What happened
On 22 May 2014, a Beechcraft Duchess 76, registration EI-BUN, was conducting a training flight at Weston Aerodrome (EIWT) in Co. Kildare. The flight was a multi-engine instructional session involving an instructor and a student pilot, marking the student's first landing in a twin-engine aircraft. Following a series of maneuvers, the aircraft returned to the aerodrome for a landing on Runway 07.
While the initial touchdown was smooth and the approach appeared normal, the aircraft began to drift toward the right side of the runway as it decelerated through 50 knots. Despite the instructor taking control and applying maximum left rudder to maintain directional control, the starboard wing began to drop. The right-hand propeller blades struck the runway surface, and the aircraft subsequently veered off the paved surface onto the grass. There were no injuries to the two crew members, though the aircraft sustained substantial damage.
The investigation
The AAIU examined the wreckage and the aircraft's maintenance history. The investigation focused on the starboard main landing gear, which had collapsed during the landing roll. Physical inspection revealed that the A-frame assembly—a critical component of the landing gear down-lock mechanism—had suffered a structural failure. Specifically, both the down tube and the diagonal tube of the A-frame were fractured.
Metallurgical analysis conducted in the UK determined that the failure originated from the development of two fatigue cracks on the external surface of the down tube, located near the fillet weld. These cracks propagated through the entire wall thickness of the tube. The analysis concluded that the cracks grew due to reverse bending loads experienced during operations such as takeoff, landing, and taxiing. The investigation also noted that while a visual and dye-penetrant inspection had been performed approximately 100 hours prior to the accident, no cracks were detected at that time.
Findings
- The primary cause of the accident was the collapse of the starboard main landing gear due to a fatigue failure of the A-frame assembly.
- The failure of the A-frame rendered the starboard landing gear down-lock mechanism ineffective.
- The fatigue cracks were driven by stresses from reverse bending loads applied to the down tube during normal flight phases.
- Although the FAA had issued a bulletin recommending inspections for these specific A-frame part numbers, such inspections were not mandatory.
Safety action
Based on the findings, the AAIU issued two safety recommendations:
- A recommendation to the FAA to consider mandating repetitive visual and non-destructive inspections for Beechcraft Duchess 76 A-frames (P/Ns 105-810023-75 and 105-810023-76).
- A recommendation to Textron Aviation to consider updating the aircraft's Maintenance Manual to include specific references to mandatory repetitive inspections for these components.