What happened
On June 21, 2018, a Bell 47G helicopter, registration F-GDRF, was performing a local leisure flight from the Colmar – Houssen aerodrome. Shortly after takeoff, at an altitude of approximately 40 meters, the pilot noticed a progressive drop in engine RPM, which fell from the recommended 3,100 rpm to 2,700 rpm.
Unable to maintain level flight, the pilot initiated an autorotation to avoid nearby vineyards, selecting a nearby rapeseed field for an emergency landing. The aircraft struck the ground heavily, causing the main rotor blades to flex, with one blade making contact with the tail boom. The pilot and passenger were able to evacuate the aircraft safely after the pilot shut down the engine.
The investigation
The BEA examined the engine, fuel system, and ignition components. While the engine was not seized and the cylinders showed no internal damage, the investigation noted that the right magneto's timing could not be verified because its drive mechanism was torn away during the impact.
Analysis of the fuel system showed that while the fuel filter contained some impurities, it was not clogged, and the carburetor functioned normally during bench testing. Although some fuel samples recovered from the lines exhibited an unusual brown color, laboratory analysis confirmed the fuel was standard Avgas 100LL.
The investigation also reviewed a similar incident that occurred one week prior, on June 14, 2018, where the same aircraft experienced a power loss during a flight near the Vosges. In that instance, the pilot performed an autorotation but was able to restart the engine and return to base, later attributing the issue to potential carburetor icing without reporting it to maintenance.
Findings
- The investigation could not identify a specific mechanical cause for the engine power loss during the initial climb.
- Meteorological data from the day of the accident indicated conditions that made carburetor icing unlikely.
- The heavy impact and resulting damage to the Bell 47G were caused by the pilot's difficulty in estimating the necessary flare height due to the tall crops in the landing field.
- The pilot's decision not to report the previous power loss incident to maintenance prevented the early detection of the underlying technical issue.