NTSB Factual Report

On November 21, 2002, at 1006 central standard time, an Airbus Industrie A319-131, N804UA, operated by United Airlines (UAL) as flight 603, received minor damage when it landed on runway 04R (8,071 feet by 150 feet, asphalt) at the O'Hare International Airport (ORD), Chicago, Illinois. The airplane landed with the nose landing gear (NLG) wheels turned 90 degrees to the direction of travel. There were no injuries to the 2 pilots, 3 flight attendants, and 77 passengers on board. The 14 CFR Part 121 flight was being conducted in visual meteorological conditions and an instrument flight rules flight plan was filed. Flight 603 departed ORD at 0918, en route to Los Angeles, California.

The captain reported that they received a "L/G shock absorber fault" on the upper display of the electronic centralized aircraft monitoring (ECAM) system when the crew attempted to retract the landing gear after takeoff. He reported they placed the gear handle in the down position after which time they also received the "AUTOFLIGHT" ECAM message and they lost use of the autopilot, autothrust, and flight directors. The captain reported that he instructed the first officer to fly the airplane while he accomplished the ECAM action items. He instructed the first officer to inform air traffic control of their problem and to request that they be able to stay near the airport in case they needed to return. The captain stated he then contacted dispatch and the system aircraft maintenance controller (SAMC) for further guidance. The captain reported the ECAM landing gear page, on the lower display, showed all three landing gear were down and locked, and the gear doors were closed, along with an amber nose wheel steering message. UAL reported that during the flight SAMC instructed the crew to interrogate the Centralized Fault Display System (CFDS), which revealed the "Nose Wheel Steering" fault. After troubleshooting the gear problem with SAMC, it was determined that the airplane should return to ORD. The captain reported that given the ECAM messages that they had, he was concerned that there would not be any nose wheel steering during the landing roll. SAMC informed the captain that if they did not have nose wheel steering it might be recovered during the landing roll when the nose strut was compressed.

The captain reported he took over flying the airplane and prepared for a landing on runway 04R. He stated the landing was smooth and the nose wheel felt normal on the wet runway. However, during the landing roll, the control tower informed him that sparks were visible from the nose gear. He reported that as the airplane slowed, they noticed a noise and vibration prior to the airplane stopping on the runway just short of the "Q" turnoff. The captain reported he informed the passengers to remain seated until he was able to assess the condition outside the airplane. He stated the Fire Department reached the airplane within one or two minutes. He was informed that there were no hazards outside the airplane and the passengers were deplaned.

Post-incident inspection revealed the NLG wheels were turned 90 degrees to the left. Both of the tires were blown. The left side tire rim was ground down to the axle. The right side tire rim was nearly ground down to the axle. There was no damage present on the airplane except for the NLG assembly.

NLG Shock Absorber Description and Operation

The manufacturer of the NLG shock absorber is Messier-Dowty. The shock absorber is a double-acting, oleopneumatic type with no separator piston.

The shock absorber assembly consists of two sections: a stationary inner cylinder (piston), which is mounted inside the shock strut assembly, and a sliding tube, which is the lower portion and contains the wheel axle. The upper portion of the inner cylinder contains anti-rotation lugs. During installation of the shock absorber, the anti-rotation lugs are aligned in slots on a backplate, which is mounted inside the top of the shock strut. A nut is then installed to secure the inner cylinder of the shock absorber/gear assembly to the backplate.

The bottom of the inner cylinder contains the lower cam. The upper part of the sliding tube contains the upper cam. This cam is held in place by eight dowel pins retained by a bushing and a retaining ring. During gear extension and retraction, the sliding tube extends down and the two cams engage, centering the nose wheel. When the strut is compressed on the ground, the cams are separated and the nose wheel is free to move for on ground steering.

Teardown Inspection

When the NLG was removed from the airplane, it was determined that the anti-rotation lugs at the top of the shock absorber were not properly seated in the backplate, which indexed and bolted to the inside of the shock strut assembly.

