Accident Overview

Photo of Sikorsky S-76C
Photo of Sikorsky S-76C
Photo copyright Lockheed Martin - used with permission

Photo of Sikorsky S-76C++ N748P wreckage
Photo of Sikorsky S-76C++ N748P wreckage

History of Flight

On January 4, 2009, a PHI Sikorsky S-76C++ departed Lake Palourde Base Heliport at 1402. The helicopter was en route to an offshore oil platform with two pilots and seven passengers. Seven minutes after departure, while in cruise flight at 850 feet mean sea level and an indicated air speed of 135 knots, the cockpit voice recorder recorded a "loud bang," followed by sounds consistent with "rushing wind." An immediate power reduction on both engines was followed by a rapid decay of main rotor RPM (revolutions per minute). The helicopter departed controlled flight and rapidly descended and crashed into marshy terrain. The pilots, and six of the seven passengers were killed. One passenger was seriously injured.

The PHI communications center did not receive a distress call or emergency transmission from the flight. A search and rescue operation was initiated at 1414 after the U.S. Air Force received an emergency locator transmitter (ELT) distress signal with the helicopter's unique identifier and location. Approximately 20 minutes after the crash, the United States Air Force Rescue Coordination Center (USAFRCC) notified PHI and the United States Coast Guard (USCG). A USCG helicopter located the wreckage and performed a rescue operation to extract the surviving passenger.

Following recovery of the wreckage, it was examined by accident investigators. Investigators determined that both sections of the cast acrylic windshield were shattered. The windshield assembly consists of left and right (copilot and pilot) sections separated by a center post. Initial visual inspections on-site did not reveal any evidence of a bird strike; however, the windscreen did exhibit concentric ring fractures which can indicate an impact to the windscreen.

Subsequent investigation determined that the helicopter had collided with a red-tailed hawk. The helicopter crashed just 17 seconds after the bird strike and seven minutes after takeoff.

An animation of the accident events is available at the following link: PHI Inc. Sikorsky S-76C++, N748P Accident Animation

Photo of a red-tailed hawk
Photo of a red-tailed hawk

Detailed examination of the airframe surrounding the windscreen revealed feathers and other bird remains. Samples were obtained from the canopy, at the apparent point of impact, the windshield and from other locations on the exterior of the helicopter. Laboratory analysis was accomplished by the Smithsonian Institution Division of Birds, which identified the remains as coming from a female red-tailed hawk. The female red-tail hawk has an average weight of approximately 2.4 pounds.

No defects in the materials, manufacturing, or construction of the windscreen were observed. A search of maintenance records did not indicate the presence of any preexisting damage that might have caused the windshield to shatter. A detailed windshield examination revealed fractures at the top of the right section of the windshield and damage to the canopy in that area. Investigators concluded that this damage was consistent with a bird impacting the canopy just above the top edge of the windshield. This resulted in structural deformation and ultimate failure of the canopy structure. All other fractures observed in other areas of the windshields were caused by ground impact.

The NTSB's Materials Laboratory examined the fractured windshields from the accident helicopter and found that all of the fractures were typical of brittle overstress. Many of the windshield fragments were large and sharp-edged. The windshield was made from monolithic cast acrylic. Investigators determined that the location of the bird impact deformed the fuselage and surrounding structure in the area of the engine control quadrant, likely jarring the fire extinguisher T-handles out of their detents and moving them aft. This aft movement then moved both engine power control levers (ECL) triggers out of their stops, moving them aft and into or near the flight-idle position.

Engine Controls

Photo of S-76 engine controls on cockpit overhead
Photo of S-76 engine controls on cockpit overhead

The S-76C++ helicopter was designed with engine controls on the overhead quadrant, accessible by both pilots. The quadrant also houses two engine fire extinguisher T-handles and two engine power control levers. The fire extinguisher T-handles are approximately four inches aft of the captain's and first officer's windshields. While in cruise flight, the fire extinguisher T-handles are positioned full-forward. Each handle is secured by a spring-loaded retaining pin which engages a detent. Moving the T-handles from the flight position detents requires an aft pull force. Aft movement of the T-handles also engages a mechanical cam on each T-handle which moves a trigger on the associated ECL out of its stop, freeing the ECL to move aft and reducing fuel to the associated engine.

