What happened
On 12 March 2021, an MBB-BK 117 C-2, registration G-MPSB, was performing a demonstration of a Category A Clear Heliport takeoff procedure at North Weald Airfield, Essex. The maneuver involved simulating an engine failure after the takeoff decision point. To simulate the loss of power, the commander reduced the throttle on Engine No 1 to idle. While the commander subsequently attempted to increase the throttle, Engine No 1 failed to respond. During the subsequent efforts to manage the situation, the throttle for Engine No 2 was accidentally reduced, leaving the aircraft with inadequate power for sustained flight. This led to a firm landing which, while not classified as a hard landing by manufacturer standards, caused damage to the aircraft's skid assembly.
The investigation
Investigators examined the mechanical and procedural aspects of the flight. It was determined that the lack of response from Engine No 1 was likely caused by the rotor rpm droop compensation being trimmed in the incorrect direction. The investigation also looked into the training equipment available. A manufacturer-provided one engine inoperative (OEI) training device, which uses collars to limit throttle movement, could have been used as an alternative method for the demonstration. However, the operator had prohibited the use of this specific device due to previous instances of overtorque events.
Findings
- The primary cause of the engine's failure to respond was the incorrectly trimmed rotor rpm droop compensation.
- The reduction in power to Engine No 2 was an inadvertent error during the crew's attempt to resolve the initial issue.
- The use of a different throttle handling technique during the simulation might have prevented the event.