Accident Overview
On October 14, 2020, about 0820 eastern daylight time, a Beechcraft 99, registration N499CZ, was substantially damaged when it experienced a landing gear collapse at Waterville Robert LaFleur Airport (WVL), Waterville, Maine. The commercial pilot, operating a Title 14 Code of Federal Regulations Part 135 cargo flight, was not injured.
Sequence of Events
The pilot reported that the landing at WVL was not firm or hard. During the landing roll, he used reverse thrust and minimal braking to reduce the airplane to a standard taxi speed. As he initiated a right turn from the runway onto the taxiway, the right main landing gear collapsed, resulting in substantial damage to the right wing.
Examination Findings
An FAA inspector who responded to the site examined the airplane and found the drag leg of the right main landing gear fractured. The National Transportation Safety Board Materials Laboratory later examined the right drag leg support fitting and the right main landing gear attach angle bracket. The examination revealed several areas of pre-existing progressive fractures on the drag leg support fitting, originating at the outer surface with radial marks 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. Smaller fatigue cracks originated at internal radii, with features resembling ratchet marks 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.
Maintenance History
According to FAA airworthiness records, the airplane was manufactured in 1969 and maintained with an approved aircraft inspection program. The most recent inspection was completed on October 6, 2020, at an airframe time of 39,544.45 hours. During that inspection, the right main landing gear was lubricated in accordance with the operator's approved program, which instructed the mechanic to add grease to the drag leg fitting but not to examine the landing gear for fatigue cracking or anomalies. An overhaul of the right main landing gear drag brace was performed every 7,500 flight cycles or 120 months; the most recent overhaul was on March 23, 2017, at an airframe total time of 38,485.5 hours. There were no nondestructive testing inspection procedures for the main landing gear drag braces prior to the accident.