Investigators theorized that the shock of the bird’s impact — the average weight of a female red-tail is 2.4 pounds — had knocked both fire extinguisher T-handles out of their detents, in turn freeing the ECLs and moving them backward as well. With power reduced, the main rotor rpm began to decay. From information provided by Sikorsky, NTSB investigators calculated that the crew had about six seconds in which to identify the problem and react to it by either increasing power or reducing cyclic or both. “Likely disoriented from the bird strike and the rush of air through the fractured windshield,” the board concluded, “they did not have time” to take action.
A bird strike right in front of the throttle quadrant might seem like a freak event not likely to be repeated in a lifetime. Helicopters collide with birds far more frequently than airplanes do, however, and the NTSB identified two similar occurrences involving S-76s within the preceding 10 years. In 1999, a bird struck another S-76 with sufficient force to dislodge the fire extinguisher T-handles from their detents, but not to move the ECLs. And about two years before the current accident, another helicopter of the same operator struck a seagull that penetrated the windshield and pushed the right-side T-handle and ECL back. The lodged remains of the bird prevented the crew from restoring power to the right engine, but they landed safely.
Despite the higher instance of bird strikes for helicopters, the FARs are less demanding and specific regarding helicopter windshields than they are for those of fixed-wing aircraft. The stock windshield for the S-76 is made of laminated tempered glass and is electrically heated by a dedicated AC generator. At the time the S-76 was originally certificated, the FAA had no bird strike requirements at all. Since Sikorsky hoped to sell the helicopter to North Sea operators, it demonstrated by a series of tests that the laminated-glass windshield complied with British requirements that the windshield resist penetration of a two-pound bird at 160 knots.
In the mid-1980s, PHI had begun replacing the stock windshields in its large fleet of S-76s with FAA-approved unheated cast-acrylic ones that were lighter in weight and also allowed the additional generators to be removed. The STC (supplemental type certificate) for the acrylic windshield involved in the current accident was issued in 1997. After the accident, the NTSB requested evidence of the tests or analysis required to show compliance with the basic standard in effect at the time, which was that a transport-category rotorcraft be able to fly and land safely after a collision with a 2.2-pound bird at high speed. Neither the FAA nor the STC holder provided it. The requirement as written was vague in any case; it might mean that a bird could penetrate the windshield and kill one pilot as long as the other survived to land the helicopter.
The NTSB noted that the U.S. Army no longer uses cast-acrylic windshields except in cases where an ejection seat is required to break through a canopy, because it found them to be much weaker, and more likely to shatter into dangerous fragments, than either laminated-glass or polycarbonate types.
