April 2002, No. 18

In recent years, there has beenan increase in the incidence of significant structural damageto commercial airplanes fromhard nosegear touchdowns. Inmost cases, the main gear touch-downs were relatively normal. The damage resulted from high nose-down pitch rates generatedby full or nearly full forward control column application beforenosegear touchdown. Flight crewsneed to be aware of the potentialfor significant structural damagefrom hard nosegear contact andknow which actions to take toprevent such incidents.

F L I G H T O P E R A T I O N S

JEFF BLAND

MANAGER

DYNAMIC LOADS

BOEING COMMERCIAL AIRPLANES

DAVID CARBAUGH

CHIEF PILOT

FLIGHT OPERATIONS SAFETY

BOEING COMMERCIAL AIRPLANES

PreventingHARD

NosegearTouchdowns

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April 2002, No. 18

nosegear contact with the runway aredescribed below.

An airplane was on approach to arelatively short runway in gusty con-ditions. The airplane experienced a normal main gear touchdown, but thefull forward column movement applied by the flight crew caused very hardnosegear contact with the runway.Resulting damage included displacednosegear, bent axles, and a buckled and cracked fuselage structure (fig. 1).In addition, the cockpit door, forwardlavatory doors, and forward passengerdoors were jammed closed.

An airplane returned to the de-parture airport following an in-flightengine shutdown. The airplane landedfirmly on the main gear. Recordings

INCIDENTS OF HARD NOSEGEAR LANDINGS

Recent incidents of hard nosegear touch-down share two characteristics. First, arelatively normal main gear touchdown is followed by full or nearly full forwardcontrol column application, which resultsin overderotation and hard nosegear contact. Second, the resulting airplanedamage is significant and requireslengthy and expensive repairs. (The location and type of damage depend on the particular model of airplane.)

Three representative incidents ofstructural damage incurred from hard

1

DAMAGE TO FORWARD FUSELAGE

FIGURE

1Hard nosegear landingscan produce heavy loads on the nosegear and its supportstructure. The resulting high stresses in the forward-fuselage upper crown andbetween the flight deck andwing front spar can causethe fuselage structure tobuckle. Appropriate actions by the flight crew can helpprevent such incidents.Understanding which actionsare appropriate requires adiscussion of the following:

1. Incidents of hard nosegear landings.

2. Structural design requirements.

3. Airplane control duringlanding and derotation.

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DISTORTION AND AFT ROTATION OF NOSEGEAR

FIGURE

2by the digital flight data recorder ended abruptly because of dam-age from the nosegear contact;however, the last data pointshowed that considerable forwardcontrol column movement hadbeen applied. The nosegear wasrotated aft and to the left of itsnormal position, resulting indamage to the lower fuselage andnosegear wheel well area (fig. 2).

An airplane landing in strongcrosswinds and turbulent condi-tions touched down on the maingear firmly, but not abnormallyfor the conditions. The airplanebounced, full forward columnmovement was applied, and thenosegear contacted the runwayvery hard, causing the nosegearto fail and rotate upward in theaft direction. The nosegear wheelassembly penetrated the electron-ics bay and caused considerabledamage (fig. 3).

NOSEGEAR COLLAPSE ON LANDING

FIGURE

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April 2002, No. 18

Redesigned metering pin

Former metering pin

Nosegear strut stroke, in

00 2 4 6 8 10 12 14 16

20

40

60

80

100

120

Nose

gear

stru

t loa

d, lb

(000

s)

NOSEGEAR LOAD VERSUS STROKE FOR PITCHOVER

FIGURE

4

In addition, the upper crown stringerson the forward fuselage of the 767-300have been strengthened in the areawhere buckling often occurs followingoverderotation. This design enhance-ment was incorporated into productionairplanes in January 1995. No retrofit isavailable for this design enhancement.

AIRPLANE CONTROL DURINGLANDING AND DEROTATION

In the last several years, there has beenan increase in the incidence of airframe

STRUCTURAL DESIGN REQUIREMENTS

Boeing first recognized that heavyloads on the nosegear could damagethe fuselage structure during the 727-200 flight-test program in the1960s. Flight-test data from variouslandings with high nose-down pitchrates led Boeing to enhance design re-quirements. These new requirementsenabled the nosegear and fuselagestructure to withstand harder nosegearcontacts. All Boeing-designed airplane

models meet these requirements.The most recent design enhance-

ments involve the 767. The 767-300nosegear metering pin has been furtheroptimized to absorb the energy producedduring overderotation events, therebylowering the load on the nosegear (fig. 4). The metering pin device con-trols the flow of hydraulic fluid withinthe nosegear oleo strut. The designenhancement was incorporated intoproduction airplanes in August 1994and is available for retrofit on earlier 767-300s.

2

3

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April 2002, No. 18

damage from hard nosegear contacts.Examination of airplane flight recorderdata from these incidents revealed that,in each case, full or nearly full forwardcolumn movement was applied betweenthe time of main gear contact andnosegear touchdown. Figure 5 showsthat enough nose-down elevator authorityexists to damage the airframe structureif the airplane is rapidly derotated following main gear touchdown. This is possible because the maximum nose-down elevator authority is designed to control go-arounds, which require

Typical for nosegear touchdown

Max

imum

nos

egea

r loa

d

Potentialairframe damage

Structural capability

Elevator column pressure

Maximumbackpressure

Maximumforwardpressure

Neutral

ELEVATOR COLUMN POSITION VERSUS NOSEGEAR LOAD

FIGURE

5

considerably more longitudinal controlthan the landing maneuver.

