland Links SH14D (1998).

Keywords • aircraft • service failures • fatigue • fracture

Abstract

A review is given of several notable aircraft and aeroen-gine service failures investigated by the NLR since the mid-1970s. These failures have been chosen partly to illustrate the specialist knowledge required for determining the physical causes of failure, but attention is also paid to the broader aspects of structural safety and preventing addi-tional failures. The selected failures are: Sikorsky S-61N helicopter rotor blade (1974); General Electric CF6-50 G40 turbine blades (1977-9); Air Tractor AT-301 propeller blade (1987): Aérospatiale Alouette III helicopter tail lug (1990); General Dynamics F-16 / Pratt & Whitney F100-PW-220 RCVV lever arm pin (1992); Boeing 747-258F engine losses owing to a wing pylon failure (1992); West-land Lynx SH14D helicopter rotor hub (1998).

UVOD

Nacionalna vazduhoplovna laboratorija (NLR) je glavna vazduhoplovna istraživačka organizacija u Holandiji. Ode-ljenje za konstrukcije i materijale, od 2004. deluje kao deo Odseka za letelice, steklo je značajno iskustvo u analizi otkaza, izvedenih tokom više od 40 godina. Najviše analiza otkaza, bukvalno na stotine, su bile posvećene vazduho-plovima i njihovim motorima, ali su izvedena i istraživanja mnogih otkaza druge opreme.

U radu je izneto stečeno iskustvo prikazom sedam značajnih otkaza vazduhoplova i njihovih motora i kako su rešeni. Izabrani otkazi su: lopatice rotora helikoptera Sikor-ski S-61N (1974); lopatice turbine Dženeral Elektrik CF6-50 G40 (1977-9); lopatice propelera Aero Traktor AT-301 (1987); uške repa helikoptera Aerospasial Aluete III (1990); osovinica poluge Dženeral Dajnamiks F-16 / Prat & Vitni F100-PW-220 RCVV (1992); Boeing 747-258F pad motora zbog otkaza pilona krila (1992); glavčina rotora helikoptera Vestland Links SH14D (1998).

Rad se završava razmatranjem znanja stručnjaka potreb-nog za utvrđivanje fizičkih uzroka koji su doveli do pojave otkaza.

INTRODUCTION

The National Aerospace Laboratory (NLR) is the princi-pal aerospace research organisation in the Netherlands. The Structures and Materials Division, since 2004 part of the Aerospace Vehicles Division, has much experience in failure analysis, covering more than 40 years. Most of the failure analyses, literally hundreds, have been for aircraft and aero-engine components, but many non-aerospace investigations have also been done.

The present paper demonstrates this experience by review-ing seven notable aircraft and aeroengine failures and their resolution. The selected failures are: Sikorsky S-61N heli-copter rotor blade (1974); General Electric CF6-50 G40 turbine blades (1977-9); Air Tractor AT-301 propeller blade (1987); Aérospatiale Alouette III helicopter tail lug (1990); General Dynamics F-16 / Pratt & Whitney F100-PW-220 RCVV lever arm pin (1992); Boeing 747-258F engine losses owing to a wing pylon failure (1992); Westland Lynx SH14D helicopter rotor hub (1998).

The paper concludes with a survey of the specialist knowledge required for determining the physical causes of the failures.

INTEGRITET I VEK KONSTRUKCIJA Vol. 9, br. 2 (2009), str. 71–87

STRUCTURAL INTEGRITY AND LIFEVol. 9, No 2 (2009), pp. 71–87

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mailto:[email protected]

Istraživanje u NLR značajnih eksploatacijskih... Some notable aircraft service failures...

LOPATICE ROTORA HELIKOPTERA SIKORSKI S-61N (1974)

Maja 1974 helikopter Sikorski S-61N se srušio u Sever-no more, šest osoba je poginulo. Slika 1. pokazuje tip leteli-ce, a sl. 2. srušeni helikopter pri spasavanju. Sve lopatice glavnog rotora su slomljene, ali je lopatica 3 bila izuzetak zbog male deformacije na mestu loma, označeno na sl. 2.

SIKORSKY S-61N HELICOPTER ROTOR BLADE (1974)

In May 1974 a Sikorsky S-61N helicopter crashed into the North Sea with the loss of six lives. Figure 1 shows the aircraft type, and Fig. 2 shows the crashed helicopter during recovery. All the main rotor blades were broken, but blade 3 was exceptional in showing little deformation at the fracture location, which is indicated in Fig. 2.

Slika 1. Sikorski S-61N PH-NZD (tip letelice)

Figure 1. Sikorsky S-61N PH-NZD (aircraft type). Slika 2. Spasavanje S-61N PH-NZD iz Severnog mora

Figure 2. Recovery of S-61N PH-NZC from the North Sea.

Slika 3a prikazuje očišćenu prelomnu površinu lopatice 3, koja obuhvata i šuplju ramenjaču izrađenu od legure alu-minijuma AA6061-T6. U eksploataciji ramenjača je zalep-ljena za oplatu u vidu orebrenog aluminijumskog džepa, datog na sl. 3b, i navedenog u listi redosleda loma utvrđe-nog fraktografskom analizom, /1/. Prva faza u redosledu je visokociklični zamor, iniciran iz pita (ujeda) korozije, na donjoj površini ramenjače ispod zalepljenog segmenta.

Figure 3a shows the recovered fracture surface of blade 3, consisting of a hollow spar made from aluminium alloy AA6061-T6. In service the spar had been adhesively bonded to an aluminium-skinned and -ribbed pocket, as shown in Fig. 3b, which also lists the failure sequence determined from fractographic analysis, /1/. The first phase in this sequence was high-cycle fatigue, initiating from corrosion pits on the spar lower surface under the bonded area.

Slika 3. Očišćena površina preloma ramenjače lopatice i faze zamornog loma Figure 3. Recovered blade spar fracture surface and phases of the fatigue life.

Uzroci otkaza Inspekcijski zahtevi za spoj ramenjača/džep: Sikorski je

propisao dopuštene tolerancije za odlepljivanje (popuštanje) spoja ramenjača/džep. U toleranciji nije bilo ograničenja zbog eksploatacije letelice. Međutim, otkaz ramenjače zbog zamora iniciranog korozijskim pitom ispod površine zalep-ljenog spoja je ukazao na nepotpunost zahteva inspekcije.

Contributing causes Inspection requirements for spar/pocket joints: Sikorsky

had set permitted tolerances for disbonding (looseness) of spar/pocket joints. Within these tolerances there was no restriction on aircraft operation. However, the spar failure from fatigue-initiating corrosion pits under the bonded area demonstrated the inadequacy of the inspection requirements.



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Postupak inspekcije lopatice: Sa sl. 3a se uočava da je ramenjača tokom eksploatacije izložena pritisku azota. To je deo inspekcije da bi se već početna zamorna prslina otkrila preko gubitka pritiska. U prvobitnom obliku sistem je imao pokazivač pritiska postavljen na svakoj lopatici rotora. Pokazivač je mogao da se proveri samo kada je letilica na zemlji, a rotor ne radi. U NLR je pokazano da ovaj sistem inspekcije ne može da otkrije prslinu sve dok ona ne prođe najbližu ivicu džepa, /2/. To ostavlja na raspolaganju samo faze 4 i 5 zamornog veka ramenjače za otkrivanje prsline, sl. 3b. Kvantitativna fraktografija (QF) je pokazala da je utvrđeni vek za ove faze bio 2,1–4,3 časa, /1, 2/, što je u pogledu bezbednosti vrlo kratko.