In the event of an engine fire, the T-handle is moved aft, which also moves the ECLs aft, reducing fuel to the engines. Based on examination of the wreckage, investigators believed the hawk struck the canopy, above the windshield on the right side near the T-handles, subsequently moving them out of their detents and pushing both ECLs out of their stops. The investigation determined that the loss of control was the result of the rapid migration of the engine power levers out of their detent position and rapid loss of rotor speed associated with the unplanned power reduction. 

Windshield

Photos of PHI N748P windshield reconstruction
Photos of PHI N748P windshield reconstruction

Investigators determined that the windshields were not the original equipment manufacturer (OEM) windshields that had been delivered with the aircraft from the factory. Maintenance records identified that PHI had replaced the original laminated glass windshields delivered on the accident helicopter with after-market cast acrylic windshields about two years before the accident. Installation of these after-market windshields on PHI's S-76 fleet provided a weight savings relative to the OEM windshields. The weight saving was due to the reduced weight of the windshield and removal of associated windshield heating equipment. Investigators also noted that PHI had again replaced several of these after-market windshields on their fleet of helicopters about one year before the accident, due to cracking at fastener holes. At the time of the accident in January 2009, PHI had a fleet of 46 S-76's, all of which were equipped with monolithic cast acrylic windshields.

Aeronautical Accessories Incorporated (AAI) designed and produced the after-market windshields, and in November 1996 applied for supplemental type certificate (STC) approval from the Federal Aviation Administration (FAA) for installation on the S-76. The STC was subsequently issued in April 1997. The certification basis for the S-76C++, based on amendments 1 through 11 of 14 CFR part 29, which were in effect at the time of the original TC, had not required bird-impact capability. In August of 1996, 14 CFR 29.631 Bird Strike was adopted. Several months later AAI applied for the STC incorporating the acrylic windshield. Investigators could not determine if compliance with 29.631 had been demonstrated during the windshield certification process.

Bird Hazards to Aircraft

Photo of a Boeing 757 struck by seagull
Photo of a Boeing 757 struck by seagull

The FAA began collecting bird strike data in 1965 in order to determine general trends related to bird encounters and to aid in development of new rules for turbine engines and transport category airplanes. In 1995, the FAA, in cooperation with the United States Department of Agriculture, Wildlife Services (USDA/WS), created the National Wildlife Strike Database in order to collect and analyze wildlife strike data. These data are annually compiled into a report, Wildlife Strikes to Civil Aircraft in the United States. The 22nd issuance of this report (1990-2015) is available at the following link: (Wildlife Strike Report).

According to the report, bird strikes continue to pose a significant threat to aviation worldwide. Between 1988 and 2014, bird and other wildlife strikes resulted in the loss of 245 airplanes and helicopters and 258 deaths internationally.

Threat Increasing

Chart of number of reported wildlife strikes with U.S. civil aircraft 1990-2015
Chart of number of reported wildlife strikes with U.S. civil aircraft 1990-2015
National Wildlife Strike Database – 2015 Report
View Larger

According to the report, between 1990 and 2015 there were 166,276 reported wildlife strikes in the U. S., distributed among all aircraft types and operations - commercial, small aircraft, and rotorcraft. Of these encounters, 95.8% involved birds, with the remainder involving bats or terrestrial mammals/reptiles. The severity of a bird encounter can be a function of aircraft size and speed, bird size, and the number of birds involved in each encounter. The threat potential for bird encounters can vary seasonally, regionally, as a function of proximity to airports, and by the number of aircraft operations conducted in high threat areas.

In the U. S., many large birds have increased in population, and adapted to life in urban areas, including near airports. Concurrent with this increase in the number of birds, commercial aviation continues to grow, resulting in an increase in the numbers and frequencies of bird encounters. As examples, the National Wildlife Strike Database illustrated the increase in strikes involving red-tailed hawks and snow geese. In addition, significant population increases have been reported for bald eagles, vultures, ospreys, peregrine falcons, and others.

Charts showing annual increases in reported strikes involving red-tailed hawks (left) and snow geese (right) - 1990-2015
Charts showing annual increases in reported strikes involving red-tailed hawks (left) and snow geese (right) - 1990-2015
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Factors Influencing Risk 

Altitude

According to data in the FAA Wildlife Strike Database, there are a number of factors that can influence the potential for a bird strike. Seventy-three percent of bird strikes occurred at 500 feet or less, and 97% occurred below 3,500 feet. For general aviation, above 500 feet, the number of reported strikes was consistently reduced by 44% per each thousand feet of altitude increase. Risk of a strike decreases exponentially with increasing altitude.

