What happened
On 3 May 2012, a Sikorsky S-76C++, registration G-WIWI, was conducting a commercial passenger flight from Battersea Heliport to a private landing site in East Sussex. During the approach to the landing site, which was surrounded by trees, the pilot flying experienced disorientation due to night conditions, low cloud, and rain. This caused the helicopter to descend toward the treetops. To recover, the commander executed a go-around maneuver, during which high torque was applied to the engines. The aircraft subsequently diverted to Lydd, where it landed safely with no injuries to the two crew members or two passengers.
The investigation
Following the incident, the operator provided flight data for analysis. While the cockpit voice recordings had been overwritten, the flight data remained available. An investigation into the data readout revealed that during the go-around, engine 1 and engine 2 recorded maximum torques of 114.5% and 113%, respectively.
However, further scrutiny of the conversion process showed that the documentation used to translate the flight data recorder (FDR) binary data into engineering units was flawed. The operator had used documentation provided by Sikorsky that was actually written by the FDAU manufacturer, Teledyne. This documentation failed to explicitly reference the specific C++ variant of the helicopter. When comparing the conversion factors, investigators found a discrepancy in the engine torque calculation, specifically a linear coefficient factor difference of approximately 6%.
Findings
- The actual torque recorded during the go-around was significantly higher than initially reported, reaching 121.5% for engine 1 and 120% for engine 2.
- Total torque exceeded the threshold requiring mandatory maintenance action, yet the operator remained unaware because of the incorrect conversion information.
- The documentation used for the conversion did not account for the specific requirements of the S-76C++ variant.
- There was a lack of a single, unified document providing clear guidance for converting FDR parameters into engineering units for this aircraft type.
Safety action
- It is recommended that the Sikorsky Aircraft Corporation issues a single, consolidated document containing the correct flight data recorder engineering unit conversion information for S-76C++ helicopters equipped with the Teledyne Control FDAU part number 2231230-10-A-1.