Casa Grande, AZ, USA
N973AE
Eurocopter AS 350 B3
The engine lost power during an attempted takeoff, and the helicopter landed hard. Just prior to the accident flight, the adjusted valve assembly on the engine was removed and replaced. Following this maintenance, the pilot flew the helicopter to a hospital where a flight paramedic and nurse boarded the helicopter. Upon liftoff, the engine lost power and the helicopter landed hard. Post accident test run and examination of the engine showed that uncommanded shutdowns of the engine would occur at higher power settings. Disassembly of the adjusted valve assembly revealed that the stop electro valve was damaged internally. The damage consisted of a deformation of the core housing which would not allow the stop electro valve to completely open during operation. In this condition, a pressure drop would occur, which would lead to an untimely activation of the stop purge valve and a fuel flow drop. The adjusted valve assembly had been overhauled by the engine manufacturer and then sold to the operator, prior to its installation on the engine. The engine manufacturer indicated that this damage occurred between the time the component was shipped from the factory and its first use, however, they could not ascertain how the damage occurred. The damage was not apparent until the unit was disassembled. Metallurgical examination showed that the damage occurred when the valve was in a disassembled condition.
HISTORY OF FLIGHT On August 27, 2006, at 0021 mountain standard time, a Eurocopter AS 350 B3, N973AE, landed hard following a loss of power during an attempted takeoff from a helipad at the Casa Grande Medical Center, Casa Grande, Arizona. The commercial pilot, a flight nurse, and a flight paramedic were not injured; the helicopter sustained substantial damage. PHI, Incorporated, owned and operated the helicopter under the provisions of 14 CFR Part 91 as a positioning flight. The flight was originating at the time, and was en route to a highway vehicle accident to pickup a medical patient. Night visual meteorological conditions prevailed and a company flight plan was in effect. In a written statement, the pilot reported that the run-up and liftoff to a hover were normal. He taxied the helicopter to the center of the helipad. As the pilot applied collective pitch at approximately 10 feet, the engine abruptly lost power and the helicopter yawed to the right. The helicopter settled to 5 feet and he applied additional collective pitch. The helicopter then landed hard and bounced once while continuing to yaw right about 20 degrees. Upon landing, the pilot heard a loud mechanical knocking noise and immediately placed the engine control switch in the off position and applied the rotor brake. After verifying that the flight nurse and paramedic were uninjured, he notified the communications center about the power loss. Eyewitnesses and the helicopter passengers reported hearing a bang noise prior to the power loss. During the hard landing, the tailboom bent downward at its attach point to the fuselage. AIRCRAFT INFORMATION The helicopter was powered by a Turbomeca Arriel 2B engine (serial number 22089). The last 100-hour inspection was completed on August 15, 2006. From August 23 to 26, the helicopter underwent various maintenance actions. On August 24, 2006, maintenance personnel performed a purge valve test on the engine, in accordance with the 500-hour engine inspection as specified by the engine manufacturer. The adjusted valve assembly (serial number 201B) did not pass the test, so it was removed and replaced with another adjusted valve assembly (serial number 778M). The adjusted valve assembly serial number 778M was manufactured in 2003 and operated 3,196 hours prior to overhaul. The assembly was sent to Turbomeca France for overhaul, and then shipped in May 2006 to Turbomeca's facilities in the United States where it was purchased by the operator. According to Turbomeca, during the overhaul process the stop electro valve portion of the adjusted valve assembly was not disassembled. This stop electro valve was installed on the adjusted valve when it was manufactured in 2003. Field disassembly of the adjusted valve assembly is not an authorized procedure. According to Turbomeca, the stop and start electro valve units can each be removed and replaced as a unit from or on the adjusted valve assembly. The helicopter was returned to service on August 26, 2006. The pilot flew the helicopter from the maintenance facility to the helipad at the Casa Grande hospital. When the accident occurred, approximately 0.5 hours had accrued on the helicopter since the adjusted valve assembly was removed and replaced. TESTS AND RESEARCH Turbomeca USA Examinations and Testing The engine was examined at Turbomeca USA in Grand Prairie, Texas. Representatives from the NTSB, FAA, PHI, Inc., Turbomeca, and Eurocopter were in attendance. An arrival