Projektno ispitivanje i analiza zamora: U vreme havarije je već došlo do drugih dvanaest otkaza ramenjače u eksplo-ataciji. Različiti uzroci otkaza su pružili čvrste dokaze da je Sikorski usvojio izuzetno neprikladne metode projektovanja prema zamoru i ispitivanja, /2/. Takođe, fraktografska analiza NRL je pokazala da pri normalnoj brzini leta od 120–130 čvorova, brzina rasta zamorne prsline je vrlo velika, /1/. Akcije sanacije drugih S-61N helikoptera, /2/

Odlepljivanje spoja ramenjača/džep: Otkrivena odleplji-vanja su odmah sanirana brzovezujućom trakom alumini-jumske folije i opravljena za dva meseca.

Postupak inspekcije lopatica: Dvojni pokazivač pritiska za sve lopatice i pokazivač pritiska lopatica u kabini su omogućili otkrivanje pada pritiska tokom leta kao i na zemlji.

Ograničenje brzine leta: Pri svakom pokazivanju pada pritiska u lopatici je trenutno smanjena brzina na 90 čvo-rova. Time su smanjeni radni naponi u lopatici i produžen zamorni vek, pa je obezbeđeno dovoljno vreme, mnogo veće od najdužeg vremena leta od 3 sata, koliko je potrebno da letelica doleti do odredišta.

LOPATICE TURBINE DŽENERAL ELEKTRIK CF6-50 G40 (1977-9)

U periodu od dve godine, od maja 1977. do maja 1979. se dogodilo 18 otkaza na osveženim motorima CF6-50 aviona MekDonel Daglas DC-10 kod većeg broja korisnika. Slike 4. i 5. prikazuju tip aviona i motor. Na sl. 6. je ozna-čen položaj otkaza, do koga je došlo zbog loma lopatice G40 prvog stepena turbine visokog pritiska.

Blade inspection system: Figure 3a notes that the spar was nitrogen-pressurised during service. This was part of an inspection system whereby early fatigue cracking would be detected via a pressure loss. In its original form this system comprised pressure indicators installed on each rotor blade. The indicators could be checked only when the aircraft was on the ground and the rotor was stationary. The NLR showed that this inspection system could fail to detect a crack until it had grown beyond the nearest pocket edge, /2/. This would leave only phases 4 and 5 of the spar fatigue life available for crack detection, Fig. 3b. Quantitative Fracto-graphy (QF) showed that the total estimate for these phases was 2.1–4.3 hours, /1, 2/, which is dangerously short.

Design fatigue testing and analysis: At the time of the crash there had been twelve other spar failures in service. The various causes of failure provided strong evidence that Sikorsky’s fatigue design and analysis methods were highly inadequate, /2/. Also, the NLR’s fractographic analysis showed that at the normal cruising speed of 120–130 knots the fatigue crack growth rates were high, /1/. Remedial actions for other S-61N helicopters, /2/

Spar/pocket disbonds: Detected disbonds to be immedi-ately sealed with High Speed aluminium foil tape and repaired within two months.

Blade inspection system: Dual pressure indicators on all blades and blade pressure indicators in the cockpit, allow-ing detection of pressure loss during flight as well as on the ground.

Speed restrictions in flight: Any cockpit indications of blade pressure drops to be immediately followed by a speed reduction to 90 knots. This lowers the blade working stresses and extends the fatigue life such that there is sufficient time, well beyond the maximum flight time of 3 hours, for the aircraft to reach a destination.

GENERAL ELECTRIC CF6-50 G40 TURBINE BLADES (1977-9)

Over a two-year period, from May 1977 to May 1979, the McDonnell Douglas DC-10 aircraft from several opera-tors experienced 18 failures of uprated CF6-50 engines. Figures 4 and 5 show the aircraft type and engine. Figure 6 indicates the location of the failures, which were due to fracture of the G40 first stage high pressure turbine blades.

Slika 4. MekDonel Daglas DC-10 (tip letelice)

Figure 4. McDonnell Douglas DC-10 (aircraft type). Slika 5. Motor Dženeral elektrik CF6-50

Figure 5. General Electric CF6-50 engine.



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Zbog habanja vrha sve su lopatice popravljene. Slika 7. shematski prikazuje tipičan lom lopatice i konfi-

guraciju unutrašnjeg hlađenja. Lopatice su bile izrađene preciznim livenjem René 80 superlegure na bazi nikla, sa prevlakom od nikal aluminida.

All blades had been repaired due to the tip abrasion. Figure 7 shows schematics of a typical blade failure and

the internal cooling configuration. The blades were made from investment cast René 80 nickel-base superalloy coated with a nickel aluminide.

Slika 6. Položaj loma motora CF6-50: lopatica prvog stepena turbine visokog pritiska (strelica)

Figure 6. CF6-50 engine failures location: first stage high pressure turbine blades (arrow).

Slika 7. Shema tipičnog loma lopatice G40 i unutrašnjeg hlađenja

Figure 7. Schematics of typical G40 blade failures and the internal cooling.

Dženeral Elektrik je istraživao problem u 1978, i utvrdio je da je otkaz posledica pregrevanja lopatica, do kojeg je došlo zbog promena unetih tokom popravki unutrašnjeg prolaza za hlađenje vazduhom, sl. 8, /3, 4/.

General Electric investigated the problem in 1978 and determined that the failures were caused by blade overheat-ing, resulting from changes in the internal air cooling passages during repair, Fig. 8, /3, 4/.



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Mere sanacije (I) Zahvaljujući do tada poznatim otkazima, Dženeral Elek-

trik je sredinom 1978. sproveo sledeće izmene:

Remedial actions (I) Owing to the then-known incidents, General Electric

introduced the following changes in mid-1978: opravka sa hemijskim skidanjem spoljnje prevlake

▼ povećanje veličine otvora za hlađenje

▼ sklop rotora sa više lopatica velike struje vazduha

▼ ukupno povećanje struje vazduha iznad granica

▼ smanjen unutrašnji pritisak napadne ivice i gubitak granica povratne

struje ▼

nepravilna količina i raspodela vazduha za hlađenje kroz otvore po napadnoj ivici što posebno dovodi do neefikasnog hlađenja sloja

▼ povećani temperaturni gradijent po napadnoj ivici

▼ povećana koncentracija napona

▼ skraćeni vek niskocikličnog (LCF) i visokocikličnog (HCF) zamora

▼ prevremene zamorne prsline i lom

repairs with chemical stripping of external coating ▼

cooling hole size increase ▼

rotor set with many high airflow blades ▼

total airflow increase beyond limits ▼

reduced internal pressure at leading edges and loss of backflow margin ▼

incorrect quantities and distribution of cooling air through leading edge holes resulting especially in ineffective film cooling

▼ higher temperature gradients at leading edges

▼ increased stress concentrations

▼ reduced LCF + HCF fatigue lives

▼ premature fatigue cracking and failure

Slika 8. Zavisnost hemijskog skidanja prevlake tokom popravke i naknadnog loma lopatice G40

Figure 8. Relation between chemical stripping during repair and subsequent G40 blade failures.

Već popravljane lopatice: „uveden je sistem upravlja-nja“, u kome ne samo da svaka lopatica prolazi pojedinačno proveru strujanja vazduha, već mora da ispuni i neke krite-rijume zadovoljavajućeg rada u sklopu rotora.

Lopatice koje treba popravljati: (a) suvo abrazivno skidanje spoljnje prevlake; (b) smanjenje površine izlaznog otvora tvrdim lemljenjem kada popravljene lopatice ne ispu-njavaju pojedinačne zahteve protoka vazduha; (c) korišće-nje uvedenog sistema upravljanja po potrebi.