The shock absorber assembly was shipped to the UAL Maintenance facility in San Francisco, California, where it was disassembled and inspected on December 5, 2002.

The shock absorber assembly was disassembled using the Job Instruction Cards (JIC) provided by UAL. These were the same JICs used when the strut was assembled during recent maintenance.

This teardown revealed the upper centering cam had been rotated 20 - 30 degrees when it was installed in the inner cylinder. With this condition, in order for the upper cam to mate properly with the lower cam, the sliding tube was rotated 20 to 30 degrees in relation to the aircraft centerline. In order for the axle to be perpendicular with the aircraft centerline when installed, the position of the anti-rotation lugs were rotated 20 - 30 degrees. This resulted in the anti-rotation lugs at the upper end of the strut not being properly engaged in the backplate slots.

Recent Maintenance

A heavy maintenance visit, C-check, on N804UA was completed on November 20, 2002. This maintenance was performed by a contract Federal Aviation Administration (FAA) Certified, Part 145 Repair Station.

The NLG dynamic seal was replaced during the maintenance. In order for the seals to be replaced, the shock absorber assembly was removed was removed from the airplane and the sliding tube was removed from the inner cylinder. The seals were replaced, the shock absorber was reassembled, and reinstalled in the aircraft.

Although the maintenance was performed by a contract facility, UAL JICs were used. The assembly tasks on the job cards were derived from the FAA-Approved Airbus Aircraft Maintenance Manual (AMM).

The airplane flew one non-revenue ferry flight and two revenue flights prior to the incident takeoff. The airplane had accumulated a total of 9 hours of flight time between the maintenance and the incident.

UAL reported they had performed about 60 NLG dynamic changes in the past. The contract maitnenance facility had previously performed six NLG dynamic seal changes. The mechanic who performed the dynamic seal change on the incident NLG shock absorber had performed this job two times prior to the assembly for N804UA. The first time was with a trainer and the second time was on his own.

Additional Incidents

On November 1, 2002, an Airbus A320, being operated by JetBlue landed at the John F. Kennedy International (JFK) Airport, New York, New York, with its NLG turned 90 degrees. This airplane had come out of maintenance where the NLG dynamic seal was replaced approximately 3 days prior to the incident. The airplane had flown 15 cycles and 23 hours between the completion of the maintenance and the incident. The investigation into this incident revealed the same findings as were found on N804UA. The upper centering cam on the inner cylinder had been mis-installed. This resulted in the anti-rotation lugs not being perpendicular to the axle and consequently the lugs were not properly seated in the backplate slots.

Messier-Dowty reported that there have been two additional incidents involving the mis-assembly of the NLG shock absorber. One of these involved a Canada 3000 A320 and the other occurred in Ireland.

Additional Information

Component Maintenance Manual (CMM)

The disassembly instructions in the CMM provided by Messier-Dowty does not contain any reference to marking any of the shock absorber components prior to disassembly.

The storage and assembly portion of the CMM contains the following Cautions:

CAUTION: TURN THE UPPER CAM (1-360) IN THE SLIDING ROD UNTIL THE

REFERENCE LINE ON THE UPPER CAM (1-360) IS ALIGNED WITH

THE ONE ON THE LOWER CAM (1-370).

REFER TO Figure 701 FOR THE POSITIONS OF THE PARTS.

CAUTION: AFTER INSTALLATION, MAKE SURE THAT THE POSITIONS OF

THE UPPER AND LOWER CAMS ARE CORRECT. MEASURE

DIMENSION "A" WITH THE SHOCK ABSORBER FULLY EXTENDED.

IT MUST BE APPROXIMATELY 377.5 MM (14.86 IN).