Chart showing general aviation bird strikes as a function of altitude (left) and general aviation number of damaging strikes above/below 1,500 feet AGL (right)
Chart showing general aviation bird strikes as a function of altitude. The specified equation defines the plotted curve, and R2 defines the relative data quality and scatter. (left)
Chart showing general aviation number of damaging strikes above/below 1,500 feet AGL (right)
National Wildlife Strike Database – 2015 Report
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Bird Mass

At any given airspeed, the mass of a bird has an effect on the energy transferred during a collision and the extent of damage that may occur. Kinetic energy (mV2/2) is a direct function of the speed and mass. The FAA Wildlife Strike Database indicates that although the probability of a bird strike is reduced as altitude increases, due to the energies involved, about 50% of bird strikes at any altitude result in damage. The kinetic energy associated with a strike is essentially unaffected by altitude, and is more directly a function of the mass and relative impact speed of the bird and aircraft. Between 1990 and 2015, 36 aircraft were destroyed (or damaged beyond repair) as a result of bird strikes.

Chart showing the effect of bird body mass – National Wildlife Strike Database
Chart showing the effect of bird body mass (left)
Chart of total number of aircraft destroyed by wildlife strikes – 68 Total 36 involving birds (right)
National Wildlife Strike Database
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Seasonal Effects

Nationally, according to the National Wildlife Strike Database, between 1990 and 2014, the highest bird strike rate occurred in late summer/early fall and the lowest rate occurred in winter. Bird migration seasons have an effect on the frequency of strikes. Fifty-three percent of bird strikes occur between July and October.

Chart showing the bird strike seasonal effects (blue bars) (left) and Photo of US Airways Flight 1549 (right)
Chart showing the bird strike seasonal effects (blue bars) (left) and Photo of US Airways Flight 1549 (right) - View Larger
Photo of US Airways flt #1549, January 15, 2009, following dual engine failure due to geese ingestion into both engines. (right)

Bird Strikes to Helicopters

Photo of black vulture that penetrated windscreen on medical helicopter at 1,000 feet AGL
Photo of black vulture that penetrated windscreen on medical helicopter at 1,000 feet AGL. Bird struck crewmember wearing helmet and visor who was only slightly injured.
2015 Wildlife Strike Report

The FAA's Wildlife Strike Database indicates a 68% increase in bird strikes since 2009 and more than a 700% increase since the early 2000's in the rotorcraft event data. These percentages represent an increase from approximately 25 reports of rotorcraft bird strikes per year in the early 2000's, to 121 strikes in 2009, and 204 strikes in 2013. Implementing a rotorcraft rate-based analysis, the reported bird strikes increased from 3.99 per 100,000 flight hours to 5.95 per 100,000 flight hours, a 49% increase in the five-year period from 2010 to 2014. An improved event reporting system accounts for a portion of this increase, but the rapid increase goes beyond the reporting improvements alone. It is often attributed to a growing population of birds in general, a growing population of larger birds, quieter aircraft, and an increase in the number of rotorcraft operations.

These recent observations reinforce previous findings from the study, Bird Strikes to Civil Helicopters in the United States, 1990-2005 (2006), by Cleary, Dolbeer, and Wright, based on 15 years of data from the FAA's National Wildlife Database. The study concluded that:

(1) Helicopters were significantly more likely to be damaged by bird strikes than airplanes

(2) Windshields on helicopters were more frequently struck and damaged than windshields on airplanes

(3) Helicopter bird strikes were more likely to lead to injuries to crew or passengers than airplane bird strikes

(4) Flight crew use of protective equipment, such as helmets and visors can aid in reducing injuries

The study from which these data are derived: Wildlife Strikes to Civil Aircraft in the United States 1990-2005 is available at the following link. (Wildlife Strikes 1990-2005). The data relevant to helicopters are contained in Appendix A of this report.