I pored ovih promena bilo je incidenata sa otkazom lopa-tica i pojavom prslina krajem 1978. i početkom 1979. NLR je utvrdio da su ovi otkazi pre svega posledica nedovoljne debljine sredine konusa i ispupčenih mesta zida, /5/. Razlog ovog stanjenja je najverovatnije pomeranje keramičkog jezgra pri livenju lopatica i skidanja metala pri popravci. Važni uticajni faktori su (a) nemogućnost da se bez raza-ranja proveri debljina zida lopatice pre ponovne upotrebe i (b) nalaz Dženeral Elektrika da lemljenje izlaznog otvora može da poremeti strujanje vazduha u serpentini.

Polazeći od toga, Dženeral Elektrik je izveo termodina-mičku analizu oblika lopatica G40 i našao četiri kritična parametra koji direktno utiču na zamorni vek lopatice. Ti parametri su struja vazduha duž napadne ivice, granice povratnog strujanja, strujanje vazduha u zmiji i debljina zida ispupčenja napadne ivice. Analizom su određeni faktori smanjenja veka za sva četiri parametra, sl. 9. Mere sanacije (II)

Poboljšan postupak povećanja veka: Na osnovu termo-mehaničke analize Dženeral Elektrik je uveo novi postupak za duži vek novih i popravljenih lopatica, /4/. Uvodi se pro-vera debljine zida ispupčenja napadne ivice i ako ima viška lema, skidanje sa izlaznog otvora na inače dobroj lopatici.

Poboljšani postupak popravke: Skidanje prevlake i nanos nove prevlake samo na vrhu popravljenog područja.

Dizajn lopatice: Novi oblik unutrašnjeg hlađenja, poseb-no na napadnoj ivici, radi boljeg sadejstva sa uspostavlja-njem rada motora.

Blades already repaired: A “set management system”, whereby each blade not only underwent individual airflow checks, but also had to meet certain criteria for satisfactory operation in a rotor set.

Blades to be repaired: (a) dry grit blast stripping of the external coating; (b) dump hole flow area reduction by brazing when repaired blades do not meet individual airflow require-ments; (c) optional use of the set management system.

Despite these changes, several incidents of blade failure and cracking occurred in late 1978 and early 1979. The NLR determined that the failures were primarily due to insufficient thickness of the nose centre and convex wall locations, /5/. In turn, this undersizing was most probably due to ceramic core shifts during casting the blades and metal removal during repair. Important contributory factors were (a) the inability to check the blade wall thicknesses non-destructively before re-use and (b) General Electric’s finding that dump hole brazing could overcorrect the ser-pentine airflow.

Following on from this, General Electric conducted a thermomechanical analysis of the G40 blade configuration and identified the four critical parameters directly affecting the blade fatigue life. These parameters were the leading edge airflow, the backflow margin, the serpentine airflow and the leading edge convex wall thickness. The analysis derived life reduction factors for all four parameters, Fig. 9. Remedial actions (II)

Improved lifing procedure: Based on thermomechanical analysis, General Electric proposed a new lifing procedure for new and repaired blades, /4/. This included leading edge convex wall thickness checks and removing dump hole brazing, if present, from otherwise satisfactory blades.

Improved repair procedure: Coating stripping and recoat-ing only at the tip repair location.

Blade design: New internal cooling configuration, espe-cially at the leading edge, to better cope with engine uprating.



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Slika 10. Air Tractor AT-301 PH-CPR približno godinu dana posle udesa

Slika 9. Faktori smanjenja veka struje vazduha napadne ivice lopatice G40, WA; ivice povratne struje, BFM, struje u zmiji, WS; i debljine ispupčene napadne ivice

Figure 9. Life reduction factors for G40 blade leading edge airflow, WA; backflow margin, BFM; serpentine airflow, WS; and leading edge convex wall thickness.

Figure 10. Air Tractor AT-301 PH-CPR about one year after the incident.

LOPATICA PROPELERA ER TRAKTOR AT-301 (1987)

Juna 1987 sa Er Traktor AT-301, izvedenog sa propele-rom od dve lopatice, vrh propelera se odlomio u letu.

Pilot je isključio motor i uspešno je sleteo. Slika 10. prikazuje letilicu godinu dana kasnije, ali ovog puta sa propelerom od tri lopatice.

Slika 11 pokazuje slomljenu lopaticu propelera i detalj površine preloma. Proizvođač propelera je Pacific Propel-lers Inc., prema licenci Hamilton Standard, a sastoji se od otkovka aluminijumske legure AA2025-T6 koji je zatim anodno nagrižen hromnom kiselinom. I pored anodno nagri-žene površine, na propeleru je uočena korozija od pitova i raslojavanja, /6/. Koroziju je verovatno izazvao atak i prodor taloga koja je sadržala morsku so kroz anodni sloj, /6/. Slomljena lopatica je otkazala zbog visokocikličnog zamora iniciranog iz korozijskog pita na ispupčenoj površini, dubi-ne 0,1 mm × dužine 0,3 mm. Uzroci otkaza Čišćenje propelera: Korisnik je zanemario preporuku

proizvođača da opere i naulji lopatice propelera nakon poslednjeg obavljenog leta u toku dana.

Radna sredina: Letelica je korišćena za prskanje letine blizu obale Holandije. Zbog toga su se na propeleru stvarale agresivne naslage. Mere sanacije

Zamena propelera: Pilot je uspeo da sleti sa letelicom. Čišćenje propelera: Pranje i nauljivanje lopatica prope-

lera posle obavljenog poslednjeg leta svakog radnog dana.

UŠKA REPA HELIKOPTERA AEROSPASIAL ALUET III (1990)

Oktobra 1990. se helikopter Aerospasial Aluet III srušio u Veluvi, u prirodnom okruženju; pilot je poginuo. Slika 12. prikazuje letilicu, A-351, koja je u sastavu tima za demonstracije Kraljevskih vazduhoplovnih snaga Holandije (RNLAF). Slike 13. i 14. prikazuju olupine helikoptera,

AIR TRACTOR AT-301 PROPELLER BLADE (1987)

In June 1987 an Air Tractor AT-301 fitted with a two-bladed propeller experienced loss of a propeller tip in flight.

The pilot managed to switch the engine off and landed safely. Figure 10 shows the aircraft about one year later, this time with a three-bladed propeller.

Figure 11 shows the broken propeller blade and a detail of its fracture surface. The propeller was manufactured by Pacific Propellers Inc., under license to Hamilton Standard, and consisted of an aluminium alloy AA2025-T6 forging that had been chromic acid anodised. Despite the anodised surface, the propeller showed pitting and exfoliation corro-sion, /6/. The corrosion was most probably due to attack and penetration of the anodised layer by deposits, which included sea salt, /6/. The broken blade failed by high-cycle fatigue initiating from a 0.1 mm deep × 0.3 mm long corro-sion pit on the convex surface. Contributing causes

Propeller cleaning: The operator neglected to follow the manufacturer’s recommendation to wash and oil the propel-ler blades after the last flight of the day.

Operating environment: The aircraft was used for crop spraying near the Dutch coast. This resulted in aggressive deposits on the propeller. Remedial actions

Replace propeller: The pilot safely landed the aircraft. Propeller cleaning: Washing and oiling the blades after

the last flight of each operating day.

AÉROSPATIALE ALOUETTE III HELICOPTER TAIL LUG (1990)

In October 1990 an Aérospatiale Alouette III helicopter crashed in the Veluwe, a nature area, with the loss of the pilot. Figure 12 shows the aircraft, the A-351, which belonged to a demonstration team of the Royal Netherlands Air Force (RNLAF). Figures 13 and 14 show the remains of the



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koji se slomio u toku leta zbog odvajanja repa. Na sl. 14. je prikazan lom uške desnog pričvršćivača koji je i doveo do loma repa.

helicopter, which crashed owing to in-flight separation of the tail. Figure 14 indicates the Right Hand (R.H.) attach-ment lug fracture that led to loss of the tail.

