Operators of Sikorsky S76C++ helicopters may be unaware of overtorque incidents because of an anomaly in existing documentation about the aircraft's flight data recorders, says the UK Air Accidents Investigation Branch.
This fact has emerged from an AAIB investigation into a 3 May incident involving significant overtorque during an emergency go-around manoeuvre, but the operator's examination did not detect this because an anomaly in the FDR guidance led to the use of the wrong conversion factors when rendering the recorded data into engineering units.
As a result the branch has issued a special interim bulletin containing the following recommendation: "It is recommended that the Sikorsky Aircraft Corporation issues, in a single document, correct flight data recorder engineering unit conversion information for S76C++ helicopters equipped with a Teledyne Control flight data acquisition unit Part Number 2231230-10-A-1. This document should follow the guidance given in Federal Aviation Administration Advisory Circular 20-141B and UK Civil Aviation Publication 731."
The event that sparked the investigation occurred during a chartered flight at night in rain and poor visibility, when the aircraft was approaching a tree-ringed private landing site in East Sussex.
The commander became disorientated during the approach, carried out a go-around, during which the overtorque occurred, and diverted to Lydd, Kent, where it landed safely.
The AAIB said that the minimum radio altitude recorded during the go-around was 2ft (61cm).