Photos of a military helicopter after bird collision (left) and a helicopter following collision with large bird (right)
Photos of a military helicopter after bird collision (left) and a helicopter following collision with large bird (right)

Aircraft Certification Requirements 

Design Certification Basis

The design certification basis is the "set" of certification regulations to which a manufacturer/modifier must demonstrate compliance. When a manufacturer/modifier applies for a Type Certificate (TC) or Supplemental Type Certificate (STC), the applicable regulatory basis is established by application of 14 CFR Part 21 - Certification Procedures for Products and Parts. Further, 14 CFR 21.17, - Designation of Applicable Regulations for type certificates and 14 CFR 21.115, Applicable Requirements for supplemental type certificates outline the methods by which the certification is established. In the case of the windshield STC, 21.115 incorporates the requirements of 21.101 to establish the certification basis for the changed area. In general, the rules in effect at the date of application for the change, that is, the amendment levels of 14 CFR Parts 23, 25, 27, or 29, etc., in effect on the application date establish the certification basis.

For changes to a TC or STC, 14 CFR 21.101 - Designation of Applicable Regulations becomes the applicable regulation for establishing the certification basis for the changed area. In general, as for the basic type or supplemental type certification, the regulatory amendment in effect at the time of application defines the certification basis for the change.

Designation of applicable regulations.

(a) Except as provided in [Secs. 23.2, 25.2, 27.2, 29.2] and parts 34 and 36 of this chapter, an applicant for a change to a type certificate must comply with either--

(1) The regulations incorporated by reference in the type certificate; or

(2) The applicable regulations in effect on the date of the application, plus any other amendments the administrator finds to be directly related.

S-76 Type Certification

During the investigation, the NTSB examined the FAA certification basis for both the S-76C++ and the monolithic cast acrylic windshield that had been installed via STC. The S-76C++ largely retained the certification basis that had been applied to the original S-76 certification in 1977. The certification basis was established as Amendments 1 through 11 of 14 CFR Part 29, Airworthiness Standards: Transport Category Rotorcraft. At the time of the original certification, there were no FAA requirements for bird strike resistance for the helicopter.  Investigators determined that neither windshield installation - the original S-76C++, or the cast acrylic windshield installed via STC, were required to meet any FAA bird impact standards. However, at a later date, in order to comply with UK import requirements, investigators learned that Sikorsky had performed bird impact tests on the original equipment windscreen. The windscreen originally delivered on the S-76C++ was demonstrated to be compliant with British Civil Aviation Requirements (BCAR), which required the windscreen to resist penetration of a two-pound bird at certain flight conditions.

Bird Strike Requirements

On August 8, 1996, the FAA adopted 14 CFR 29.631, Bird Strike, as amendment 40 to 14 CFR Part 29, which required the helicopter to withstand the impact of a 2.2-pound bird at a specified flight speed.

Bird strike.

The rotorcraft must be designed to ensure capability of continued safe flight and landing (for Category A) or safe landing (for Category B) after impact with a 2.2-pound (1.0 kg) bird when the velocity of the rotorcraft (relative to the bird along the flight path of the rotorcraft) is equal to VNE or VH (whichever is the lesser) at altitudes up to 8,000 feet. Compliance must be shown by tests or by analysis based on tests carried out on sufficiently representative structures of similar design.

Monolithic Cast Acrylic Windshield

In November 1996, AAI applied for an STC to replace the OEM windshield on the S-76 with a monolithic cast acrylic windshield. The STC was issued in April 1997. Investigators noted that, based on the application date of the STC, compliance with 14 CFR 29.631 should have been included in the certification basis of the STC. It was concluded that compliance would have required that the cast acrylic windshield be capable of withstanding the impact of a 2.2-pound bird.

Investigators could not establish that compliance with this regulation had been demonstrated during the STC windshield certification process. The accident report stated, "the STC data package...revealed no documentation to indicate ...compliance with 14 CFR 29.631."

Illustration of Sikorsky S-76 certification timeline
Illustration of Sikorsky S-76 certification timeline

Conclusion

Birds continue to pose a significant threat to helicopter operations. In recent years, bird populations for many species have been increasing, as has the numbers of aircraft and aircraft in operations. This combination results in an ever-increasing bird strike threat. Bird strike data also indicate that helicopters are more frequently involved in bird strikes than other types of aircraft, and those strikes generally result in more damage to the aircraft and injury to crew or passengers. In order to reduce the hazard, later certification regulations have adopted bird impact standards for transport category helicopters.  In addition to continual adoption of newer (or improved) certification standards, multiple data studies related to bird strikes have concluded that operational mitigations, such as flight at higher altitudes, and flight crew use of protective equipment for head and eyes, can be employed to help reduce the threats or consequences of bird strike events./p>

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