Slika 11. Slomljena lopatica propelera i detalj površine preloma

Figure 11. Broken propeller blade and detail of the fracture surface.Slika 12. Aerospasial Aluet III A-351 (strelica)

Figure 12. Aérospatiale Alouette III A-351 (arrowed).

Slika 15. daje detalje loma uške, koji se pojavio u blizini zavarenog spoja uške i okvira aviona. Lom je iniciran zamorom iz intergranularnih prslina na gornjoj strani. Inter-granularne prsline su termički obojene. Detaljno istraživa-nje je pokazalo da su one povezane sa filmom na granicama zrna i mrežom (Mn + Fe)S, i da su, prema tome, likvacijske prsline od zavarivanja, /7/.

Figure 15 gives details of the lug fracture, which was close to welds joining the lug into the airframe. Failure occurred by fatigue initiating from intergranular cracks on the upper side. The intergranular cracks were heat-tinted. Detailed investigation showed that they were associated with grain boundary films and rosettes of (Mn + Fe)S, and were therefore due to liquation cracking during welding, /7/.

Aluet III A-351 Alouette III A-351

Slika 13. Olupina helikoptera

Figure 13. Airframe wreckage. Slika 14. Odvojeni rep i mesto loma desne uške za pričvršćenje

Figure 14. Separated tail and the Right Hand attachment lug fracture.

Slika 15. Detalji loma uške repa koji pokazuju intergranularne prsline praćene zamorom: DOF = smer leta

Figure 15. Details of the tail lug fracture, showing intergranular cracking followed by fatigue: DOF = Direction of Flight



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Uzroci otkaza Hemijska analiza atomskom emisionom spektrometrijom

(AES) je pokazala da je uška izrađena od ploče srednje ugljeničnog čelika (0,4%), umesto od ugljeničnog čelika sa 0,2–0,3% ugljenika prema specifikaciji. Zato je otežano zavarivanje uške za okvir. Sadržaj sumpora je takođe viši od propisanog, što olakšava pojavu likvacijskih prslina. Mere sanacije za druge helikoptere Alouette III, /7/

Hemijska analiza uške: Mali uzorci su odsečeni sa kraje-va leve (L.H.) i desne (R.H.) uške za pričvršćivanje sa osta-lih 66 letilica RNLAF. Uzorci su analizirani na sadržaj ugljenika pomoću disperzije energije pri X zračenju (EDX). Na jednom uzorku je utvrđen previše visok sadržaj ugljeni-ka (0.4%).

Zamena: Uške koje odstupaju su zamenjene drugim, sa propisanim sadržajem ugljenika.

OSOVINICA POLUGE DŽENERAL DAJNAMIKS F-16 / PRAT & VITNI F100-PW-220 RCVV (1992)

Februara 1992. se jedan Dženeral Dajnamiks F-16 srušio u stambenom bloku grada Hengelo, bez ljudskih žrtava. Do rušenja je došlo zbog otkaza motora.

Na sl. 16. su data dva pogleda na mesto rušenja, uključu-jući ostatke motora Prat & Vitni F100-PW-220.

Slika 17. prikazuje letelicu, J-054, sa oznakama RNLAF. Na sl. 18. je označen položaj otkaza motora, do koga je

došlo posle loma osovinice pričvršćene za polugu pokretne lopatice poslednjeg prstena kompresora (RCVV).

Contributing cause Atomic Emission Spectrometry (AES) chemical analysis

showed that the lug was made from a medium-carbon steel (0.4%) plate, instead of the specified 0.2–0.3% carbon steel. This would have made the lug more difficult to weld into the airframe. The sulphur content was also higher than spe-cified and hence facilitated liquation cracking. Remedial actions for other Alouette III helicopters, /7/

Lug chemical analyses: Small samples were cut from the ends of the Left Hand (L.H.) and R.H. attachment lugs of the 66 remaining RNLAF aircraft. Samples were analysed for carbon content using Energy Dispersive analysis of X-rays (EDX). One sample was found to have a too-high carbon content (0.4%).

Rework: The anomalous lug was replaced by another having the specified carbon content.

GENERAL DYNAMICS F-16 / PRATT & WHITNEY F100-PW-220 RCVV LEVER ARM PIN (1992)

In February 1992 a General Dynamics F-16 crashed between housing blocks in Hengelo city, without loss of life. The crash was caused by engine failure.

Figure 16 shows two views of the crash site, including the remains of the Pratt & Whitney F100-PW-220 engine.

Figure 17 shows aircraft, the J-054, in its RNLAF livery. Figure 18 indicates the location of the engine failure,

which was due to fracture of a pin attached to a Rear Com-pressor Variable Vane (RCVV) lever arm.

Slika 16. Mesto udesa F-16 J-054 i položaj motora

Figure 16. Crash site of F-16 J-054 and indication of the engine.

Slika 17. Dženeral dajnamiks F-16 J-054 Figure 17. General Dynamics F-16 J-054.

Slika 18. Položaj loma motora F100-PW-220 Figure 18. F100-PW-220 engine failure location: RCVV lever arm assembly.

Slika 19. je shema sklopa RCVV, sa označenim lomom osovinice. Materijal kraka poluge je Inconel 718, superle

Figure 19 is a schematic of the RCVV assembly, indicat-ing the pin fracture. The lever arm material was Inconel 718,



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gura na bazi nikla. Materijal osovinice je Nitronic 60, nerđajući čelik. Detaljno istraživanje je pokazalo da je na osovinici došlo do prslina od naponske korozije (SCC), /8, 9/. Slike 20. i 21. pokazuju površinu preloma i dokazuju pojavu korozije i SCC. Stvarni (fizički) uzrok otkaza moto-ra je prikazan kroz redosled događaja, sl. 22.

a nickel-base superalloy. The pin material was Nitronic 60, a stainless steel. Detailed investigation showed that the pin had undergone stress corrosion cracking (SCC), /8, 9/. Figures 20 and 21 show the pin fracture surface and evidence of corrosion and SCC. The actual (physical) cause of engine failure involved a sequence of events, Fig. 22.

Slika 19. Shema RCVV i sklop poluge, sa oznakom loma osovinice za pričvršećenje poluge; RCVV sklop poluge Figure 19. Schematic of RCVV and lever arm assembly, indicating fracture of the pin attached to the lever arm.

Korozijski pitovi i zarezi koji polaze od otvora na glavi osovinice Corrosion pits and slots running from hole in pin head

Transgranularne prsline od naponske korozije Transgranular stress corrosion cracking (SCC) Slika 20. Površina preloma primljene

osovinice poluge Figure 20.As-received fracture surface

of lever arm pin. Slika 21. Prsline od korozije i naponske korozije osovinice poluge

Figure 21. Corrosion and stress corrosion cracking of the lever arm pin.

so se taloži na poluzi u bajpasu vazduha i apsorbuje vlagu tokom prekida rada

▼ koncentrisani rastvor soli prodire u prostor između poluge i

osovinice ▼

korozija u međuprostoru, naponska korozija (SCC) i lom osovinice▼

poremećaj položaja petog stepena RCVV ▼

aerodinamička pobuda lopatica šestog stepena kompresora ▼

zamorni lom uške za pričvršćenje lopatice šestog stepena rotora ▼

ispad lopatice i potpuni raspad motora

salt in by-pass air deposits on lever arms and absorbs moisture during shutdowns

▼ concentrated salt solution penetrates crevices between arm and

pin ▼

crevice corrosion, SCC and fracture of pin ▼

mispositioned 5th stage RCVV ▼

aerodynamic excitation of 6th stage compressor blades ▼

fatigue failure of blade-retaining lugs on 6th stage rotor ▼

blade loss and internal disintegration of engine Slika 22. Zavisnost hemijskog skidanja tokom popravke i

naknadnog loma lopatice G40 Figure 22. Relation between chemical stripping during repair and

subsequent G40 blade failures.



