What happened
On 10 November 1997, a Bell 206 BII helicopter, registration EI-BYJ, was performing a local test flight north of Balbriglegan, Co. Dublin. The flight was intended to verify the correct settings for maximum and minimum autorotation RPM as part of an annual inspection process. The crew consisted of the pilot and a qualified engineer acting as a passenger to record technical data.
During the flight, the pilot initiated an autorotation at 2,000 feet by lowering the collective and closing the throttle. As the rotor RPM rose toward 107%, the pilot applied collective pitch to arrest the increase. During the subsequent descent, the rotor RPM dropped sharply to 80%, triggering low RPM warnings. The pilot experienced two intense vibrations before stabilizing the aircraft and returning to base. While there were no fatalities or injuries, the aircraft sustained severe damage, including a wrinkled tail boom upper skin, wrinkled aft fuselage skin, and evidence that the engine-to-transmission driveshaft had struck the isolation mount.
The investigation
The investigation focused on the discrepancy between the flight test instructions and the manufacturer's maintenance recommendations. The operator was utilizing a flight test schedule that required the aircraft to be loaded to its maximum gross weight of 3,200 lbs. However, the manufacturer's Maintenance and Overhaul Instructions Manual recommended that autorotation RPM checks be performed at low gross weights to ensure safety, as low RPM is the most critical flight condition.
Furthermore, the investigation found that the operator was using a UK CAA-derived document rather than an approved flight test programme specifically produced by the manufacturer or the IAA. It was also noted that the manufacturer's Flight Manual and Maintenance Manual contained conflicting information regarding the recommended weights for these specific tests.
Findings
- The pilot narrowly avoided a total loss of control through rapid corrective action.
- The operator's flight test document required the aircraft to be flown at maximum gross weight, contradicting manufacturer recommendations.
- The manufacturer's maintenance manual suggests checking autorotation RPM at low weights to prevent dangerous RPM decay.
- The operator lacked an officially approved Flight Test Programme.
- There was a lack of standardized guidelines between the manufacturer's Flight Manual and the Maintenance and Overhaul Instructions Manual regarding weight requirements for RPM checks.