Circumstances
On 25 May 1999 at 1550 local time, a Piper PA-24-260 Comanche, registered HB-OTF, was on a VFR instructional flight from Les Eplatures aerodrome. The aircraft, built in 1970 (serial number 24-4915), was piloted by the owner, a Polish private pilot with 393 total flight hours (3 on type), accompanied by an Italian instructor holding a professional pilot and instructor license with 11,635 total hours (20–30 on type). The flight was intended to train the pilot on autopilot use as part of his transition to the aircraft.
Shortly after takeoff, while heading toward the Fribourg VOR, the instructor noted a total electrical failure. In coordination with the pilot, they diverted to Yverdon-les-Bains aerodrome, where the aircraft's maintenance company was located. The instructor assumed responsibility for the emergency landing gear extension procedure, while the pilot flew the aircraft.
The instructor reported that he extended the gear using the emergency system, strictly following the checklist. However, he suspected that the gear might not be locked. He chose not to inform the pilot to avoid additional stress and also decided not to shut down the engine, keeping it available for a possible go-around. During landing, the landing gear retracted, and the aircraft slid on its belly for 50–60 meters. Neither occupant was injured.
Investigation Findings
Examination of the landing gear confirmed that it was not locked at touchdown. The damage to the attachment of the electric drive system indicated that the electric motor-driven actuator had not been disengaged. The emergency gear extension procedure requires disengaging the actuator by fully pushing a lever forward, then manually extending the gear with a handle. The lever likely was not moved to its full extent, as significant force is needed at the end of travel.
The electrical system investigation revealed that the battery was completely discharged, with excessive water levels in each cell, and could not be recharged. No short circuits were found. The engine started without issue using a replacement battery, and the ammeter showed charging current. The alternator had a slight zero-current deviation due to a faulty diode, but this alone would not cause battery discharge. The alternator circuit breaker was a toggle switch without a protective cover, and the checklist did not explicitly state that this switch must be ON during start.
Analysis
The exact cause of the total electrical failure could not be determined; it may have been due to a depleted or defective battery. The emergency landing gear extension procedure was likely incomplete because the disengagement lever was not fully pushed forward, leaving the gear unlocked.
Probable Cause
The accident was caused by incomplete manipulation of the emergency landing gear extension system, necessitated by a failure of the onboard electrical power supply.