Bird Strike Resistant Windshield Contributed To Helicopter Crash


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Bird Strike Resistant Windshield Contributed To Helicopter Crash

By Eddy Metcalf

December 1, 2010 - The National Transportation Safety Board released a final report on a fatal crash involving a transport-category helicopter caused by a bird strike. The Board said the lack of requirements for bird strike-resistant windshields contributed to the crash, and called on the FAA to develop such requirements. 

On January 4, 2009, a dual-engine Sikorsky S-76C++ helicopter (N748P), registered to and operated by PHI, Inc., crashed into marshy terrain near Morgan City, Louisiana approximately 7 minutes after takeoff from Amelie, Louisiana, on a charter flight to an oil rig in the Gulf of Mexico. Both pilots and 6 of the 7 passengers were killed in the crash. 

The aircraft had reached level cruise flight at 850 feet mean sea level and 135 knots when the cockpit voice recorder recorded a loud bang, followed by sounds consistent with rushing wind and a power reduction on both engines. The aircraft crashed several seconds later.

Feathers and other bird debris were collected from the canopy and windshield of the aircraft. Laboratory analysis identified the remains as coming from a female red-tailed hawk; the average weight of such a bird is 2.4 pounds. The investigation revealed that the impact of the bird on the canopy just above the windshield near the engine control quadrant likely jarred the fire extinguisher T-handles out of their detents and moved them aft, pushing both engine control levers into or near the flight idle position, reducing fuel to both engines. The pilots were probably disoriented from the broken windshield and rushing air and were unable to react in time to maintain control of the helicopter. 

The helicopter was originally equipped with laminated glass windshields that complied with European bird-strike resistance standards. PHI replaced the windshields with lighter weight, aftermarket cast acrylic windshields that did not have any bird-strike resistance standards. 

In 1978, when the S-76 was certificated, there were no bird-strike requirements. Currently, 14 Code of Federal Regulations 29.631 (in effect since August 8, 1996) states that, at a minimum, a transport-category helicopter, such as the S-76C++, should be capable of safe landing after impact with a 2.2-pound bird at a specified velocity, this requirement includes windshields. Current FAA requirements for transport category helicopter windshields also state that ?windshields and windows must be made of material that will not break into dangerous fragments.? 


About 4 months after this accident, Sikorsky issued a safety advisory to all operators of the S-76C++ regarding the reduced safety of acrylic windshields (both cast and stretched) compared to the helicopter?s original windshield. According to the advisory, the S-76C++?s laminated glass windshield demonstrated more tolerance to penetrating damage from in-flight impacts (such as bird strikes) compared to acrylic windshields.  

Sikorsky expressed concern in the safety advisory that the presence of a hole through the windshield, whether created directly by object penetration or indirectly through crack intersections, may cause additional damage to the helicopter, cause disorientation or injury to the flight crew, increase pilot workload, or create additional crew-coordination challenges. The investigation revealed that, following this accident, PHI is replacing all of the windshields in its S 76 helicopters with windshields that meet European bird-strike standards. 

Based on main rotor speed decay information provided by Sikorsky, the accident flight crew had, at most, about 6 seconds to react to the decaying rotor speed condition. Had they quickly recognized the cause of the power reduction and reacted very rapidly, they would likely have had enough time to restore power to the engines by moving the ECLs back into position. However, the flight crewmembers were likely disoriented from the bird strike and the rush of air through the fractured windshield; thus, they did not have time to identify the cause of the power reduction and take action to move the ECLs back into position. 

The accident helicopter was not equipped with an audible alarm or a master warning light to alert the flight crew of a low-rotor-speed condition. An enhanced warning could have helped the accident flight crew quickly identify the decaying rotor speed condition and provided the flight crew with more opportunity to initiate the necessary corrective emergency actions before impact. The NTSB determined that the helicopter crashed because of the sudden loss of power to both engines following the bird strike and the subsequent disorientation of the crewmembers.


Contributing to the accident, the Board said, were the lack of FAA regulations and guidance requiring helicopter windshields to be resistant to bird strikes, the lack of protections that would prevent the T-handles from inadvertently dislodging out of their detents, and the lack of a master warning light and audible system to alert the flight crew of a low-rotor speed condition. 

Recommendations were issued to the FAA dealing with, among other things, the design of S-76C++ fire extinguisher T- handles and engine control quadrants, and similar designs of other helicopters, and of audible low-rotor alarm systems; certification standards for helicopter windshields; and simultaneous dual-engine power loss training for helicopter pilots.


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