Boeing supplements the standard procedures in its aircraft maintenancemanuals (AMM) with conditional maintenance inspection procedures. These procedures address unscheduled maintenance situations thatresult from non-normal flight and landing conditions. The procedures arecontinually being updated to standardize inspection procedures across allmodels and to provide more explicit inspection content. Currently, quan-titative vertical acceleration thresholds are being added to all Boeing-designed airplane conditional inspections to assist operators in decidingwhether to initiate hard landing inspections. Similar information is beingadded to the AMMs for Douglas-designed airplanes. Boeing also isadding qualitative data to the 737 AMM conditional inspection mainte-nance procedures based on operators’ requests and service experience.Although specific to 737-100/-200/-300/-400/-500, the rationale andsome of the data may be applied to other Boeing airplane models.

UPDATES TO 737

CONDITIONAL MAINTENANCE

RALPH MICHAEL GARBER

ASSOCIATE TECHNICAL FELLOW

SERVICE ENGINEERING – STRUCTURES

BOEING COMMERCIAL AIRPLANES GROUP

LAWRENCE VAN KIRK

ASSOCIATE TECHNICAL FELLOW

FLIGHT OPERATIONS ENGINEERING

BOEING COMMERCIAL AIRPLANES GROUP

M A I N T E N A N C E

INSPECTIONPROCEDURES

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oeing is updating its conditional maintenanceprocedures for 737

non-normal landings, such as a hard landing, a high-drag-load or side-load landing, an off-runway excursion, or a tail strike. The changes are basedlargely on airline service experience (see “Non-NormalLanding Service Experience”on p. 20) and are part of an ongoing effort by Boeing to update recommendedunscheduled inspections afternon-normal events for all itsmodels. The changes involve

1. Determination of non-normal landings.

2. More explicit inspection procedures.

DETERMINATION OF NON-NORMAL LANDINGS

Flight crew judgment. Boeing com-mercial airplanes have been designed for a 10-ft/s sink rate at or below the maximum design landing weight and a 6-ft/s sink rate at more than the maximum design landing weight. A hard landing inspection is recommendedif these values are approached or areexceeded. Because the sink rate is notdirectly measured, however, the flightcrew must rely on its own judgment orpeak vertical center-of-gravity (CG)acceleration from the flight data recorder(FDR) after the flight to determinewhether an inspection is warranted.

Service experience indicates that most flight crews report a hard landingwhen the sink rate exceeds approxi-mately 4 ft/s. Past experience also indi-cates that the flight crew’s determinationof a hard landing is the most reliable criterion because of the difficulty in interpreting recorded acceleration values at the CG of the airplane.

Vertical acceleration values. Using vertical acceleration values as the sole criterion for initiating unscheduledinspections is generally not advisablebecause of the location and design considerations of the FDRs andaccelerometers. In most instances, thereis no absolute way of knowing whetherthe recorded accelerations are a mini-mum, maximum, or some intermediatevalue relative to the entire airframestructure. This is because the onboardaccelerometer located near the air-plane CG is limited in its capability to capture actual loads that may beoccurring in the entire airplane struc-ture during the landing impact.

with ACMS capability that can be tailored to meet each operator’s unique needs: Honeywell Aerospace, Redmond,Wash., USA; SFIM, Paris, France; and Teledyne Technologies, Inc.,Los Angeles, Calif., USA.

Bulk flight data from the ACMS are stored in the quick-access recorder(QAR), located in the electrical andelectronic equipment bay on the airplane. The QAR operates much like the floppy disk drive on a desktop computer. It uses a standard optical diskor solid-state memory card as the storagemedia. In service, the operator takes theQAR disk from the airplane and loadsthe data into a maintenance ground station computer for further processing.

Because the ACMS is not required for airplane certification, the operator determines which information to record.Some operators find the FDR data suf-ficient for their maintenance program andput the same data on both the QAR and the FDR. Other operators specify a different set of parameters for theQAR for more detailed performancedata on systems that tend to drive main-tenance costs such as engines and onoperational practices during landing,takeoff, and taxi.

