NTSB interim factual report of PHI Sikorsky S-76C Helicoper crash raises many questions
The following is the interim factual report from the NTSB regarding the January 4, 2009, helicopter crash that killed 8 and seriously injured 1 person. The interim report suggests that a hawk bird strike to the windshield might have been the first of a series of events that brought down the sophisticated aircraft. The aircraft reportedly underwent a replacement of the original production laminated glass windscreens by PHI about two years prior to the accident as part of normal procedures for S-76 fleet maintenance. This helicopter’s windscreens were replaced a second time due to cracking approximately one year prior to the accident. The interim report suggests that the bird strike created the sound of a bang and a loud air noise followed by a substantial increase in the background noise level that was recorded on both intercom microphones and cockpit area microphone. Less than a second after the bang and loud air noise, the CVR captured the sound of decreasing rotor and engine rpm. Seventeen seconds later, the recording ended.
What remains to be learned is why the laminated glass windscreens could not withstand a bird strike. In addition, why was there no low rotor r.p.m. horn on this helicopter, like there are on the great majority of helicopters.
HISTORY OF FLIGHT
On January 4, 2009, at 1409 Central Standard Time (CST), a Sikorsky S-76C++ helicopter, N748P, registered to and operated by Petroleum Helicopters , Inc.(PHI), as a CFR 49 Part 135 air taxi flight using day visual flight rules (VFR), crashed into marshy terrain approximately 7 minutes after take-off and 12 miles southeast of the departure heliport. Both pilots and six of the seven passengers on board were killed. One person was critically injured. The helicopter departed Lake Palourde Base Heliport, a PHI base (7LS3), in Amelia, Louisiana, en route to the South Timbalier oil platform ST301B to transport workers from two different oil exploration companies. No flight plan was filed with the Federal Aviation Administration (FAA), nor was one required. A company flight plan was filed with the PHI communications center that provided weather updates and flight following for the helicopter crew.
According to representatives of PHI, the flight was being tracked via Outerlink, a satellite based fleet-tracking system used by the PHI communications center. The helicopter departed from 7LS3 at 1402. The Outerlink track suddenly ended about 7 minutes after departure at 1409. There were no reports of any problems from the flight crew on the PHI radio frequencies or emergency transmissions on any monitored air traffic control frequencies. A search and rescue operation was initiated at 1414 after the US Air Force notified PHI and the United States Coast Guard of a distress signal being transmitted with the unique identifier that is part of the Emergency Locator Transmitter (ELT) signal that transmits the Aircraft registration number and latitude and longitude coordinates. The helicopter wreckage was found shortly thereafter near the location of the loss of the track and transmitted ELT signal location by the U.S. Coast Guard. The wreckage was found partially submerged and exhibited very little main rotor blade damage.
The twin-engine, 14-seat, 2-year-old helicopter was equipped with glass cockpit instrumentation, a combination cockpit voice recorder (CVR) and flight data recorder (FDR), an enhanced ground proximity warning system (EGPWS), solid state quick access recorder (SSQAR), and a VXP vibration recorder. The two Turbomeca Ariel turbo shaft engines were equipped with digital engine control units (DECU). All of these devices were recovered and evaluated.
The weather conditions reported at Amelia, Louisiana at 1430 CST were; scattered cloud layers at 1,500 feet and 3,500 feet, a broken cloud layer at 10,000 feet, visibility 10 miles, winds at 160 degrees at 6 knots, temperature of 24 degrees Celsius, and dew point of 19 degrees Celsius.
WRECKAGE AND IMPACT INFORMATION
A detailed examination of the wreckage and components did not revealed any evidence of pre-impact engine, transmission, hydraulic servo, or systems failures. The main rotor transmission had no external damage and the rotor shafts were free to rotate. The transmission case was opened and all internal components appeared normal with no damage. Additionally, no evidence of a midair collision, or in- flight rotor blade failure was found. Fuel was found in the wreckage.
All three main rotor hydraulic servos and the tail rotor servo were found in good condition with no external leakage or damage. Functional tests and tear downs revealed no problems. Hydraulic reservoirs were full and no leakage was found.
FLIGHT RECORDER INFORMATION
Data from the Penny & Giles combination flight data recorder (FDR) and cockpit voice recorder (CVR) were analyzed at the NTSB’s Recorders Laboratory with download assistance from the manufacturer’s facility in England, and the US Army Safety Center in Fort Rucker, Alabama. Both recorders captured the accident flight.
The CVR captured the sound of a bang and a loud air noise followed by a substantial increase in the background noise level that was recorded on both intercom microphones and cockpit area microphone. Less than a second after the bang and loud air noise, the CVR captured the sound of decreasing rotor and engine rpm. Seventeen seconds later, the recording ended.
The non-volatile memory (NVM) from the engines’ digital Engine Electronic Control Units (EECUs) was successfully downloaded, and no faults were recorded.
TESTS AND RESEARCH
The engines were examined. No anomalies were noted that would have prevented normal operation.
A bird specialist with the U.S. Department of Agriculture (USDA) examined the helicopter for evidence of a bird strike. A visual examination did not detect any such evidence, but a swab was taken from the pilot-side windscreen. The sample was sent to the Smithsonian Institution Feather Identification Lab for identification. Results from DNA testing on that sample showed that microscopic remains of a hawk variety were present. The swab was taken from an area of the windscreen that exhibited concentric ring fractures. Similar concentric rings were visible in the gel coat of the fuselage area just above the windscreen.
Additional swabs for bird remains were taken from the fuselage; empennage; various inlets, including the engines; and from the rotor hub and main rotor blades. Examination revealed the presence of small parts of feathers under a right side windscreen seal and in the folds of the right side engine inlet filter.
The original production laminated glass windscreens from the accident helicopter had been removed and replaced by PHI about two years prior to the accident as part of normal procedures for S-76 fleet maintenance This helicopter’s windscreens were replaced a second time due to cracking approximately one year prior to the accident. PHI replaces all of the windscreens in their S-76 fleet with a lighter weight, cast acrylic windscreen that was approved by the Federal Aviation Administration (FAA) via a Supplemental Type Certificate (STC) upon delivery.