Airbus

On December 13, 2002, Airbus issued an Operator Information Telex (OIT). The purpose of the OIT was to inform A319/A320/A321 operators of both NLG incidents. The OIT cautioned operators to strictly adhere to the instructions in the AMM. The OIT also stated that Airbus was adding an additional step in the installation instructions. That step recommends "…rotating the shock absorber around its vertical axis to ensure that it correctly engaged in the slots of the backplate … before tightening the shock absorber upper bolt. If installation is correct then rotation of the shock absorber will not be possible."

On March 13, 2003, Airbus issued another OIT. This OIT clarified the one issued on December 13, 2002, by stating that the shock absorber needed to be extended and pressurized prior to rotating it in order to assure that the lugs are engaged in the backplate slots. The OIT also stated that items in the AMM that were listed as "NOTES" were going to be changed to "WARNINGS." Finally the OIT stated that Airbus was reviewing the design of the backplate to further reduce the possibility of installation errors.

AMM

Airbus issued AMM Temporary Revision No. 32-048 dated December 23, 2003. The filing instructions for this revision stated, "Insert this Temporary Revision immediately before 32-21-13, Page 425, with the issue date Nov 01/02." This revision changed the procedures for installing the shock absorber in the airplane by adding the following Note and steps under Section B:

NOTE: When the shock absorber (11) is fully engaged in the top of the

shock strut (8), the length of the thread (16) that you can see is between

17 and 18 mm (0.67 and 0.71 in.)

(9) Install and tighten the nut (7) with you hand.

(10) Try to turn the NLG axle to make sure that the anti-rotation lugs are fully

engaged in the slots of the back plate.

Following this incident, Airbus revised the shock absorber disassembly and installation sections in the AMM.

The AMM Subtask 32-21-13-020-053 addresses the removal and disassembly of the shock absorber.

Revision date Aug 01/02 states in part:

(1) Before you remove the cylinder (3), make a mark with a felt tip marker

to show the position of the sliding cool (9), upper cam (7), cylinder (3).

Revision date May 01/03 contains an additional warning, which says:

WARNING: BEFORE YOU REMOVE PARTS FROM THE SHOCK ABSORBER,

MAKE SURE THAT YOU MAKE MATCHMARKS WITH A FELT-TIP

MARKER. THIS IS VERY IMPORTANT. IT WILL LET YOU INSTALL

THE PARTS IN THE CORRECT POSITION WHEN YOU ASSEMBLE

THE SHOCK ABSORBER AGAIN. INCORRECT ASSEMBLY OF THE

SHOCK ABSORBER IS DANGEROUS FOR AIRCRAFT SAFETY.

The AMM Subtask 32-21-13-020-054 addresses the assembly of the shock absorber. Revision date Aug 01/02 states, in part:

(2) Install the upper cam (7) on the cylinder (3).

NOTE: You must align the reference line on the upper cam (7) with the reference

line on the lower cam (8). (Ref. Fig. 406/TASK 32-21-13-991-007)

(4) Carefully engage the cylinder (3) with the upper cam (7) in the sliding

rod (9) until it touches the bottom of the bore.

NOTE: During the installation of the cylinder, try to keep the axis XX' parallel

to teh axis YY' and the axis ZZ' perpendicular to the axis YY'.

(Ref. Fig. 406/TASK 32-21-13-991-007)

(5) Put the upper cam (7) in the position that agrees with the mark you

made at removal.

The following warning was added to the task in revision May 01/03:

WARNING: DURING INSTALLATION OF THE CYLINDER, MAKE SURE THAT

YOU KEEP THE AXIS XX' PARALLEL TO THE AXIS YY' AND THE

AXIS ZZ' PERPENDICULAR TO TH AXIS YY'. IF YOU DO NOT DO THIS,

THE ASSEMBLY WILL NOT BE CORRECT. THIS IS DANGEROUS

FOR AIRCRAFT SAFETY.

