The accident helicopter had a visual low-rotor-rpm warning system, but not an audio one. In 1978, when it was certificated, no audio system was required, but a requirement was soon added that single-engine helicopters and twins that did not have a system for automatically increasing power on one engine when the other failed must have an audio warning for low engine rpm. The S-76 did have the automatic system, however, and so it was not required to add an audio warning.
The pilots were highly experienced — more than 15,000 hours in rotorcraft between them, and more than 6,000 in the S-76. Their training was up to date. But it transpired that, while PHI’s annual recurrent training syllabus included dual engine failures, the engines always failed one after another, not simultaneously. Simultaneous loss of both engines, followed by auto-rotation to a landing, was practiced in initial training, but not afterward.
In its finding of probable cause, the NTSB identified the bird strike and the subsequent “disorientation” of the crew as the primary causal elements. “Had they quickly recognized the cause of the power reduction and reacted very rapidly,” the board wrote, “they would likely have had enough time to restore power to the engines by moving the ECLs back into position.” The board did not mention lowering the collective, which is the first response to a loss of power; it seemed to accept that an appropriate reaction to the emergency in the time available and in the sensory chaos of the moment could not be expected. The sudden rush of wind into the cockpit may have masked the sound of the rotor losing speed; eight seconds passed between the impact and the first pilot saying “low rotor.” At that point, by the board’s calculation, the possibility of saving the helicopter had already passed.
Among contributing causes, the board cited the lack of FAA regulations and guidance regarding bird strikes on rotorcraft at the time the S-76 was certificated; the design of the fire extinguisher T-handles, which could be inadvertently knocked out of their detents; and the lack of “a master warning light and audible system to alert the flight crew [to] a low-rotor-speed condition.”
Subsequent to this accident, the FAA proposed new standards for cockpit alerting systems that would require, among other things, that newly certificated aircraft provide “redundant sensory cueing” — in other words, both visual and aural warnings at once.
Sikorsky published a safety advisory warning of the reduced resistance to bird strikes of aftermarket acrylic windshields and of the danger, if the windshield is penetrated, of “crew-coordination challenges” and of “disorientation or injury to the flight crew.”
And PHI began replacing the cast-acrylic windshields in its fleet with laminated-glass ones.
This article is based on NTSB reports of the accident and is intended to bring the issues raised to our readers’ attention. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory.