What happened
On 3 April 2021, a Guimbal Cabri G2, registration F-HPTI, was conducting a training flight near Saint-Georges-de-Reneins, France. The flight, operated by Savoie Hélicopt and involving an instructor and a student-pilot, departed from a dirt track for a flight toward Marnaz.
During the initial climb, approximately 15 meters above the ground, the aircraft experienced an abrupt jolt and yaw accompanied by a clacking noise. The instructor, believing the issue was a minor failure, took control and initiated a turn to return to the original takeoff site. However, while flying south at an altitude of roughly five meters and a speed below 30 kt, the engine power decreased significantly, and a continuous aural warning signal sounded in the cabin.
To avoid a high descent rate, the instructor opted for an immediate emergency landing in a nearby field. During the running landing, the front of the helicopter's skids made contact with a tarpaulin covering the ground and sank into loose soil. This caused the aircraft to tip forward, resulting in the tail boom breaking and the main rotor being damaged. The helicopter eventually came to rest on its left side, causing substantial damage to the airframe.
The investigation
The investigation focused on the engine's performance and the mechanical state of the aircraft. Investigators examined the Lycoming O-360-J2A engine, including the cylinder compression, oil filters, and fuel filters. While the oil and fuel filters showed no signs of metal contamination, a compression test revealed that cylinder No. 1 failed to show the expected pressure increase compared to the other three cylinders.
An endoscopic examination of the combustion chambers showed no abnormal debris or striations. However, a subsequent complete teardown of the engine by a maintenance workshop identified significant carbon deposits on the piston and valve seats of cylinder No. 1, as well as coking on the intake valves and damage to the exhaust valves across all cylinders.
Findings
- The engine power reduction was likely caused by a sealing fault at the valves of one of the cylinders.
- The aircraft's landing maneuver was complicated by the presence of loose soil and a tarpaulin, which led to the forward tip and subsequent structural damage.
- The instructor's decision to return to the takeoff site was based on the initial perception of a minor malfunction, but the secondary power loss necessitated an immediate landing.