N609AA

On August 24, 2003, about 1110 eastern daylight time, a Boeing 757-223, N609AA, operated by American Airlines Inc. as flight 1163, a Title 14 CFR Part 121 scheduled domestic passenger flight, had a failure of the left main landing gear truck beam, while taxiing for takeoff at Miami International Airport, Miami, Florida. Visual meteorological conditions prevailed. An instrument flight rules flight plan was filed. The airplane received minor damage. The two airline transport-rated pilots, four flight attendants, and 163 passengers reported no injuries. The flight was originating at the time, and was en route to Chicago, Illinois.

The first officer stated that during preflight examination of the airplane he didn't see anything wrong with the aircraft and it was deemed suitable for flight. They closed the doors and blocked out at 1055. After receiving the taxi clearance, they proceeded on "Mike" taxiway to runway 9L. During taxi, the aircraft suddenly dipped left and skidded to a stop. They assessed a possible tire blowout and after contacting ground control the aircraft behind them confirmed it was the left outboard main tire and it was smoking. They immediately called for fire equipment, which responded to the aircraft. They shut down the aircraft and called ramp control, maintenance, and flight dispatch to coordinate. After the fire crews visually inspected the aircraft they reported the left main landing gear had failed and there was no fire or smoke to be seen. Maintenance confirmed that the bogie on the left main gear had failed. There was no threat to safety and they deplaned all of the passengers and crew uneventfully and were bussed to the terminal.

The failed landing gear components were initially examined by NTSB on the taxiway on the day of the incident and at the American Airlines maintenance facility at Miami International Airport on August 25, 2003. The left main landing gear truck beam had fractured in two pieces between the aft axle and the truck pivot pin. The fracture occurred about 14 inches forward of the aft axle centerline. The truck beam is P/N 161N1611-6 and S/N 16PE2 according to the aircraft records. According to American Airlines the landing gear was delivered new with the aircraft on July 29, 1996, and had accumulated 7,517 cycles since new. There was no truck beam shield present with the wreckage although the attach straps were present. The brake rods were recovered with the beam and show abrasion damage on the lower portion of the lug where they attach to the clevis of the inner cylinder.

The aft section of truck beam had shear lips and river patterns present on the fracture face from about the 7 o'clock position around to the 5 o'clock position. The inner diameter had cosmoline covering about 15 percent of the surface and grease covering about 25 percent of the surface area. A big ball of detached cosmoline was present at the lower end of the beam plugging up the drain hole. There was an apparent impact mark at the 6 o'clock position on the outer surface of the beam. There was no apparent degradation of the primer on the interior surface. On the forward section of the truck beam the fracture was located about 10.5 inches aft of the pivot centerline. There was some cosmoline evident on the inner surface but a majority of the surface was covered by grease. The lower third of the beam was filled with grease. There was no apparent internal primer degradation. Again at about the 6 o'clock position on the outer surface there was an apparent impact mark. The fracture surface matched that for the aft section.

Metallurgical examination of the failed left main landing gear truck beam was performed at the NTSB Materials Laboratory, Washington, D.C. The truck beam assembly was transported to the NTSB Materials Laboratory and initially examined on September 5 and 6, 2003, with representatives from the FAA and American Airlines present. Initial inspections found the truck beam circumferentially fractured between the aft axle and the central pivot point. The mating forward and aft portions of the beam were received along with the installed forward and aft axles and five attachment straps. The following markings were found stamped into the exterior of the forward section of the beam; 161N1611-7 ASSY, 161N1611-6, 16PE2 and 17576. The Illustrated Parts Catalog (IPC) indicated that one strap was intended to hold electrical wiring and the other four to attach a truck beam shield, p/n 161N1710-1 to the underside of the truck beam. The truck beam shield was not received in the laboratory and was reportedly not found on Miami International Airport. The interior surfaces of the beam were entirely covered by an intact layer of primer paint. Further, a corrosion preventative compound on top of the primer covered an estimated 15 percent of the interior surface and a grease like substance on top of the corrosion preventative compound covered between 25 percent and 35 percent of the interior surface. During initial on-scene examinations a large ball of corrosion preventative compound was reportedly found covering and blocking the beam drain hole at the aft bottom of the beam.

