FAA STC On Replacement Windshield Did Not Require Bird-Strike
Testing
A helicopter accident in Louisiana in 2009 which fatally
injured eight people was caused by a red-tailed hawk striking the
windshield of a Sikorsky S-76C++ helicopter. The NTSB probable
cause report indicates that the impact caused both throttles to be
pushed to flight idle, and the crew did not have sufficient time to
react and recover before impacting terrain.
NTSB Identification: CEN09MA117
Nonscheduled 14 CFR Part 135: Air Taxi & Commuter
Accident occurred Sunday, January 04, 2009 in Morgan City, LA
Probable Cause Approval Date: 11/24/2010
Aircraft: SIKORSKY S-76C, registration: N748P
Injuries: 8 Fatal, 1 Serious.
A Sikorsky S-76C++ departed on an air taxi flight from PHI,
Inc.’s heliport en route to an offshore oil platform with two
pilots and seven passengers. Data from the helicopter’s
flight data recorder indicated that the helicopter established
level cruise flight at 850 feet mean sea level and 135 knots
indicated air speed. About 7 minutes after departure, the cockpit
voice recorder recorded a loud bang, followed by sounds consistent
with rushing wind and a power reduction on both engines and a decay
of main rotor revolutions per minute. Due to the sudden power loss,
the helicopter departed controlled flight and descended rapidly
into marshy terrain.
Examination of the wreckage revealed that both the left and
right sections of the cast acrylic windshield were shattered.
Feathers and other bird remains were collected from the canopy and
windshield at the initial point of impact and from other locations
on the exterior of the helicopter. Laboratory analysis identified
the remains as coming from a female red-tailed hawk; the females of
that species have an average weight of 2.4 pounds. No defects in
the materials, manufacturing, or construction were observed. There
was no indication of any preexisting damage that caused the
windshield to shatter. Thus, the fractures at the top of the right
section of the windshield and damage to the canopy in that area
were consistent with a bird impacting the canopy just above the top
edge of the windshield. The fractures in the other areas of the
windshields were caused by ground impact.
The S-76C++ helicopter has an overhead engine control quadrant
that houses, among other components, two engine fire extinguisher
T-handles and two engine power control levers (ECL). The fire
extinguisher T-handles, which are located about 4 inches aft of the
captain’s and first officer’s windshields, are normally
in the full-forward position during flight, and each is held in
place by a spring-loaded pin that rests in a detent; aft pulling
force is required to move the T-handles out of their detents. If
the T handles are moved aft, a mechanical cam on each T-handle
pushes the trigger on the associated ECL out of its wedge-shaped
stop, allowing the ECL to move aft, reducing fuel to the engine
that the ECL controls. (Flight crews are trained to move an
engine’s fire extinguisher T-handle full aft in the event of
an in-flight fire so that the ECL can move aft and shut off the
fuel flow to the affected engine.)
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 ECL triggers out of their stops and allowing them to
move aft and into or near the flight-idle position, reducing fuel
to both engines. A similar incident occurred on November 13, 1999,
in West Palm Beach, Florida, when a bird struck the windshield of
an S-76C+ helicopter, N276TH, operated by Palm Beach County. The
bird did not penetrate the laminated glass windshield, but the
impact force of the bird cracked the windshield and dislodged the
fire extinguisher T-handles out of their detents; however, in that
case, the force was not great enough to move the ECLs.
Maintenance records indicated that PHI replaced the original
laminated glass windshields delivered on the accident helicopter
with after-market cast acrylic windshields about 2 years before the
accident. The after-market windshields provided a weight savings
over the original windshields. PHI again replaced the windshields
(due to cracking) with cast acrylic windshields about 1 year before
the accident. Aeronautical Accessories Incorporated (AAI) designed
and produced the after-market windshields and obtained supplemental
type certificate (STC) approval from the Federal Aviation
Administration (FAA) in April 1997. AAI did not perform any
bird-impact testing on the cast acrylic windshields supplied for
the S-76C++, and the FAA’s approval of the STC did not
require such testing.
Sikorsky S-76C++ File Photo
PHI also replaced the original windshields on other helicopters
with the cast acrylic windshields; one of these helicopters
experienced a bird-strike incident about 2 years before the
accident. Postincident examination revealed a near-circular hole
with radiating cracks near the top center of the right windshield.
The bird penetrated the windshield and pushed the right-side
T-handle. The trapped remains of the bird prevented the right-side
throttle from being reengaged, but the pilot was able to land the
helicopter safely.
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 National Transportation Safety Board determines the probable
cause(s) of this accident as follows:
(1) the sudden loss of power to both engines that resulted from
impact with a bird (red-tailed hawk), which fractured the
windshield and interfered with engine fuel controls, and (2) the
subsequent disorientation of the flight crewmembers, which left
them unable to recover from the loss of power. Contributing to the
accident were (1) the lack of Federal Aviation Administration
regulations and guidance, at the time the helicopter was
certificated, requiring helicopter windshields to be resistant to
bird strikes; (2) the lack of protections that would prevent the T
handles from inadvertently dislodging out of their detents; and (3)
the lack of a master warning light and audible system to alert the
flight crew of a low-rotor-speed condition.