ERA21LA021

While taxiing to the ramp, the airplane’s right main landing gear collapsed, resulting in substantial damage to the right wing. Examination of the right main landing gear drag leg support fitting and angle bracket revealed several areas of fatigue cracking with multiple origins. The attach angle bracket fracture exhibited features consistent with fatigue. The reason for the crack initiation could not be determined. A review of the maintenance procedures performed on the right main landing gear 8 days prior to the accident found that they instructed the mechanic to add grease to the drag leg fitting. The procedures did not specify to examine the landing gear for fatigue cracking or anomalies; therefore, it is unlikely that the mechanic would have looked in the area of the cracked main landing gear drag leg support fitting and angle bracket. Furthermore, there was no nondestructive testing procedures for examining the main landing gear drag brace, so it’s unlikely an inspection would have detected the multiple areas of fatigue cracking.

On October 14, 2020, about 0820 eastern daylight time, a Beechcraft 99, N499CZ, was substantially damaged when it was involved in an accident at Waterville Robert LaFleur Airport (WVL), Waterville, Maine. The commercial pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 135 cargo flight. According to the pilot, the landing at WVL was not “firm or hard” and during the landing roll, he used reverse thrust and minimal braking to slow the airplane to a “standard taxi speed.” As he initiated a right turn to exit the runway onto the taxiway, the airplane’s right main landing gear collapsed, resulting in substantial damage to the right wing. An examination of the airplane by the Federal Aviation Administration (FAA) inspector who responded to the site revealed the drag leg of the right main landing gear was fractured. According to FAA airworthiness records, the airplane was manufactured in 1969. It was maintained 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. It did not instruct the mechanic 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 performed 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. The National Transportation Safety Board Materials Laboratory examined the right drag leg support fitting and right main landing gear attach angle bracket. 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.