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Windshield Not Resistant to Bird Strikes in Fatal Helicopter Crash

Tue, Dec 14, 2010 — David Evans

Featured, Safety Issues

A bird weighing about 2.4 pounds hitting a cruising helicopter can inflict fatal damage, especially if the windshield isn’t certified to resist bird strikes.

S-76C++

S-76C++

Such is the case involving a PHI-operated Sikorsky S-76C++ struck by a red tailed hawk on 4 January 2009 at an altitude of 850 feet over Louisiana marshland.

The name of red tailed hawk is obvious from this picture

The name of red tailed hawk is obvious from this picture

The report of investigation was just released by the National Transportation Safety Board (NTSB). The helicopter was cruising at a speed of 135 knots at the time.

The bird carcass not only broke through the windshield, at struck the throttle controls mounted near the top in the cockpit. When the bloody carcass struck the levers, the force moved them aft to the flight idle position. The two pilots, probably disoriented from the broken windshield and rushing air, were unable to react in time to maintain control of the helicopter. It crashed just 17 seconds after the bird strike and seven minutes after takeoff for a flight to an oil rig, killing the pilots and six of the seven passengers. The surviving passenger was seriously injured.

The NTSB determined that the Federal Aviation Administration (FAA) has inadequate standards for helicopter windshield resistance to bird strikes.

Of the NTSB’s 12 recommendations emanating from this crash, the first dealt with the vulnerable location and design of the engine controls.

The Sikorsky S-76C++ features an overhead engine control quadrant. Among other things, the quadrant houses two engine power control levers (ECLs) and two T-handles for the fire extinguishers. These T-handles are just 4 inches aft of the captain’s and first officer’s windshields. They are normally in the full-forward position in flight and a held in place by a spring loaded detent.

S-76C++ engine control quadrant

S-76C++ engine control quadrant

In the event of an engine fire, the crew is supposed to move the T-handle aft, which also moves the ECLs aft, reducing fuel to the engines.

Based on the evidence, the NTSB believes the hawk struck the windshield on the right side near the T-handles, moving them out of their detents and pushing both ECLs out of their stops.

S-76C++ fire extinguisher T-handle and ECL with trigger mechanism

S-76C++ fire extinguisher T-handle and ECL with trigger mechanism

A similar train of events occurred in a November 1999 incident in West Palm Beach, FL. A bird struck the windshield of a Palm Beach County S-76C+ helicopter. The helicopter was outfitted with a glass windshield, which the bird did not penetrate. But the bird did crack the windshield. The force was sufficient to dislodge the T-handles but not great enough to move the ECLs. This helicopter was able to make a safe landing.

The difference was in the strength of the windshield. The Palm Beach County helicopter was outfitted with the original glass windshield installed during manufacture.

The helicopter that crashed in Louisiana had had the glass windshield replaced in 2006 with a lighter weight cast acrylic windshield. As the NTSB noted, the helicopter’s certification was “grandfathered” to an earlier period, when there were no standards for bird strike resistance. Thus, no bird-impact testing was performed on the cast acrylic windshields installed on PHI helicopters, nor was such testing required by the FAA.

All that was left of the PHI helicopter that fatally crashed in the swamp

All that was left of the PHI helicopter that fatally crashed in the swamp

Sure enough, a bird hit the acrylic windshield of a PHI S-76A++ in 2006. The bird punched a near-circular hole with radiating cracks near the top of the right windshield. The trapped remains of the bird prevented the right throttle from being reengaged, but the pilot was able to land the helicopter safely.

The NTSB observed that helicopters certificated after August 1996 must have windshields that resist 2.2-lb bird strikes. To meet a European airworthiness requirement, Sikorsky tested the glass windshield against 2-lb bird carcasses hitting at a speed of 160 knots. In some of the tests, the exterior glass layer cracked, but the dead birds did not penetrate through the windshield.

Note that an adult female red tailed hawk weighs 2.4 pounds, which is greater than either FAA or European standards. The current FAA requirement for helicopter windshields requires resistance to impacts of 2.2-lb birds and that windshields “must be made of materials that do not break into dangerous fragments.” No definition is provided for “dangerous fragments” and there is no guidance as to how a manufacturer would show compliance with the requirement.

In contrast, transport category airplanes have well defined certification requirements. Windshields “must withstand, without penetration, the impact of a 4 pound bird” and must be designed to minimize the danger to pilots from flying windshield fragments.

The standards for fixed wing transports are more demanding than for helicopters, despite the fact that helicopters operate at altitudes where collisions with birds are more likely.

The NTSB recommended that the engine control quadrant on the S-76C++ and other similar helicopter controls be designed so that ECLs and T-handles be designed so that they are less likely to be dislodged out of their detents. The NTSB was silent on the wisdom of locating the controls in close proximity to the windshield (instead of 4-inch separation, at least 8-inches seems prudent).

And the NTSB recommended that the FAA:

“Issue a certification policy to require that operators of helicopters with windshields with demonstrated bird-strike resistance not be permitted to replace those windshields with ones that have not been demonstrated to be resistant to bird strikes.”

The question is why does it take an NTSB recommendation to stimulate such obvious FAA action? Dozens or more helicopters were retrofitted with the acrylic windshields, yet the FAA allowed the practice of grandfathering to a period when no bird strike requirements existed. The regulators’ torpor is amazing.

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