What happened
On March 22, 2016, a Piper Aircraft PA3/39, registration F-BXPX, was performing a flight for instruction following a helicopter training session. The aircraft departed from the Plessis-Belleville airfield on runway 07. During the takeoff roll, an eyewitness observed the aircraft lift off short of the intended distance, pitch up sharply, and bank to the right. The aircraft drifted toward the right edge of the runway, where the right wing struck the ground approximately 60 meters from the runway centerline. The aircraft came to a halt between the paved and unpaved portions of the runway. The accident resulted in two injuries (the instructor and the student) and the total destruction of the aircraft.
The investigation
The investigation examined the wreckage and the aircraft's systems. The airframe remained largely intact, though the aircraft pivoted clockwise after the initial wing strike. The flaps were found in the takeoff position, and the landing gear was extended. Both engines showed similar propeller damage, ruling out asymmetric power as a factor.
Investigators focused on the Century 2000 three-axis autopilot (AP) system. Upon inspection, the AP mode annunciators (AP, HDG, and ATT) were found to have broken filaments. Because these filaments showed no signs of deformation typically caused by impact, investigators concluded the lights were likely non-functional prior to the crash. This meant the crew could not visually confirm if the autopilot was engaged.
Testimony from the student pilot indicated that while the autopilot had been tested and worked at the previous airfield (Lognes), it was not retested for the departure from Plessis-Belleville. The instructor testified that during the takeoff rotation, he momentarily looked down for less than three seconds to retrieve a mobile phone and a document that had slipped under the seat. When he looked back, the aircraft was already in a right-hand bank and pitching down.
Findings
- The investigation could not definitively establish a single cause, but identified two primary possibilities.
- The autopilot may have been inadvertently activated without the crew's knowledge due to the failure of the mode indicator lights. If engaged, the system could have commanded a right turn toward a pre-selected heading of 200° and a nose-down pitch to counter the pilot's rotation.
- Alternatively, the instructor may have inadvertently moved the flight controls while reaching for the fallen items under the seat.
- The lack of visual feedback from the autopilot annunciators prevented the crew from recognizing an uncommanded mode engagement.