In addition to using flight data forunusual flight and landing conditions,operators may use the data to evaluatemaintenance requirements by collectingQAR data weekly and keeping per-manent records. This can be done for an operator’s entire fleet or a samplingof airplanes.

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Several accelerometers placedthroughout the airplane have shown significant variations in both time andmagnitude of vertical acceleration values, or structural loads. These varia-tions are the result of airplane weight,CG, motion (e.g., sink rate; forward and side velocity; roll, pitch, and yaw angles; and corresponding rates),external forces (e.g., gust loads, groundeffect, and runway contact loads), andstructural dynamics (e.g., vibrations and harmonics). Also the sampling frequency of the recorded vertical acceleration data — which is subject tothe specific flight recorder installation and varies from 4, 8, or 16 samples per

second — can cause wide variation in re-corded peak vertical acceleration values.

After reviewing some limited datagenerated by operators, Boeing decidedto perform more extensive analysis tocorrelate vertical acceleration with thedesign sink rate. Figure 1 shows anexample comparing the recommendedinspection threshold to analytical digitalFDR accelerations (based on an eight-samples-per-second digital FDR) for the 737 family at the design sink rate of10 ft/sec. The figure compares the peakCG acceleration to the filtered digitalFDR sensor acceleration (both the calculated peak and the lowest possiblerecorded peak value at eight samples per second). The inspection threshold is slightly less than the lowest data pointto minimize the number of nuisanceinspections without exceeding the design sink rate.

With this information, Boeing devel-oped vertical acceleration thresholds to trigger operator review of flight datawhen these acceleration values areexceeded in service. The thresholds, inaddition to flight crew judgment, giveoperators the option of initiating AMMconditional inspections after review of FDR data.

AMM changes. Boeing is modifying the 737 AMM, section 05-51, and willrevise the AMMs for other Boeing-designed airplane models, to include these vertical acceleration values (table 1).For Douglas-designed airplanes, similarvalues are being generated and will beavailable in the AMMs in early 2001.The values are intended as thresholds thatcan be used to help determine whether a hard landing inspection is necessary. If the flight crew concludes that it has experienced a hard landing, the AMMconditional inspections should be per-formed even if the acceleration readingsdo not exceed the values added to theAMM. The vertical acceleration values are to be used by operators, in addition to or in lieu of flight crew judgment, toinitiate conditional maintenance inspections.

The values shown in the AMM have a relatively low threshold to ensure thatthe airplane has not exceeded the design

sink rate and that the structural integrityof the airplane has been protected. Thismay lead to some unnecessary inspec-tions; however, it should reduce thenumber of inspections performed over-all because many inspection thresholdsestablished by airlines at this time are for very moderate sink rate landings.Upon retrieval of flight data, when certain CG acceleration thresholds areexceeded, operators can decide whentheir maintenance crews should perform a hard landing inspection.

Note that if the acceleration values are recorded during a hard nose landing or accompanied by more than 2 deg of roll at the time of main landing gearimpact, a hard landing may be experi-enced at significantly smaller verticalacceleration values. This information isincluded in the AMM revisions.

Data retrieval. In addition to the FDR,many airplanes have an optional onboardmaintenance data system. Maintenancedata can be more easily retrieved from such a system than from the FDR.

