What happened
On June 9, 2011, a Piper PA-36/375, registration CS-AUC, was performing flight test maneuvers at Aeródromo Cosme Pedrógão, Santarém, to verify its airworthiness following a major overhaul. The aircraft had been out of service for a prolonged period since September 2009. During a series of touch-and-go landings, the pilot experienced an abnormal sensation in the left main landing gear during a landing on runway 05.
Sensing a gear collapse, the pilot immediately shut down the engine and maneuvered the aircraft to the right side of the runway, attempting to maintain lift on the left wing for as long as possible. The aircraft eventually came to rest off the runway, supported by the right gear and the remaining debris of the left gear. The pilot was uninjured, and the aircraft sustained minor damage to the left gear and lower fuselage.
The investigation
The GPIAAF investigation focused on the structural integrity of the landing gear and the effectiveness of the recent maintenance inspections. Investigators examined the aircraft's history, noting it had accumulated over 3,600 flight hours and approximately 8,400 landings, some of which involved high loads and unpaved runways.
Upon inspection of the wreckage, investigators discovered a total fracture of the left main wheel axle. Analysis of the fracture surface revealed that a pre-existing crack had already compromised approximately two-thirds of the axle's thickness. The investigation also reviewed the recent annual inspection, which had included visual checks of the gear, lubrication, and tire pressure, but had not utilized non-destructive testing (NDT) methods.
Findings
- The primary cause of the incident was a total fracture of the left main gear axle.
- The fracture was a progressive failure; a pre-existing crack had been growing over time due to the stresses of takeoff and landing cycles.
- The aircraft's long period of inactivity may have contributed to oxidation within the crack, increasing material brittleness.
- The recent maintenance inspection failed to detect the crack because it relied solely on visual observation rather than advanced diagnostic methods such as ultrasonic or electromagnetic testing.
- The stresses applied during the recent flight test maneuvers were sufficient to propagate the existing fatigue crack to the point of final failure.