Accident Overview
On August 19, 2020, at approximately 2150 mountain standard time, a Diamond DA-40 airplane, registration N386MA, sustained substantial damage during a landing accident at Benson, Arizona. The flight instructor and a pilot under instruction (PUI) were not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as an instructional flight.
Flight Details
The flight departed from Tucson, Arizona, with the aim of conducting a night cross-country flight to Benson, fulfilling a requirement for the PUI's commercial pilot license. The PUI executed a normal approach to runway 10 and initiated the landing flare at about 65 knots. The main landing gear touched down first, and as the nose lowered, a loud noise was heard. The PUI held the control stick aft to keep the nose up, but soon heard the strut scraping along the runway. After exiting the aircraft, they observed that the nose gear had separated at the pivot axle.
Nose Landing Gear Examination
The nose landing gear assembly, part number D41-3223-10-00_1, was installed before the operator purchased the airplane in 2014, when the airframe total time was 1,427.7 hours. At the time of the accident, the airframe had accumulated 8,528.3 hours. The nose gear fork assembly collapsed when a pivot axle welded to the arm fractured. Metallurgical examination revealed two opposite-facing fatigue cracks on the pivot axle, propagating inward. The cracks were consistent with reverse bending from landing forces and cycles. They grew until the remaining cross-section could not withstand the final landing loads, leading to overstress fracture. Subsequent separation of the pivot axle resulted in overstress fractures of adjacent components.
Inspection and Service Bulletin
Examination determined that the cracks could have been detected using nondestructive inspection techniques such as fluorescent penetrant inspection (FPI) or magnetic particle inspection (MPI), though disassembly would be required. Diamond Aircraft reported awareness of nine other cases of cracking on this part, with some aircraft having about 5,000 hours total time. Following the accident, Diamond issued Mandatory Service Bulletin (MSB) 40-091, requiring dye penetrant inspection of the nose landing gear leg for aircraft with total time airframe of 3,000 hours or greater, within the next 25 hours if operated on unprepared surfaces, or within 100 hours if operated on paved surfaces. The inspection must be repeated every 100 hours, and the terminating action is installation of part D41-3223-10-00_2 or later.
Probable Cause
The National Transportation Safety Board determined the probable cause of this accident was fatigue cracking of the nose landing gear pivot axle, resulting in its fracture during landing.