One type of onboard maintenancedata system is the airplane conditionmonitoring system (ACMS) installed on many 737, 747, 757, 767, and 777 airplanes. ACMS is hosted in either the data management unit (DMU) or the digital flight data acquisition unit (DFDAU) on 737, 757, and 767 airplanes and in the DMU on 747s. It is provided by the airplane informationmanagement system (AIMS) on the 777. Three companies build DFDAUs

CG acceleration by analysis

Peak at CG8 samples per sec at CGPeak at sensor8 samples per sec at sensorPeak at CG, 737-300/-400

1.7

1.6

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.860,000

737 hard landing inspection threshold Lowest indicated

vertical acceleration, 8 samples per sec

Gross weight, lb

Incr

emen

tal v

ertic

al a

ccel

erat

ion

(g)

80,000 100,000 120,000 140,000 160,000 180,000

737-

400

hgw

737-

400

hgw

737-

600

737-

600

737-

700

737-

700

737-

800

737-

800

737-

300

737 10 FT/S LANDING ANALYSIS

FIGURE

1

1

Airplane model

737 (all)747 (all)757 (all)767-200/-300767-400777

Net vertical CG load factor

8 samples per sec 16 samples per sec

2.1 2.21.7 1.81.8 1.91.8 1.91.9 2.01.9 2.1

B

Note: Similar data for Douglas-designed models, such as the MD-11 and 717, will be released in the aircraft maintenance manuals in early 2001.

VERTICAL ACCELERATION THRESHOLDS

TABLE

1

Outboard link, forward pin

Inboard link, forward pin

Shear fracture Shear fracture

Fracture

will involve both phase I and II structuralinspections. The most significant changeswill involve inspection of the main and nose landing gear and supporting structure.

These changes are as follows:During the phase I

inspections of the mainlanding gear, operatorsshould check for shock strutleakage and examine theinside diameter of the fusepins of the drag strut and theoutboard end of the mainlanding gear beam for distortion. This involveschecking for visible damageto the specific componentwithout removing it.

Operators also shouldexamine the main landinggear beam to the inboardrear spar stabilizing link fordamage to the link or thecrank shafting of the forwardand aft attach bolts (figs. 2and 3). This is accom-plished by loosening the nuton the stabilizing link boltand turning the bolt to deter-mine whether it is deformedor crank-shafted.

Damage at this locationwill warrant further actionduring phase II inspections;specifically, the trunnionlink should be removed inaccordance with the AMMand the forward trunnionfuse bolt inspected (fig. 4).

OUTBOARDFORWARD

Forward trunnionfuse bolt

Trunnion link

Forward trunnionbearing assembly

Lockwasher

Main gear shock strut

Drag bracefuse bolt

FORWARD TRUNNION FUSE PIN

FIGURE

4

Operators review these data to seewhether changes in operation coincidewith changes in frequency of conditionalmaintenance. For instance, in theabsence of flare during landing, there is a tendency for three-point landingsand generally higher sink rates at touch-down. If severe enough or done often,this can damage or wear nose landinggear components. This damage or wear usually is found at scheduled gearoverhaul. Initial touchdowns on themain gear during flared landing allowthe airplane to straighten out before nose gear contact, such as in crosswind

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conditions. This will avoid putting sideloads on the nose gear at high speedsand create less wear on some nose gear components, such as tires, steer-ing components, and internal shock strutcomponents, including upper and lowerbearings, centering cam components,and their anti-rotation devices.

MORE EXPLICIT INSPECTION PROCEDURES

In addition to vertical accelerationthresholds being added to the AMMs,the conditional maintenance inspectionsare being updated in the 737 AMM

to provide more explicit inspection procedures. The conditional inspectionsare constantly being updated and geared toward ensuring continued serv-iceability and structural integrity of the airplane.

Changes to the 737 AMM, section 05-51, involve the conditional maintenance inspections for hard,high-drag-load, and side-load landings,as well as off-runway excursions. Thesechanges will permit operators to makethe most effective use of the two-phasedconditional maintenance inspectionprocess. The changes also will keep

2

View 1

View 2 View 3

(See view 2)Gear beam stabilizing link(see view 3)

(See view 1)

Inboard link pins

Outboard link pins

Outboard link

Wing rear spar

Main landing gear beam

OUTBOARD FORWARD OUTBOARD FORWARD

OUTBOARD FORWARD

Inboard link

MAIN LANDING GEAR BEAM LINKATTACHMENTS TO WING REAR SPAR

FIGURE

2

FRACTURED LINK ATTACH PINS

FIGURE

3phase I inspections as simple as possiblewith minimal access and disassembly requirements.

