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23rd of May 2001, Production Error, Blog #653

A 9-year-old Fokker 100 (Fokker F28 Mk.100) was operating a scheduled passenger flight between Charlotte-Douglas Airport (North Carolina, USA) and Dallas/Fort Worth International Airport (Texas, USA) on the 23rd of May 2001.

The aircraft in its final position (Source; © DWS)

Onboard the aircraft was a crew of four and 88 passengers for the ~1500 kilometres (810 nautical miles) flight. The flight progressed smoothly after departing Charlotte-Douglas Airport at 12:28 lt (Local Time) and was approaching Dallas/Forth Worth around 15:00 lt. after an uneventful flight. ATC cleared the flight for a visual approach to runway 17C. The visual approach was flown as cleared, visually, backed up with the localizer and Precision Approach Path Indicator (PAPI). After a stable approach, the aircraft touched down normally on Runway 17C. Shortly after the touchdown a loud bang-like sound was heard throughout the aircraft, simultaneously the right wing dropped and the aircraft started to the right edge of the runway. Initially, the crew was able to maintain control of the aircraft, using rudder inputs and applying the left wheel brakes. As the speed decayed rudder authority became less and the Captain used "gentle" tiller (nose wheel steering control) inputs to assist in aligning the airplane with the runway centerline. The aircraft came to a stop on the runway with the airport fire department on the scene in less than a minute. After consulting with the cabin crew and after contact between the captain and the fire brigade, it was decided that an emergency evacuation was not required and that the passengers could deplane using steps, which were brought to the aircraft. There were no injuries sustained by the passengers and crew.

An examination of the aircraft revealed that the lower portion of the right main landing gear assembly had failed and separated from the airplane. The aircraft had travelled approximately 4300 feet before coming to a stop, with the aircraft centreline approximately 12,5 feet to the right of the runway centerline. Damage to the aircraft was limited to the right main landing gear and the right wing. The forward spar of the right-wing, between stations 12,375 and 13,550, was damaged.

The aircraft in its final position (Source; © M. McDowell)

As part of the National Transportation Safety Board (NTSB) investigation, the landing gear parts were sent to the NTSB Materials Laboratory in Washington, D.C., for further examination. During this examination segments of the landing gear were sent to the landing gear manufacturer's materials laboratory for analysis under the supervision of the Air Accidents Investigation Branch of the United Kingdom.

During this investigation, it became apparent that a forging fold was produced during the forging of the gear, during the preform stage of the forming. (Folding is formed by the confluence of already oxidized surface metal during the forging process. On free forgings, folding is mainly due to too small a feed amount during elongation, too large a pressing amount, or too small a radius of the anvil block corner; on die forgings, folding is mainly caused by metal convection or backflow during die forging).

An example of a fold occurred during forging Preforming is a "hand operation," therefore the occurrence of folds is partially dependent on the operator. A review of the manufacturing process revealed that there have been no changes to the forging process that would relate to the development of a fold.

During a fleet campaign by the operator, three more gears were found with positive crack indications when the eddy current inspections were performed. The aircraft manufacturer issued a service bulletin and the NTSB issued a number (four) Urgent Safety Recommendations to the FAA to prevent re-occurrence. As a result, the FAA issued an Airworthiness Directive (AD) The AD required an initial eddy current inspection of the MLG main fittings as cited in Fokker Service Bulletin, within 1,000 total landings or 30 days, whichever occurs later. The AD further required operators to repeat the inspection every 500 landings or 6 months, whichever occurs first.

The NTSB concluded that the probable cause of this accident was;

A forging fold that was introduced during the manufacture of the right main landing, which resulted in a fatigue crack in the right main landing gear cylinder, and its subsequent failure during landing.

The NTSB report, which served as the source for this blog, which contains more details on the investigation, can be accessed by clicking on the .pdf file below;

23may2001 Gear Failure F100
Download PDF • 115KB

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