Sikorsky has alerted operators that certificated aftermarket cast acrylic windscreens built by Bell Helicopter affiliate Aeronautical Accessories are less tolerant to impact damage than the heavier original equipment glass and stretched acrylic windscreens built by Sikorsky.
The action is a result of a 4 January accident in which a Petroleum Helicopters S-76C++ crashed en route to an oil platform in Louisiana. Both pilots and six of the seven passengers on board were killed after the 14-seat helicopter descended into a swamp after a loud noise was heard in the cockpit and the torque to both engines subsequently dropped to zero.
According to the flight data recorder, the twin-engined helicopter was cruising at 138kt (255km/h) and about 700ft (215m) altitude at the time of the loud noise picked up on the cockpit voice recorder.
Bird specialists and US National Transportation Safety Board investigators later found remnants of a "hawk variety" on the pilot-side windscreen and other parts of the helicopter, leading to speculation that the birdstrike may have caused the windscreen to shatter or implode, incapacitating one or both pilots and causing, directly or indirectly, the engine power reduction.
As a result of the findings, Sikorsky on 17 February issued an "all operators" letter revealing that the cast acrylic windscreens, available to operators under a supplemental type certificate held by Aeronautical Accessories, provide the "equivalent" level of impact tolerance as Sikorsky-provided glass and STC stretched acrylic windscreens only at speeds below 109kt.
The NTSB says Petroleum, per normal procedure, had removed the original laminated glass windscreens after the helicopter was purchased two years ago to convert the aircraft to "operational mission status", which includes the lighter cast acrylic replacement.
The speed threshold is not a limit to helicopter operations by law because there was no US Federal Aviation Administration requirement for bird strike resistance when the helicopter was certificated, says Sikorksy.
Given its need to certificate helicopters for North Sea operators, however, Sikorsky had designed and tested the glass and stretched acrylic windscreens to handle collisions with 1kg (2.2lb) birds at the helicopter's top speed of 155kt according to more stringent UK civil airworthiness standards.