NTSB now says S-76 crash cause may be bird strike

 - February 23, 2009, 3:51 PM

DNA analysis of material taken from the pilot-side windscreen of a Sikorsky
S-76C++ helicopter that crashed January 4 near Morgan City, La., suggests that
the accident might have been caused by a bird strike.

On February 5 the NTSB said the preliminary investigation showed no evidence of a bird strike, but it also ruled out fuel starvation; a midair collision; the failure of the engine, transmission, hydraulic servo or systems; or an in-flight rotor blade failure.

The Safety Board released an update of the accident investigation on Monday and stated that although a “visual examination [during the initial investigation] did not detect any such evidence [of a bird strike],” the subsequent DNA test found “microscopic” remains of a hawk. The Smithsonian Institution Feather Identification Lab performed the DNA test. Investigators also found “small parts of feathers” under a right side windscreen seal and in the folds of the right seat engine inlet filter.

According to the NTSB, the swab of material was taken from “an area of the windscreen that exhibited concentric ring fractures.” Additional swabs were taken this past week from the fuselage; empennage; various inlets, including the engines; the rotor hub and the main rotor blades.

According to the initial NTSB investigation, an analysis of the flight data recorder indicated that the helicopter was cruising at 138 knots at an altitude of about 700 feet agl when the cockpit voice recorder indicated “a loud noise followed by a substantial increase in the background noise level that was recorded on both intercom and area microphones. About one second after the loud noise, the torque of both engines dropped simultaneously to near zero. The engine and rotor parameters recorded by the FDR and recorded sounds from the CVR showed a simultaneous drop in RPMs over the next several seconds.”

The investigation also notes that PHI, the helicopter operator, had replaced the original production windscreen and replaced it with a lighter weight, cast acrylic windscreen.

The Safety Board said ongoing investigation will include further analysis of the voice and data recorders, as well as examination and analysis of the damage to the cast acrylic and composite center post; investigation into the windscreen STC issuance and specifications; research into the potential scenarios that could cause the loss of engine torque and electrical anomalies noted on the flight recorders; and the flight crew’s response to the event.