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Uzroci otkaza Materijal osovinice: Osetljivost Nitronic 60 na SCC nije

bila poznata, /9/, dok nije utvrđena u NLR. Zaostali naponi: Obradom glave osovine na hladno radi

montaže na RCVV polugu uneti su veliki zaostali naponi. Kombinacija ovih napona i koncentrisanog rastvora soli u prostoru između poluge ručice i osovinice dovela je do pojave SCC. Mere sanacije ostalih motora F100-PW-220

Materijal osovinice: Kod mnogih osovinica je utvrđena pojava naponske korozije, /8, 9/. Proizvođač motora je zamenio materijal osovinice superlegurom Inconel 625 na bazi nikla, koja je otporna na SCC u rastvoru soli, /8/.

Sklop RCVV poluga + osovinica: Svi sklopovi poluga + osovinica Inconel 718/Nitronic 60 su postepeno zamenjeni sklopovima Inconel 718/Inconel 625. “Stari“ sklopovi su do zamene češće kontrolisani.

GUBITAK MOTORA BOING 747-258F ZBOG OTKAZA NOSAČA NA KRILU (1992)

Oktobra 1992, jedan El Al teretni avion Boing 747-258F, 4X-AXG, je uzleteo sa aerodroma Skipol. Posle oko trinaest minuta on se srušio na stambeni blok u predgrađu Amster-dama, uz 43 ljudske žrtve i brojne povređene. Slika 23. prikazuje avion kratko pre rušenja, a sl. 24. daje izgled na mestu nesreće.

Contributing causes Pin material: The SCC susceptibility of Nitronic 60 was

unknown, /9/, until identified by the NLR. Residual stresses: Cold-upsetting the pin head to attach

it to the RCVV lever arm resulted in high residual stresses. The combination of these stresses and a concentrated salt solution in the crevice between the lever arm and pin resulted in SCC. Remedial actions for other F100-PW-220 engines

Pin material: Many more pins were found to contain stress corrosion cracks, /8, 9/. The engine manufacturer changed the pin material to the nickel-base superalloy Inconel 625, which is immune to SCC in salt solutions, /8/.

RCVV lever arm + pin assemblies: All Inconel 718/ Ni-tronic 60 lever arm + pin assemblies were gradually replaced by Inconel 718/Inconel 625 assemblies. The “old” assemblies were subjected to frequent inspection until replacement.

BOEING 747-258F ENGINE LOSSES OWING TO A WING PYLON FAILURE (1992)

In October 1992 an El Al Boeing 747-258F cargo air-craft, the 4X-AXG, took off from Schiphol airport. Some thirteen minutes later it crashed into apartment blocks in a suburb of Amsterdam, with the loss of 43 lives and many injured. Figure 23 shows the aircraft some time before the accident, and Fig. 24 is a view of the crash site.

Slika 23. Boing 747-258F 4X-AXG

Figure 23. Boeing 747-258F 4X-AXG. Slika 24. Mesto udesa 747-258F 4X-AXG

Figure 24. Crash site of 747-258F 4X-AXG.

Ovako je izgledao redosled događaja koji je doveo do otkaza, /10/. Pet minuta posle poletanja motor br. 3 i njegov nosač (pajlon) su se odvojili od desnog krila usmereni prema spolja i unazad. Motor br. 3 je udario u motor br. 4, što je dovelo do njegovog odvajanja od krila zajedno sa nosačem. Oba motora i nosači su pali u jezero na oko 25 km istočno of Skipola. Pri odvajanju motora od krila napadna ivica je veoma oštećena. Slika 25. pokazuje konačni položaj u toku leta. Ovo oštećenje i odvajanje dva motora su prouzrokovali izuzetno teško upravljanje letilicom. Načinjen je pokušaj povratka u Skipol, ali bez uspeha.

Za istraživanje koje je usledilo angažovane su mnoge organizacije, uključujući proizvođače aviona i motora, operater letelice, državni vazduhoplovni organi, saveti za istraživanje udesa, Tehnički tehnološki institut Izraela i nekoliko odeljenja NLR.

The sequence of events leading to the crash was as follows, /10/. Five minutes after take-off the No. 3 engine and its pylon separated from the right wing in an outboard and rearward direction. The No. 3 engine hit the No. 4 engine, causing this engine and its pylon also to separate from the wing. Both engines and pylons fell into a lake about 25 km east of Schiphol. During the engine separations the wing leading edge was extensively damaged. Figure 25 shows the in-flight final situation. This damage and loss of the two engines made control of the aircraft extremely difficult. An attempt was made to return to Schiphol, with no success.

Subsequent investigation of the accident involved many organisations, including the aircraft and engine manufactur-ers, the aircraft operator, airworthiness authorities, accident investigation boards, the Technion Israel Institute of Tech-nology, and several NLR divisions.



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Odeljenje za konstrukcije i materijale NLR je bilo zadu-ženo za istraživanje komponenti za spajanje nosača motora br. 3 sa desnim krilom, /10, 12/. Pokazalo se da su upravo te komponente bile ključne za objašnjenje ovog vrlo komplikovanog udesa.

The NLR’s Structures and Materials Division was assigned the task of investigating the components connect-ing the No. 3 engine pylon to the right wing, /10, 12/. These components turned out to be the key to explaining this very complicated accident.

Slika 25. Konačni položaj u toku leta 747-258F 4X-AXG Figure 25. In-flight final situation of 747-258F 4X-AXG.

Slika 26. Shema dostupnih komponenti veze nosača motora broj 3 sa krilom nosača

Figure 26. Schematic of recovered components from the No. 3 engine pylon-to-wing connections.

Dostupne komponente za spajanje nosača motora br. 3 za krilo

Slika 26. daje shematski prikaz dostupnih komponenti za spajanje nosača motora br. 3 sa krilom. Treba uočiti da nedostaje unutrašnja sigurnosna osovinica srednje ramenja-če, a da je dostupan spoljni deo sigurnosne osovinice sred-nje ramenjače. Makroskopski pregled je pokazao da su

Recovered components from the No. 3 engine pylon-to-wing connections

Figure 26 is a schematic of the recovered components from the No. 3 engine pylon-to-wing connections. Note especially that the inboard mid-spar fuse pin was missing, but part of the outboard mid-spar fuse pin was recovered. Macroscopic inspection indicated that the upper link and



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gornja veza i dijagonalni podupirač odlomljeni zbog pre-opterećenja kada su se motori i nosači odvojili od krila. Zato je pažnja usmerena na komponente srednje ramenjače, naročito na unutrašnje i spoljnje spojnice nosača i parcijal-nu sigurnosnu osovinicu, sve izrađene od niskolegiranog čelika AISI 4330M. Unutrašnje spojnice nosača su od čeli-ka visoke čvrstoće, 1517–1655 MPa. Sigurnosna osovinica je od čelika srednje čvrstoće 869–958 MPa. Spojnice nosača srednje ramenjače

Slika 27. pokazuje spojnicu nosača srednje ramenjače sa pregledom uočenih oštećenja, /10, 11/. Na osnovu ovih rezultata izvedeni su sledeći zaključci: – Nedostupna unutrašnja sigurnosna osovinica je otkazala

tako da je spoljnja uška unutrašnje spojnice nosača sred-nje ramenjače preuzela čitavo opterećenje u tom spoju, pa je otkazala zbog preopterećenja zatezanjem i savijanjem.

