What happened
On 3 May 2012, a Sikorsky S-76C++ helicopter, registration G-WIWI, was performing a commercial passenger flight from Battersea Heliport to a private landing site in East Sussex. The flight took place at night under challenging conditions, including rain, low cloud, and poor visibility.
While maneuvering near the landing site, which was surrounded by trees, the commander became disoriented. This caused the aircraft to descend toward the treetops in a forested area. To avoid the trees, the pilot executed a go-around maneuver. During this high-power maneuver, the aircraft's radio altimeter recorded a minimum height of only two feet. The helicopter subsequently diverted to Lydd, where it landed safely. There were no injuries and no damage to the aircraft.
The investigation
Following the incident, the AAIB analyzed flight data provided by the operator. The investigation focused on the engine torque levels recorded during the go-around. The operator had utilized a third-party service to convert the flight data into engineering units. However, the investigation found that the conversion process relied on documentation that was inaccurate for this specific aircraft variant.
The investigation established that the Flight Data Acquisition Unit (FDAU) installed on G-WIWI required different conversion factors than those provided in the documentation the operator was using. Specifically, the documentation used was intended for an older version of the unit. When the AAIB performed its own conversion using the correct parameters, it revealed that the engine torque had actually reached much higher levels than previously thought. A subsequent calibration check confirmed that the recorded torque during the maneuver had exceeded the manufacturer's limits.
Findings
- The engine torque recorded during the go-around was significantly higher than initially reported due to an incorrect linear conversion factor for engine torque.
- While initial readings suggested torque levels of 114.5% and 113%, the corrected data showed levels of 121.5% and 120%.
- The total torque exceeded 240%, a threshold that necessitates mandatory maintenance actions.
- The operator was unaware that the engines had exceeded the required maintenance limits because the conversion documentation used was for a different FDAU version.
- The correct conversion information was not contained within a single, easily accessible document.
Safety action
It is recommended that the Sikorsky Aircraft Corporation publishes a single, consolidated document containing the correct flight data recorder engineering unit conversion information for all S-76C++ helicopters equipped with the Teledyne Control FDAU part number 2231230-10-A-1.