The AMM Subtask 32-21-13-420-052 addresses the installation of the shock absorber. Revision dates Aug 01/02 and Aug 01/03 both state in part:

CAUTION: WHEN YOU INSTALL THE SHOCK ABSORBER, MAKE SURE THAT

YOU ALIGN THE PISTON LUGS WITH THE SLOTS IN THE SHOCK

STRUT BACKPLATE. THE SHOCK ABSORBER WILL THUS BE IN

THE CORRECT POSITION (THE WHEEL AXLE PERPENDICULAR

TO THE AIRCRAFT CENTERLINE). AN INCORRECT INSTALLATION

CAN CAUSE IMPORTANT DAMAGE TO THE NLG.

Airbus issued further revisions dated May 01/03.

The subtask was revised to read:

(14) With the shock absorber fully extended, make sure that the wheel axle

is perpendicular to the aircraft centerline.

NOTE: In the fully extended position of the shock absorber, the NLG is automatically

centered (the nose wheel steering is in zero position).

(a) Make sure that the dimension H of the extended part of the NLG sliding

tube between the lower area of the NLG lower bearing and the upper

surface of the towing fitting lug is between 454.6 mm (17.8976 in.) and 445.6

mm (17.5433 in.). (Ref. Fig. 404/TASK 32-21-13-991-009)

(15) Try to turn the NLG shock absorber around its vertical axis to make

sure that the anti-rotation lugs are engaged in the slots of the back plate:

(a) The shock absorber must not turn.

(b) If you can turn the shock absorber, this shows that the anti-rotation lugs

are not engaged in the slots because the centering cams are not

correctly installed. You must remove the shock absorber to install

the cams in the correct position.

UAL

Prior to the incident the UAL job cards for removing and disassembling the NLG shock absorber instructed maintenance personnel to draw a reference mark with a grease pencil to show the position of the sliding tube, upper cam, and lower cam prior to removing the inner cylinder. This is in accordance with the Airbus AMM, with the exception that Airbus states to use a felt tip marker.

During the teardown inspection of the incident NLG, on December 5, 2002, it was difficult to determine which part of the assembly was the upper cam by using the drawings in the AMM. The marks were made with a felt tip marker on what was thought to be the sliding tube, the upper cam, and the retaining bushing. When the retaining bushing was removed it became obvious which part was actually the upper cam. During the reassembling of the NLG, it was discovered that the marks made at disassembly had been wiped off during handling of the components. Based on this observation, UAL removed the item requiring the use of a marker to index the parts and instead added the following procedure to the JICs used to remove and disassemble the NLG in preparation for replacing the dynamic seals.

CAUTION: BEFORE SEPARATING THE UPPER CAM FROM THE SLIDING

TUBE, SELECT AN "INDEX" DOWEL HOLE IN THE SLIDING

TUBE AND ITS CORRESPONDING UPPER CM DOWEL HOLE.

IMMEDIATELY AFTER SEPARATING THE UPPER CAM FROM

THE SLIDING TUBE, LOOP A LARGE TIE-WRAP THROUGH EACH

"INDEX" DOWELHOLE TO FACILITATE PROPER PARTS

ORIENTATION DURING RE-ASSEMBLY. DO NOT TRIM THE

EXCESS TIE-WRAP. IF TIE-WRAPS ARE NOT AVAILABLE,

SELECT AN EQUIVALENT TEMPORARY DURABLE METHOD

OF IDENTIFYING THE "INDEX" DOWEL HOLES.

The following note was added to Item 14 on the same JICs.

NOTE: DO NOT REMOVE THE TIE-WRAPS (OR EQUIVALENT) FROM

THE DOWEL HOLES IN THE SLIDING TUBE OR UPPER CAM

DURING THE CLEANING PROCESS.

(2) Exercise care d

NTSB Probable Cause

The maintenance facililty improperly assembled and installed the nose landing gear shock absorber assembly. Factors were the improper assembly which allowed the nose gear to turn 90 degrees to the direction of travel.