In response to recent incidents,Boeing has produced a training video to increase flight crew awareness of the potential for both nosegear andairframe damage as a consequence ofoverderotation. Based on a successfultraining effort in 1994 and 1995 thatsignificantly reduced hard nosegearlandings worldwide for several years,the video serves as a refresher for flightcrews. The nine-minute video has beensent to all Boeing airline customers.

(For information on how to obtain additional copies, refer to the editorsnote at the end of this article.)

Many factors influence a successfullanding and derotation. First, the approach must be stabilized, as defined by the Flight Safety Foundation (table 1).If these criteria are not met at any time before touchdown, the flight crewshould initiate a go-around.

On approach, the speed-brake lever should be armed for landing and the autobrakes should be set for therunway surface conditions. The landing

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The Flight Safety Foundation suggests that operators consider adopting the following definition of a stabilized approach: All flights shall be stabilized by the 1,000-ft height above touchdown (HAT) in instrument meteorological conditions and by the 500-ft HAT in visual meteorological conditions.

An approach is considered stabilized by the Flight Safety Foundation whenthe following criteria have been met:

The airplane is on the correct flight path.

Only small changes in heading and pitch are required to maintain that path.

The airplane speed is not higher than Vref + 20 kt indicated airspeed and not lower than Vref.

The airplane is in the proper landing configuration.

The sink rate is no more than 1,000 ft/min. If an approach requires a highersink rate, a special briefing should be performed.

The power setting is appropriate for the configuration and not below the minimum power for approach as defined by the airplane operations manual.

All briefings and checklists have been performed.

Specific types of approaches are considered stabilized if they also fulfill the following:

Instrument landing system The airplane must be flown within one dot of the glideslope or localizer.

Category I or I I The airplane must be flown within the expanded localizer band.

Visual The wings must be level on final approach when the airplane reaches the 500-ft HAT.

Circling The wings must be level on final approach when the airplane reaches the 300-ft HAT.

Unique A special briefing is required.

ELEMENTS OF A STABILIZED APPROACH

TABLE

1

Source: Flight Safety Foundation Approach and Landing Accident Reduction (ALAR) Task Force

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derotation should be performed so thatthe flight crew immediately starts flyingthe nosewheels smoothly onto the run-

way when the main wheels touch down.Flight crews can accomplish this bycontrolling the airplane pitch rate while

relaxing aft column pressure. Whenheavy brake applications are needed,with and without autobrakes, increasedaft column pressure may be required toslow the derotation rate. Flight crewsshould not hold the nose up in thetouchdown attitude or allow the nose to rise because either could result in a tail strike. Control column movementforward of the neutral position shouldnot be needed. Figure 6 illustrates thissmooth relaxation of column force asthe nose is lowered. The figure com-pares the radio altitude, pitch angle, andcontrol column forces for both normallandings and landings during which airframe damage occurred.

With the nose down, spoilers up, andthrust reversers deployed, the airplane is in the correct stopping configuration.This should be established as soon as is practical during landing. Forward column movement should not beapplied to lower the nose rapidly in aneffort to improve landing performanceor directional control. The rudder pro-vides the required directional controluntil the airplane is at a relatively lowspeed, then rudder pedal nosewheelsteering is used to complete the landingrollout. Large forward column dis-placement does not improve the effec-tiveness of nosewheel steering and mayreduce the effectiveness of main-wheelbraking because it reduces the amountof weight on the main gear.

If the airplane bounces, the flightcrew should hold or reestablish anormal landing attitude and add thrustas necessary to control the rate ofdescent. Thrust need not be added fora shallow bounce or skip. When a high,hard bounce occurs, the flight crewshould initiate a go-around.

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200

100

0

Landings during which airframe damage occurred Normal landings

100

4

12 16 20 24 28 32 36 40

12 16 20 24 28 32 36 40

12 16 20 24 28 32 36 40

8(Nose up)

(Nose down)

(Pull)

(Push)

4

0

40

40

0

Elapsed time, sec

Landings are positioned so that all reach 0 ft radio altitude at the same elapsed time (28.5 sec).

Radi

o al

titud

e, ft

Pitc

h an

gle,

deg

Cont

rol c

olum

n fo

rce,

lb

RADIO ALTITUDE, PITCH ANGLE, AND CONTROLCOLUMN FORCES NORMAL AND HARD LANDINGS

FIGURE

6

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SUMMARYFlight crews can reducethe chances of airplanedamage from hardnosegear contact byavoiding high derotationrates and excessive forward column inputs. In the event of a hardlanding, the flight crewshould report the event to the engineering andmaintenance departmentsso that the airplane canbe inspected for potentialstructural damage.

Editors note: A Boeing trainingvideo, Airplane Derotation: AMatter of Seconds, covers thematerial presented in this article.Copies of the nine-minute videohave been sent to all commercialairplane customers. Additionalcopies are available from the di-rector of Flight Technical Services,Boeing Commercial Airplanes, P.O. Box 3707, Mail Code 20-97,Seattle, WA 98124-2207, USA; telephone 206-662-7800. Additional information on hardnosegear contact is available in Boeing Commercial Airplanes Flight Operations Technical Bulletins nos. 757-48 and 767-47,Feb. 1, 1993.