What happened
On 17 March 2021, a Cessna 172 S, registered F-HFBR, was conducting a standardization instruction flight from Bordeaux-Mérignac airport toward Mimizan. The flight was being conducted by a trainee pilot accompanied by an instructor.
During the return leg of the flight, while flying at an altitude between 1,0- and 1,500 ft, the crew experienced a sudden, uncommanded reduction in engine speed. This was immediately followed by a drop in oil pressure and an increase in cylinder temperature, confirming an engine failure. The propeller subsequently stopped in a horizontal position.
The instructor took control of the aircraft and identified a field containing low crops approximately 2 nm from the runway 05 threshold. After extending the flaps, the instructor performed a forced landing in the field. Upon touchdown, the nose landing gear sank into the loose soil and broke, causing the aircraft to pitch forward and overturn.
Both occupants, the trainee pilot and the instructor, were unharmed and evacuated the aircraft without assistance. A nearby helicopter pilot, having heard the emergency transmissions, landed nearby to assist.
The investigation
The BEA examined the engine and the aircraft's maintenance history. The investigation focused on the cause of the engine in-flight shutdown and the validity of the engine's maintenance intervals. Investigators analyzed the engine's internal condition, the oil filtration records, and the application of manufacturer-recommended downtime procedures.
Findings
Technical examination of the engine revealed extensive contamination of the lubrication system, consisting of a mixture of engine oil and fine metal particles. This contamination resulted from significant wear on the camshaft and several hydraulic lifters, which was driven by corrosion on the engine casing and cylinder barrels. This degradation caused a marked decrease in the oil's lubricating effectiveness, leading to damage to the front half-bearings and the incipient fusion of the number 1 cylinder piston.
Several contributing factors were identified:
- Maintenance intervals for 50-hour inspections were repeatedly exceeded.
- Manufacturer recommendations for engine care during lengthy periods of inactivity (downtime) were not applied, which likely allowed corrosion to develop.
- There was insufficient attention paid to the inspection of the oil filter during routine changes, which could have detected the presence of metallic particles and the ongoing deterioration of internal components.
- The extension of the engine's Time Between Overhaul (TBO) was improperly granted, as it failed to account for the 12-year calendar age limit of the engine.