The majority of the circumferential fracture was located at approximately 14 inches forward of the aft axle centerline and about 10.5 inches aft of the central pivot centerline. Examinations found that the fractures on the forward and aft piece matched and that no pieces of the beam were missing from the fracture area. Optical examinations of the fracture surfaces, after light cleaning, uncovered shear lips and river patterns indicating that the fracture initiated at the bottom of the beam between the 5 and 7 o'clock positions. Close examinations found features consistent with overstress fractures stemming from both sides of a 2.6 inch-long (circumferential) granular area at the bottom of the beam. From the granular area, the overstress portions of the fracture propagated circumferentially around the sides of the beam and forward near the top, meeting at the top of the beam just aft of the center pivot boss. The overall geometry of the fracture was consistent with beam bending loads producing tension stress on the bottom. The lower surface of the truck beam had several areas of disturbed paint and damaged finishes (top coat paint, primer and plating) due to apparent contact with other objects.

The granular portion of the fracture was located within the largest disturbed area measuring about 3 inches circumferentially and 4 inches longitudinally. The disturbed area was mostly on the forward piece of the beam but also extended onto the aft piece. At different points within this area, the paint, primer and cadmium plating were variously missing. Bare base metal was exposed adjacent to and forward of the fracture but no significant base metal appeared to have been removed. Most of the damage to the finishes was in the longitudinal direction on both sides of the fracture. Scratches, scuff marks and other signatures on the aft side clearly indicated a rearward displacement of the coating materials. Heavy randomly oriented scratches and gouges on the forward side of the fracture overlaid and obscured the longitudinal marks. In addition to the surface damage at the fracture, a portion of the edge of the forward fracture face was dented and deformed upward with the mating area on the aft fracture not damaged. A longitudinal crack was also visible in the damage area on the forward side of the fracture. This crack will be described in more detail later.

In a second area of finish damage the paint was missing and the underlying plating was damaged and displaced at several circumferentially elongated spots in a band between about 4 and 6 inches aft of the central pivot. The damaged spots were generally less than 0.3 inch wide but the largest area measured 1.2 inches long by 0.3 inch wide. Close examinations found the paint missing and the plating displaced mostly in the circumferential directions with flaking paint noted around the edges of the damage areas. A third area of finish damage was noted on the bottom of the beam just forward of the jacking knob (approximately 5 inches forward of the aft axle centerline). In this area the coatings were damaged/missing over a large area with indicated movement of the contacting object in the forward direction To allow more in-depth examinations, sections were saw cut from both pieces of the beam. The sections included the forward and aft granular fracture areas, and the surrounding disturbed finish areas. The removed sections were cleaned of grease and corrosion preventative compound, inspected optically with a stereo microscope, and electronically with a scanning electron microscope (SEM).

Backscattered electron imaging of the outer surface of the beam adjacent to the granular fracture location on the forward side of the fracture uncovered a mottled surface with distinct elemental regions. Elemental maps acquired by energy dispersive x-ray spectroscopy (EDS) established the regions to be predominately cadmium, iron or aluminum. The aluminum regions were found either on top of iron or cadmium. Neither the beam material (AISI 4340M) nor the finishes contained significant amounts of aluminum. However, the base plate of the truck beam shield normally mounted under the truck beam was specified as an aluminum alloy (AA 2024).

Visual examinations of the damaged area adjacent to the fracture also uncovered a "y" shaped longitudinal crack in the bare metal area forward of the fracture. The crack was about 0.7 inches long and both arms of the "y" intersected the fracture near the center of the granular area. The granular area extended completely through the wall thickness of the beam and did not exhibit shear lips at either the inner or outer diameter surfaces. The granular area was made up of fracture regions on two offset planes connected in part by a shear ridge. The outboard region of the granular ...