The two-phased inspection startswith a close visual inspection of variousstructural components, especiallythose most vulnerable to damage, todetermine whether further inspections are warranted. A second phase ofinspections is conducted if any damageis found during phase I.

In some instances, operators mayrequest that Boeing review their findings if structural damage or fuel or hydraulicleakage is detected during phase I orphase II inspections. Boeing also may be asked to review flight recorder data.(See “Analyzing QAR Data” on p. 23.)The reviews may indicate that furtherinspections are warranted.

AMM changes for non-normal landings. Changes to the unscheduledmaintenance procedures in the 737-100/-200/-300/-400/-500 AMM

structural anomalies. In addition to fuse pin deformations, axle and truckdeformations may be discovered during close inspection of the gear.

When an airplane goes into soft orwet turf or the gear picks up debris

(fig. 5), in addition to high-drag-load conditionalinspections, the wheeland tire assemblies shouldbe replaced because wateror dirt may have contami-nated the wheel bearings.Also, the wheel speed trans-ducers should be removedand inspected; brakesshould be washed, exam-ined for obvious damage,and operationally checked;and the entire gear shouldbe cleaned of debris,especially under the axlesleeves, and relubricated.

In one instance, an axlefracture was attributed tomoisture and mud underthe axle sleeve following an off-runway excursion.According to the main-tenance records, no inspection or cleaning wasdone, and the contamina-tion resulted in corrosionand crack initiation.

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On the outboard attach fuse pin for themain landing gear beam, the retentionbolt should be removed and the pinrotated to check for crank shafting.

During phase II inspections of themain and nose landing gear, operatorsshould ensure proper hydraulic fluidlevels are in the shock struts by per-forming a two-point service check,or by completely servicing the shockstruts in accordance with the AMM.Operators also should remove the land-ing gear inner cylinders if shock strutservicing was found to be incorrect or if both a hard and a high-drag-load or side-load landing occurred at thesame time. The barrel of the inner cylinders and axles also should bedimensionally checked for distortion or bending and examined for cracking.

Airline technical and opera-tional staff may be consultedfollowing phase I and II in-spections, depending oninspection findings. Boeingis often requested to providetechnical assistance duringsuch reviews.

AMM changes for off-run-way excursions. Off-runwayexcursions occur either onhard, even surfaces that do notcreate higher-than-normalloads or on uneven surfaceswith depressions and obstruc-tions that also may include soft

and muddy conditions. The latter situa-tion can create high vertical, high drag,and side loads when the gear goes overrough terrain or when the airplane stopssuddenly in soft terrain.

Maintenance procedures for off-runway excursions are being added tothe AMM for the 737-300/-400/-500 and revised for the 737-100/-200 AMM as follows.

Travel onto surfaces with depressionsor obstructions will generally requireclose inspection of all fuse pins duringthe two-phased inspection process outlined in the AMM. The gear then may be removed for closer inspectiondepending on flight crew judgment,FDR/QAR data review, consultationbetween the operator and technicalexperts, or the discovery of any

OFF-RUNWAY EXCURSION

FIGURE

5

ABOVE: Examples ofengine and airplanedamage as the resultof hard landings

SUMMARYOperators need adequate data after a non-normal flight or landing or an off-runway excursion to determine whether to conduct anunscheduled inspection of the airplane. Operators are being provided information to supplement flight crew reports of such conditions in the Boeing AMM. This supplemental informationincludes vertical acceleration thresholds for unusual landings,which will be available in the AMMs for all Boeing- and Douglas-designed airplanes later this year.

Also, more explicit procedures for airline maintenance crews to use in initiating conditional structural inspections will be added to the 737-100/-200/-300/-400/-500 AMM by third-quarter 2001.Boeing is standardizing these inspections across all models to theextent possible, given differences in structure that will require different inspections in some instances.