inspection was performed. The engine was borescoped and slight rub marks were noted on the high-pressure turbine blade housing and blade tips. All of the fuel, oil, and air fittings were tight and secure. The oil strainers, oil filter, fuel filter, and magnetic plugs were free from debris. Continuity of the drive train and correct operation of the freewheel assembly were confirmed. The total time on the engine was 3,720.58 hours. Initial test runs on the engine resulted in uncommanded engine shutdowns at higher power settings. The adjusted valve assembly (part number 0292958060, serial number 778M) was removed and replaced with a new adjusted valve assembly (part number 0292958050, serial number 734M). With the new adjusted valve installed, the engine checked within tolerances. No anomalies were noted. The adjusted valve is made up of the start electro valve, the stop electro valve, the pressurizing valve, and the stop purge valve. The pressurizing valve is set at 300 kPA (43.5 PSI) and the stop purge valve is pressurized at 1,300 kPA (188.5 PSI). The adjusted valve provides start fuel to the fuel nozzles until approximately 45 percent Ng (gas generator turbine speed defined as percent). When fuel pressure reaches 300 kPa, the pressurization valve opens and fuel is supplied to the injection wheel. Shut down is commanded through the digital engine control unit (DECU) through the stop electro valve and excess fuel is purged through the stop purge valve. Turbomeca France Examinations and Testing The DECU and adjusted valve assembly were sent to the Bureau d'Enquêtes et d'Analyses pour la Sécurité de l'Aviation civile (BEA) for further examination at Turbomeca facilities in France. During the examination of the adjusted valve assembly, it was noted that the stop electro valve was not opening completely. Disassembly of the stop electro valve showed a deformation of the core housing which restricted the movement. In this condition, a pressure drop would occur, which would lead to an untimely activation of the stop purge valve and a fuel flow drop. According to Turbomeca, this damage to the core most likely occurred after shipment from the factory. The damage to the stop electro valve was not apparent until the unit was disassembled. At the time of this report, the manufacturer stated that there had been no similar events reported and that the damage was due to the iron core striking the housing. Reports from Turbomeca representatives indicated that the damage to the valve occurred at an unknown time, but following the adjusted valve's shipment to Turbomeca USA. The NTSB investigator requested photographs of the damage identified during the examination but none were supplied. No related discrepancies were noted in the examination of the DECU. The Turbomeca representative indicated that the valve was damaged after it left the overhaul facility in France, and prior to the accident, and that Turbomeca did not know the origin of the damage. The representative further indicated that had the damage been present during the overhaul acceptance tests, the operational discrepancy would have been identified. Upon departure from Phoenix, the helicopter weight was 4,207 pounds. Upon departure from Casa Grande, the helicopter weight was 4,692 pounds. Performance calculations from Turbomeca showed that the takeoff from Phoenix required less than 92.5 percent NG, which did not reach the level where the adjusted valve assembly discrepancy would have resulted in an uncommanded engine shut down. The takeoff from Casa Grande required 92.5 percent NG or higher, which would have reached the level where the adjusted valve assembly discrepancy resulted in an uncommanded engine shutdown. NTSB Materials Laboratory Examination The NTSB Materials Laboratory examined the adjusted valve assembly (to include the stop electro valve) and reviewed the TMF examination report. The disassembled stop electro valve components were optically examined at low magnifications and a small dent was uncovered at the edge of the core housing. With an assembled stop electro valve, the location of the dent was internal and not exposed. Higher magnification inspections revealed features in the dent consistent with two adjacent impacts by the sharp corner of a machined object. None of the valve components had similar corner or machining mark configurations. The impact deformed the edge inward making the housing locally out of round. The edge deformation prevented exit of the core from the housing until the flat side of the core was aligned with the dent. Once installed, the deformation limited movement of the core in the housing. ADDITIONAL INFORMATION All retained components were released to PHI, Inc.
Internal damage to the stop electro valve which resulted in fuel starvation and subsequent loss of engine power during takeoff.
Source: NTSB Aviation Accident Database
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