– U toku otkaza unutrašnjeg spoja srednje ramenjače muška poluga unutrašnje spojnice krila se pomerila prema spolja od vrata uške na spojnici nosača. Sa druge strane, zbog otkaza spoljnjeg spoja srednje ramenjače došlo je do pome-ranja muške uške spoljnje spojnice krila prema vratu na uški na spojnici nosača, što je uslovilo ogrebotine.

diagonal brace had broken away owing to overload as the engine and pylon separated from the wing. Attention then focussed on the mid-spar components, notably the inboard and outboard pylon fittings and the partial fuse pin, all of which were made from AISI 4330M, a low alloy steel. The inboard pylon fittings were made of high strength steel, 220–240 ksi (1517–1655 MPa). The fuse pin was made of medium strength steel, 126–139 ksi (869–958 MPa). Mid-spar pylon fittings

Figure 27 shows the mid-spar pylon fittings with sum-maries of the observed damage, /10, 11/. From these results two main conclusions were drawn: – The missing inboard fuse pin must have failed such that

the outer lug of the inboard mid-spar pylon fitting took the entire load at this connection and subsequently failed by tensile + bending overload.

– During failure of the inboard mid-spar connection the male lugs of the inboard wing fitting moved outward from the throat of the lugs on the pylon fitting. On the other hand, failure of the outboard mid-spar connection resulted in the male lugs of the outboard wing fitting moving into the throat of the lugs on the pylon fitting, thereby causing scraping.

Savijena uška sa unutrašnje strane

Inner lug bent Spoljna slomljena zbog preopterćenja zatezanjem i savijanjem

Outer lug failed by tensile + bending overload

Ogrebonite u vratu (strelica) Scraping (arrowed) in throat

Slika 27. Uške veze spojnice nosača motora 3 sa srednjom ramenjačom i ukupna oštećenja Figure 27. The lugs of the No. 3 engine mid-spar pylon fittings and damage summaries.

Spoljnja sigurnosna osovinica srednje ramenjače Slika 28. pokazuje shemu spoljnjeg spoja srednje rame-

njače sa poprečnim presekom neoštećene sigurnosne osovi-nice, kao i dostupni ostatak sigurnosne osovinice, još uvek u jednoj od muških uški spojnice krila.

Sigurnosna osovinica se slomila na tankom zidu prikaza-nom u poprečnom preseku. Na spoljnjoj strani je sigurnosna osovinica otkazala zbog prekomernog smicanja, a na unut-rašnjoj zbog zamora praćenog prekomernim smicanjem. Na sl. 29. se vidi spoljnja površina preloma. Zamor je počeo na više mesta duž žljebova od obrade. QF je pokazala da je brzina rasta zamorne prsline verovatno bila velika, /10, 12/.

Outboard mid-spar fuse pin Figure 28 shows a schematic of the outboard mid-spar

connection with a cross-section of the intact fuse pin, and also the recovered fuse pin remnant, still in one of the male lugs of the wing fitting.

The fuse pin broke at the thin-walled locations shown in cross-section. On the inboard side, the fuse pin failed by shear overload, but on the outboard side it failed by fatigue followed by shear overload. Figure 29 shows the outboard fracture surface. Fatigue initiated at multiple origins along machining grooves. QF showed that the fatigue crack growth rates were probably high, /10, 12/.



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Slika 28. Shema i ostatak sigurnosne osovinice spoljnog dela srednje ramenjače sa nosača motora 3 za vezu sa krilom

Figure 28. Schematic and remnant of the outboard mid-spar fuse pin from the No. 3 engine pylon-to-wing connections.

Slika 29. Prelom spoljnje sigurnosne osovinice srednje ramenjačeFigure 29. Fracture surface of the outboard mid-spar fuse pin.

Uzroci otkaza Konstrukcija spojne osovinice: Nepristupačan prilaz za

obradu tankozidnog dela uslovio je unutrašnje žljebove od obrade (koncentraciju napona) što je doprinelo zamoru. Redosled toka razdvajanja motora br. 3 i nosača i osnovni uzrok

Slika 30. pokazuje najverovatniji redosled razdvajanja motora br. 3 i nosača. Osnovni (fizički) uzrok je visok lokal-ni dinamički napon koji je doveo do zamora i loma zbog preopterećenja i unutrašnje i spoljnje sigurnosne osovinice.

Contributing causes Fuse pin design: Less-than-optimum access for machin-

ing the thin-walled locations resulted in internal machining grooves (stress concentrations) that facilitated fatigue. Separation sequence for the No. 3 engine and pylon and basic cause

Figure 30 shows the most probable separation sequence for the No. 3 engine and pylon. The basic (physical) cause was high local dynamic stresses resulting in fatigue and overload failure of both the inboard and outboard fuse pins.

Slika 30. Najverovatniji redosled odvajanja motora broj 3 i nosača

Figure 30. Most probable separation sequence for the No. 3 engine and pylon.



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Netipično savijanje sigurnosne osovinice: Boing je poka-zao analizom konačnim elementima (FEA) da se sigurnosna osovinica pod opterećenjem savija netipično, u vidu „kole-nastog vratila“, sl. 31. Savijanje kolenastog vratila poveća-va koncentraciju napona na mestu tankog zida.

Konstrukcija nosača: U vreme udesa nije bilo potrebnog znanja o dinamičkom opterećenju nosača motora. Mere sanacije ostalih aviona Boing 747

Boing je uveo više izmena na spoju nosač-krilo, /10/. Slika 32. prikazuje dve od tih izmena: – Nova konstrukcija sigurnosne osovinice izrađene od

nerđajućeg čelika otpornog na koroziju i bez tankog zida. Ovaj tip osovinice je mnogo otporniji na zamor.

– Dva dodatna spoja između spojnica nosača srednje ramenjače. Sem toga, spojnice nosača srednje ramenjače, dijagonal-

ni podupirač i gornja veza su ojačani.

Non-classical fuse pin bending: Boeing showed by Finite Element Analysis (FEA) that the fuse pins would bend under load in a non-classical manner referred to as “crank-shafting”, Fig. 31. The crankshaft bending would have increased the stress concentrations at the thin-walled locations.

Pylon design: At the time of the accident there was inade-quate knowledge of the dynamic loads on engine pylons. Remedial actions for other Boeing 747 aircraft

Boeing introduced several changes for the pylon-to-wing connections, /10/. Figure 32 illustrates two of these changes: – A new design fuse pin made from corrosion-resistant

stainless steel and without thin-walled locations. This type of pin is much more resistant to fatigue.

– Two extra connections between the mid-spar pylon fittings. In addition, the mid-spar pylon fittings, the diagonal

brace and upper link were strengthened.

Analiza klasičnog savijanja Classical bending analysis

Boing analiza konačnim elementima:

savijanje tipa "kolenastog vratila" Boeing Finite Element Analysis (FEA): "crankshaft"

bending Slika 31. Savijanje sigurnosne osovinice pod

radnim opterećenjem Figure 31. Fuse pin bending under service

loads. Slika 32. Dve faze popravke veze nosača srednje ramenjače sa krilom, uporediti sa sl. 30

Figure 32. Two of the remedial actions for the mid-spar pylon-to-wing connections; compare with Fig. 30.

Slika 33. Vestland Links SH14D (tip letelice)

Figure 33. Westland Lynx SH14D (aircraft type). Slika 34. Olupina helikoptera Links -282

Figure 34. Airframe wreckage of Lynx-282.

GLAVČINE ROTORA HELIKOPTERA VESTLAND LINKS SH14D (1998)

Novembra 1998. helikopter Vestland Links SH14D, Links-282, je ostao bez lopatice rotora i zatim i bez glavči-ne rotora prilikom priprema za uzletanje. Posada Kraljevske holandske mornarice (RNLN) se bezbedno evakuisala pre nego što je požar zahvatio helikopter. Slika 33. pokazuje tip letilice. Slike 34. i 35. pokazuju ostatak helikoptera i glavčinu rotora posle ugašenog požara.

