A 1969-built Beechcraft 99A Airliner was operating a cargo flight on this day, the 14th of October 2020, between Manchester-Boston Regional Airport (KMHT - New Hampshire, USA) and Waterville-Robert Lafleur Airport (KWVL - Maine, USA). The operating pilot was the only person onboard for the Title 14 Code of Federal Regulations Part 135 cargo flight.
Aft View of Right Wing (Source NTSB, Courtesy of FAA)
After an uneventful flight, the aircraft landed at runway 23 ( a 5500 x 100 ft, asphalt Runway) at approximately 08.20 lt (local time). Reverse thrust was selected and the brakes were applied lightly during the roll-out to slow the aircraft to taxi speed.
When a right turn was initiated to turn off the runway, the right-hand main landing gear suddenly collapsed. The pilot initiated an emergency shutdown of the aircraft and informed ATC. The pilot did not sustain any injuries, but damage to the aircraft was substantial.
The aircraft was inspected by a Federal Aviation Administration (FAA) inspector at the accident location. This inspection revealed a fractured drag leg of the right-hand main landing gear. The aircraft was then recovered from the runway and an investigation was launched by the National Transportation Safety Board.
The Right Support Drag Leg as found (Source NTSB, Courtesy of FAA)
A review of the aircraft maintenance records showed that with an approved aircraft inspection program. The airplane’s most recent inspection was completed on October 6, 2020, at an airframe time of 39,544.45 hours. At that time, the right main landing gear was lubricated in accordance with the operator’s approved aircraft inspection program. The guidance instructed the mechanic to add grease to the drag leg fitting. There was no instruction or requirement to inspect the landing gear at that time for anomalies like fatigue cracks. An overhaul of the landing gear was required at an interval of 7500 flight cycles or 12 months. The last overhaul of the landing gear was performed in March 2017.
Right Support Drag Leg as found on the airplane (Source NTSB, Courtesy of FAA)
The right drag leg support fitting and right main landing gear attach angle bracket were examined by the National Transportation Safety Board Materials Laboratory.
Examination of the right drag leg support fitting revealed the presence of several areas of pre-existing progressive fractures originating at the outer surface of the part with radial marks emanating inward, visible curved crack arrest marks, and microscopic striations consistent with fatigue. The largest fatigue crack originated at an external radius feature where parallel gouges and periodic depressions were observed on the adjacent surface. The smaller fatigue cracks originated at internal radii and features resembling ratchet marks were observed at each location, consistent with multiple initiation sites. The attach angle bracket fracture exhibited features consistent with fatigue emanating from the nut plate side and the opposite drag leg support fitting interface side.
The aircraft in its final position (Source Kathrynsreport.com, © Unknown)
The National Transportation Safety Board determines the probable cause(s) of this accident was;
A failure of the right main landing gear drag leg support fitting and angle bracket as a result of multiple areas of fatigue cracking.
The NTSB report which served as the source for this blog can be accessed by clicking on the .pdf file below. This report will also provide access to the NTSB Investigation Docket with more details on the investigation.
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