NYC99IA024

The Boeing 737-300 touched down under the control of the first officer, and the pilots felt a vibration or shimmy. The captain reported that he took control of the airplane, and stopped it on the runway. Examination disclosed that the lower torsion link on the right main landing gear had failed, and the wheels had rotated 45 degrees outboard. Metallurgical examination of the failed torsion link revealed it had failed in overstress. According to a Boeing 737 Service Letter, this had happened before and was traced to excessive play at the torsion link apex joint, which rendered the shimmy dampers ineffective. The shaft on the shimmy damper was bent about 20 degrees rearward, and the apex nut which held the upper and lower torsion links together was loose on the shaft. The damage precluded a check of the apex nut for proper torque.

On November 6, 1998, about 1820 eastern standard time, a Boeing 737-3TO, N12318, operated by Continental Airlines as flight 1924, experienced a twisted right main landing gear while landing at Newark International Airport (EWR), Newark, New Jersey. The airplane received minor damage, and there were no injuries to the 2 certificated cockpit crewmembers, 4 flight attendants, or 104 passengers. Visual meteorological conditions prevailed for the scheduled passenger/cargo flight that originated from Sky Harbor International Airport (PHX), Phoenix, Arizona, about 1405. Flight 1924 was operated on an instrument flight rules (IFR) flight plan under 14 CFR Part 121.

The airplane had landed on Runway 4R under the control of the first officer, and was on its landing roll when a vibration was felt. According to a written statement from the captain:

"...Landing was on centerline with no crab or side forces. Autobrakes were selected at position #1. As the nose was apparently beginning to contact the runway normally, severe vibration was felt. Although selected immediately, thrust reversers delayed extension apparently until wheel spin-up and just prior to nose wheel contact RW. Vibration increased, and I assumed control of AC to utilize nose steering and differential thrust to maintain control of AC. AC required considerable left nose wheel steering and left reverse to maintain centerline. AC continued to vibrate severely and appeared to be 'skipping' on the right main gear, although at the time I was unable to identify the cause. The vibration continued until AC was stopped on the RW...."

The airplane was stopped on the runway, about 8,300 feet from the approach end of Runway 4R, and was examined by emergency personnel. The lower portion of the right main landing gear was rotated about 45 degrees to the right, and further taxi was not possible. The engines were shut down, and after about 30 minutes, the passengers were deplaned using portable stairs, and were bussed to the terminal.

Examination of the landing gear revealed that the inboard tire on the right main landing gear was punctured on the inboard sidewall, and had deflated. Small pieces of debris associated with the right main landing gear were found on the runway, up to 300 feet behind the airplane.

The airplane was taken to a Continental Airlines maintenance hanger where the right main landing gear was replaced, and the airplane was returned to service. The removed landing gear was examined at the Continental Airlines maintenance hanger on November 24, 1998.

The investigation revealed that wheels and brakes were attached to a hydraulically dampened piston that moved up and down, inside the main landing gear strut. Torsion links were attached to the lower portion of the main landing gear strut, and the hydraulically dampened piston, between the main landing gear wheels. An apex joint connected the two torsion links, and allowed the joint to flex and maintain proper alignment, as the piston moved up and down inside the main landing gear strut. The lower torsion link was fractured between the wheel strut, and the apex joint, through the large lightening hole.

According to Boeing Service Letter 737-SL-32-057:

"...Since 1989, eight operators have reported thirteen main landing gear lower torsion link fractures. Fractures have occurred on 737-100/-200 airplanes as well as new model 737 airplanes. The fractures typically occur by ductile fracture across the larger lightening hole. There is an accompanying shimmy damper piston fracture in some instances...An investigation of the latest fractures...has determined that excessive play was present at the torsion link apex joint, rendering the shimmy dampers ineffective. Loss of proper dampening resulted in torsion link loads in excess of design, followed by ductile fracture of torsion links on both main gear...."

According to the report from the Safety Board Material Laboratory:

"...One of the fractures contained features typical of overstress separation. No preexisting crack features were noted on this fracture. The other fracture exhibited severe damage that obliterated the fracture features...."

The lower torsion link was found to be loose on the shaft of the shimmy damper connected to the upper torsion link. The apex nut was loose, and the shimmy damper shaft was bent rearward about 20 degrees. The damage from the occurrence precluded a check of the apex nut for proper torque.

The shimmy damper piston was found to be intact and operative.

Following the incident, Continental Airlines conducted a fleetwide examination of the Boeing 737 fleet for proper torque on the torsion link apex nut. None have been reported as loose.