WESTLAND LYNX SH14D HELICOPTER ROTOR HUB (1998)

In November 1998 a Westland Lynx SH14D helicopter, the Lynx-282, lost a rotor blade and then the rotor head during preparation for take-off. The Royal Netherlands Navy (RNLN) crew exited safely before the airframe caught fire. Figure 33 shows the aircraft type. Figures 34 and 35 show the remains of the airframe and rotor head after the fire was extinguished.



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Na sl. 36. je data shema položaja loma lopatice rotora, koja se nalazi na tzv. žutom kraku glavčine rotora. To je monolitni otkovak izrađen od (α + β) prerađene i termički obrađene legure titana Ti-6Al-4V, /13/. Glavčina rotora je otkazala posle 3591,9 sati u eksploataciji, pri čemu je ukupan siguran vek ocenjen na najmanje 5000 sati i na 8600 sati na mestu loma, /14/.

Figure 36 is a schematic of the rotor blade failure loca-tion, which was in the so-called yellow arm of the rotor hub. This was a monolithic forging made from (α + β) processed and heat-treated titanium alloy Ti-6Al-4V, /13/. The rotor hub failed after 3591.9 service hours, whereas the overall safe-life had been estimated as 5000 hours minimum and 8600 hours at the failure location, /14/.

Slika 36. Shema monolitnog otkovka glavčine rotora sa označenim mestom loma

Slika 35. Odvojena glavčina rotora Links-282 Figure 35. Separated rotor head of Lynx-282.

Figure 36. Schematic of monolithic rotor hub forging, indicating the failure location.

Slika 37. pokazuje spoljnu površinu preloma neposredno posle uzimanja, a sl. 38. daje pregled rezultata QF frakto-grafske analize, /13/. Do otkaza je došlo zbog visokociklič-nog zamora, koji je iniciran ispod površine zbog postojanja površinskog sloja obrađenog sačmarenjem.

U obimnom ispitivanju nije nađen dokaz da je lom pos-ledica loših osobina materijala, /13/. Međutim, fraktograf-ska analiza i analiza mehanike loma eksploatacijskog otka-za, lom ispitivanjem na realnom delu i lom adhok ispitiva-nja epruveta su pokazali da je naponska istorija eksploata-cijskog zamora možda oštrija od pretpostavljene. Ovo je podržano posebnim ispitivanjima pretpostavljenih i stvarnih zamornih opterećenja i napona, /15/, kasnije diskutovanih.

Figure 37 shows the outboard fracture surface shortly after recovery, and Fig. 38 summarises the QF fractographic analysis results, /13/. Failure occurred by high-cycle fatigue, which initiated sub-surface owing to the presence of a shot-peened surface layer.

An extensive investigation found no evidence for failure due to poor material properties, /13/. However, fractogra-phic and fracture mechanics analyses of the service failure, a full-scale test failure and ad hoc specimen test failures indicated that the service fatigue stress history could have been more severe than anticipated. This possibility was sub-sequently supported by a separate investigation of the assumed and actual fatigue loads and stresses, /15/, discussed next.

Slika 37. Spoljna površina preloma žutog nosača lopatice na licu mesta (videti sl. 36)

Figure 37. Outboard fracture surface of the yellow arm in situ (see Fig. 36).



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Slika 38. Spoljnja površina preloma sa shemom područja inicijacije zamora i fazama zamornog veka

Figure 38. Outboard fracture surface with schematic of fatigue initiation area and phases of fatigue life.

Uzroci otkaza Neprikladna konstrukcija prema zamoru: Proizvođač

helikoptera, GKN Vestland, je utvrdio da u uslovima opte-rećenja na tlu, minimalni nagib na tlu (MPOG), nije uklju-čen pri konstrukcijskoj analizi glavčine rotora. Primenom poznate hipoteze Palmgren-Majner o zbirnom oštećenju, usvojenoj i u prvobitnoj analizi sigurnog veka, GKN Vest-land je našao da MPOG uslovi opterećenja mogu biti i do 70% ukupnog zamornog oštećenja glavčine rotora, /15/. Mere sanacije za ostale Links helikoptere

Novi projekt glavčine rotora: GKN Vestland je projekto-vao i izradio glavčinu rotora koja nije izjedna, sa vijcima na polugama radi bolje raspodele opterećenja. Materijal je zamenjen β legurom titana Ti-10V-2Fe-3Al. Nova glavčina rotora je postepeno uvedena u flotu RNLN.

POTREBNO ZNANJE EKSPERATA

Istraživanje tekućih eksploatacijskih otkaza zahteva širok opseg znanja i ekspertiza. Slika 39 daje pregled korišćenih specijalizovanih znanja i tehnika. Među njima su možda najvažnije tačke: – Otkazi se odnose na brojne klase metalnih materijala za

letelice: legure aluminijuma, niskolegirane čelike visoke i srednje čvrstoće, ugljenične čelike, nerđajuće čelike, legure titana, i superlegure na bazi nikla.

– Fraktografska analiza je uvek potrebna. Fraktografijom se utvrđuje mehanizam loma, uključujući zamor, naponsku koroziju i preopterećenje, i uzima u obzir uticaj sredine. Kad god je to bilo moguće, kvantitativna fraktografija (QF) je korišćena za analiziranje brzine rasta zamorne prsline.

– Metalografija je obično potrebna da pomogne ili da potvrdi fraktografsku analizu.

– Hemijska analiza se obično koristi radi provere materi-jala.

Contributing cause Inadequate fatigue design: The helicopter manufacturer,

GKN Westland, found that a ground load condition, mini-mum pitch on ground (MPOG), had not been included in the rotor hub design analysis. Using the well-known Palm-gren-Miner cumulative damage hypothesis, also used in the original safe-life analysis, GKN Westland found that the MPOG load condition could have contributed up to 70% of the total fatigue damage to the rotor hub, /15/. Remedial action for other Lynx helicopters

New design rotor hub: GKN Westland designed and manufactured non-monolithic rotor hubs with bolted-on arms to provide multiple load paths. The material was changed to the β titanium alloy Ti-10V-2Fe-3Al. The new rotor hubs were gradually introduced into the RNLN fleet.

REQUIRED SPECIALIST KNOWLEDGE

Investigating the foregoing service failures required a broad range of knowledge and expertise. Figure 39 summa-rises the specialist knowledge and techniques used. Perhaps the most important points are: – The failures cover many classes of aircraft metallic

materials: aluminium alloys, high and medium strength low alloy steels, carbon steels, stainless steels, titanium alloys and nickel-base superalloys.

– Fractographic analyses were always needed. Fractogra-phy had to identify the fracture mechanisms, including fatigue, stress corrosion and overload, and take account of environmental effects. Whenever possible, Quantita-tive Fractography (QF) was used for fatigue crack growth analyses.

– Metallography was usually needed to aid or confirm the fractographic analyses.

– Chemical analyses were usually done to check on the materials.



86


S-61N CF6-50 AT-301 Alouette F100 B747 Lynx Fraktografija Fractography Priprema površine loma Fracture cleaning NDI NDI Metalografija Metallography Tvrdoća Hardness Hemijska analiza Chemical analysis

Testing zatezanje tensile zamor fatigue SCC SCC

Ispitivanje

KIc KIc Fatigue analysis HCF HCF Analiza zamora rast prsline crack growth Metallurgy aluminijum aluminium čelici steels titan titanium

Metalurgija

superlegure superalloys Slika 39. Pregled potrebnog znanja specijalista i tehničkih postupaka Figure 39. Summary of required specialist knowledge and techniques.

ZAHVALNOST

Brojne kolege, veoma mnogo njih da bih im se zahvalio pojedinačno, sudelovali su u slučajevima prikazanim u ovom radu. Njihov doprinos je suštinski bitan.

ACKNOWLEDGEMENTS

Many colleagues - too many to acknowledge individu-ally - assisted in the cases reviewed in this paper. Their contributions were essential.

LITERATURA – REFERENCES

1. Wanhill, R.J.H., Graaf, E.A.B. de, Vet, W.J. van der, Investiga-tion into the cause of an S-61N helicopter accident, Part I: fractographic analysis and blade material tests, NLR Technical Report NLR TR 74103 C, National Aerospace Laboratory NLR, Amsterdam, the Netherlands, 1974.

2. Wanhill, R.J.H., Graaf, E.A.B. de, Delil, A.A.M., Significance of a rotor blade failure for fleet operation, inspection, mainte-nance, design and certification, Fifth European Rotorcraft and Powered Lift Aircraft Forum, Amsterdam, the Netherlands, 1979, pp. 38-1–38-15.

3. Veer, F. van der, High pressure turbine stage one blade failures, KLM Report No. G-180, KLM Royal Dutch Airlines Engineering and Maintenance Division, Schiphol, the Nether-lands, 1979.

4. Wanhill, R.J.H., Mom, A.J.A., Contribution of turbine blade service failures to a damage tolerance approach, Damage Tolerance Concepts for Critical Engine Components, AGARD Conference Proceedings No.393, Advisory Group for Aero-space Research and Development, Neuilly-sur-Seine, France, 1985, pp. 22-1–22-11.

5. Wanhill, R.J.H., Mom, A.J.A., Hersbach, H.J.C., Investigation of cracking and failure of first stage turbine blades from CF6-50 engines, NLR Technical Report NLR TR 79091 C, National Aerospace Laboratory NLR, Amsterdam, the Netherlands, 1979.

6. Kolkman, H.J., On the cause of failure of a propeller blade of Air Tractor PH-CPR, NLR Technical Report NLR TR 87086 C, National Aerospace Laboratory NLR, Amsterdam, the Neth-erlands, 1987.

7. Oldersma, A., Wanhill, R.J.H., Failure analysis of the RH upper attachment lug in an Alouette III fuselage-tail boom connec-tion, NLR Contract Report NLR CR 90344 C, National Aero-space Laboratory NLR, Amsterdam, the Netherlands, 1990.

8. Kool, G.A., Kolkman, H.J., Wanhill, R.J.H., Aircraft accident F16/J-054: F100-PW-220 6th stage disk and 5th stage RCVV pin investigation, NLR Contract Report NLR CR 92424 C, National Aerospace Laboratory NLR, Amsterdam, the Nether-lands, 1992.

9. Kolkman, H.J., Kool, G.A., Wanhill, R.J.H., Aircraft crash caused by stress corrosion cracking, International Gas Turbine and Aeroengine Congress and Exposition, the Hague, the Neth-erlands, 1994, American Society of Mechanical Engineers Paper 94-GT-298, 1994.

10. Wanhill, R.J.H., Oldersma, A., Fatigue and fracture in an aircraft engine pylon, Engineering Against Fatigue, Eds. J.H. Beynon, M.W. Brown, R.A. Smith, T.C. Lindley, B. Tomkins, A.A. Balkema, Rotterdam, the Netherlands, 1999, pp. 721-727.

11. Wanhill, R.J.H., Investigation of the inboard mid-spar fitting from the pylon of engine #3 of El Al Flight 1862, NLR Contract Report NLR CR 92454 C, National Aerospace Labo-ratory NLR, Amsterdam, the Netherlands, 1992.

12. Oldersma, A., Wanhill, R.J.H., Investigation of the outboard mid-spar fuse pin from the pylon of engine #3 of El Al Flight 1862, NLR Contract Report NLR CR 93030 C, National Aero-space Laboratory NLR, Amsterdam, the Netherlands, 1993.

13. Wanhill, R.J.H., Material-based failure analysis of a helicopter rotor hub, Practical Failure Analysis, 3 (2003), pp. 59-69.

14. Falconer, J.M., Goddard, P.N., GKN Westland Helicopters letter JMF/KP/1036 to the Royal Netherlands Navy, 14 Novem-ber 1998.

15. King, S.P., Lynx hub MPOG fatigue damage assessment, Report CD/SPK/JTWP20-3555, Issue 1, GKN Westland Heli-copters Limited, Yeovil, United Kingdom, 15 November 1999.



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ISTRAŽIVANJE U NACIONALNOJ VAZDUHOPLOVNOJ LABORATORIJI (NLR) ZNAČAJNIH EKSPLOATACIJSKIH OTKAZA VAZDUHOPLOVA SOME NOTABLE AIRCRAFT SERVICE FAILURES INVESTIGATED BY THE NATIONAL AEROSPACE LABORATORY (NLR)UVODINTRODUCTIONLOPATICE ROTORA HELIKOPTERA SIKORSKI S-61N (1974)SIKORSKY S-61N HELICOPTER ROTOR BLADE (1974)Uzroci otkazaContributing causesAkcije sanacije drugih S-61N helikoptera, /2/

LOPATICE TURBINE DŽENERAL ELEKTRIK CF6-50 G40 (1977-9)Remedial actions for other S-61N helicopters, /2/

GENERAL ELECTRIC CF6-50 G40 TURBINE BLADES (1977-9)Mere sanacije (I)Remedial actions (I)Mere sanacije (II)Remedial actions (II)

LOPATICA PROPELERA ER TRAKTOR AT-301 (1987)Uzroci otkazaMere sanacije

UŠKA REPA HELIKOPTERA AEROSPASIAL ALUET III (1990)AIR TRACTOR AT-301 PROPELLER BLADE (1987)Contributing causesRemedial actions

AÉROSPATIALE ALOUETTE III HELICOPTER TAIL LUG (1990)Uzroci otkazaMere sanacije za druge helikoptere Alouette III, /7/

OSOVINICA POLUGE DŽENERAL DAJNAMIKS F-16 / PRAT & VITNI F100-PW-220 RCVV (1992) Contributing causeRemedial actions for other Alouette III helicopters, /7/

GENERAL DYNAMICS F-16 / PRATT & WHITNEY F100-PW-220 RCVV LEVER ARM PIN (1992)Uzroci otkazaMere sanacije ostalih motora F100-PW-220

GUBITAK MOTORA BOING 747-258F ZBOG OTKAZA NOSAČA NA KRILU (1992)Contributing causesRemedial actions for other F100-PW-220 engines

BOEING 747-258F ENGINE LOSSES OWING TO A WING PYLON FAILURE (1992)Dostupne komponente za spajanje nosača motora br. 3 za kriloRecovered components from the No. 3 engine pylon-to-wing connectionsSpojnice nosača srednje ramenjačeMid-spar pylon fittingsSpoljnja sigurnosna osovinica srednje ramenjačeOutboard mid-spar fuse pinUzroci otkazaRedosled toka razdvajanja motora br. 3 i nosača i osnovni uzrok Contributing causesSeparation sequence for the No. 3 engine and pylon and basic causeMere sanacije ostalih aviona Boing 747Remedial actions for other Boeing 747 aircraft

GLAVČINE ROTORA HELIKOPTERA VESTLAND LINKS SH14D (1998)WESTLAND LYNX SH14D HELICOPTER ROTOR HUB (1998)Uzroci otkazaMere sanacije za ostale Links helikoptere

POTREBNO ZNANJE EKSPERATAContributing causeRemedial action for other Lynx helicopters

REQUIRED SPECIALIST KNOWLEDGEZAHVALNOSTACKNOWLEDGEMENTSLITERATURA – REFERENCES

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