La Monte, MO, USA
N549AM
EUROCOPTER AS350-B2
The emergency medical services flight crew was departing from their home base helipad on the fourth flight of the day. The pilot reported that he began to lift straight up in a slow climb to about a 100- to 125-foot-high hover. As he completed a left pedal turn to depart into the wind, he confirmed that he was clear vertically of a tree that was directly east of the helipad. As he began a slow transition to forward flight, he heard a loud "bang." The pilot immediately reacted to the sound by slightly lowering the collective and turning to the right to get back to the helipad. He lowered the nose briefly and then began to flare the helicopter as he pulled up on the collective. The helicopter hit hard on the helipad, which spread the helicopter's landing skids so that the helicopter was lying on its belly, and the fuel tank ruptured during the impact. There was no postimpact fire. The pilot turned off the main electrical push button and began to yell "get out" to the flight crew. The pilot exited the helicopter unassisted and crawled about 15 to 20 feet away; medical crewmembers on board were assisted out of the helicopter by first responders. Postaccident examination revealed that damage to the helicopter was consistent with a high vertical velocity impact. Examination of the main rotor and tail rotor drive systems revealed evidence of low rpm on impact. Recorded nonvolatile data indicated that the engine experienced a flameout before ground impact. During the on-site examination, all fuel, oil, and air lines and connections were found to be tight. Additionally, the fuel flow lever was found in the flight detent, and the fuel shutoff lever was found lock-wired in the open position. There was no indication that the fuel filter annunciator light had illuminated during the accident flight, which would indicate that the fuel filter was clogged or that the filter bypass had opened. The on-site examination revealed that the fuel filter cartridge exhibited no anomalies, and the bypass open indicator was not extended. The bypass open indicator operated normally during subsequent tests. The engine was run on a test cell and operated within the original equipment manufacturer's specifications for all tests performed. Tests performed on the fuel control unit, starting drain valve, overspeed drain valve, pressurizing valve, bleed valve, start electrovalve, and the fuel injection manifold, as well as a three-dimensional check of the axial compressor, revealed no malfunctions or failures that would have precluded normal operation. The fuel was examined and no contamination was present. There was no evidence of ingestion of water, snow, or ice that would lead to an engine flameout. During the accident flight, the weather conditions required for ice buildup in the air intake filter were not present, and the temperatures in the intake plenum and air filter before engine start were not conducive to ice formation. Although examination of the helicopter's fuel circuit revealed that it operated correctly, during testing air was drawn into the fuel circuit between the fuel filter assembly and the fuel pumps when the fuel filter assembly drain valve was depressed. Numerous tests were conducted to determine if the air in the fuel lines and the fuel filter assembly could be purged during the engine start sequence and/or by normal engine operation. The tests revealed that some air would still remain trapped in the upper cavity of the fuel filter assembly (the volume of the cavity measured 33 milliliters). The additional tests, which simulated the fuel flow rates at flight idle, maximum takeoff power, and maximum engine generator speed, revealed that the volume of trapped air was about 1 to 2 cubic centimeters. An exemplar engine was subjected to 16 tests on a test cell to determine the effects on the engine if the trapped air was released through the fuel bypass valve located on top of the fuel filter assembly. The engine did not flame out during any of the tests, although one of the tests using a pre-clogged cartridge resulted in a fuel flow interruption lasting 5 milliseconds, a transient disturbance of the engine parameters, and a transient slower rate of power increase for about 5 seconds. The tests did not account for variables such as a "cold soaked" aircraft, cold fuel, air flow disturbances due to rotor wash, the effects of air filters certified under supplemental type certificates and the possible degraded condition of those air filters, no use of compressor outlet pressure bleed air, and no electrical load on the engine. Analysis of the faults that resulted in the uncommanded in-flight shutdown of the engine indicated that the most likely reason for the flameout was an interruption of fuel flow to the engine. Although the investigation found that air could be introduced into the fuel system through the fuel filter assembly, the mechanism that would allow air to disrupt the fuel flow to the engine and result in an engine flameout could not be determined.
HISTORY OF FLIGHT On December 19, 2010, about 1000 central standard time, a Eurocopter AS350-B2 helicopter, N549AM, sustained substantial damage when it landed hard after a loss of power during takeoff from a helipad at La Monte, Missouri. The pilot, flight nurse, and flight paramedic received serious injuries. The helicopter was operated by Air Methods Corporation under the provisions of 14 Code of Federal Regulations (CFR) Part 91 as a medical flight and was departing for an on-scene patient transfer at Fristo, Missouri. Visual meteorological conditions prevailed at the time of the accident. The flight was on a company visual flight rules flight plan. The pilot reported that he arrived about 0645 for his shift, which started at 0700. He performed the required preflight planning activities, which included obtaining a flight release, checking the aircraft maintenance report, reviewing the logbook, checking weather, conducting a crew brief, and performing a preflight inspection of the helicopter. The pilot reported that he conducted a thorough preflight of the helicopter, which included checking the air filter for ice and snow. He reported that there was no visible moisture on the helicopter during the preflight. Air Methods trip sheet documents indicated that the pilot was notified of an emergency medical flight (EMS) flight about 0711, and the helicopter departed about 0724. The helicopter arrived at the patient location in Lincoln, Missouri, at 0737, and departed with the patient on board about 0800. The helicopter arrived in Columbia, Missouri, about 0827. While in Columbia, the pilot fueled the helicopter with 60 gallons of Jet A fuel before the return flight to La Monte. The helicopter departed Columbia about 0906 and arrived at La Monte about 0934. After landing at La Monte, the flight crew went into the office to complete the required paperwork and flight debrief. The pilot reported that they did not encounter any rain or snow showers during the three flight legs, and it was not raining or snowing at La Monte when they landed. The Air Methods trip sheet documents indicated that the pilot was notified of a second EMS mission, the accident flight, about 0948. The pilot reported that he did a weather check and accepted the flight. The flight crew boarded the helicopter and the pilot completed the start and takeoff procedures without incident. The helicopter was pointed directly west as the pilot began to lift straight up in a slow climb to about 100 to 125 feet. He applied left pedal to point the aircraft to the southeast. As he completed the pedal turn, he confirmed that he was clear vertically of a tree that was directly east of the helipad. As he began a slow transition to forward flight he heard a loud "bang." The pilot immediately reacted to the sound by slightly lowering the collective and turning to the right to get back to the helipad. He lowered the nose briefly and then began to flare the helicopter as he pulled up on the collective. The helicopter hit hard on the helipad, which spread the helicopter's landing skids so that the helicopter was lying on its belly. The fuel tank ruptured during the impact. The pilot noticed a flame about 2 inches in height about 3 feet from the helicopter, but the flame went out quickly and there was no postimpact fire. The pilot turned off the main electrical push button and began to yell "get out" to the flight crew. He crawled about 15 to 20 feet away from the helicopter. One of the first responders told the pilot that there was still electrical power to the helicopter. The pilot explained to a fireman how to turn off the "DC Bat" button that was located next to the main electrical button, which the fireman then turned off. In her account of the accident, the flight nurse, who was sitting in the right rear seat, reported that the helicopter lifted off the pad normally to about 100 feet, then the engine went quiet. She did not hear any loud noise or bang. She heard the pilot yell, "Brace yourself" and she got into the crash position. She recalled that she was in extreme pain after the helicopter impacted the ground. She refused assistance in getting out of the helicopter until trained medical personnel arrived and placed her on a long board before exiting the helicopter. She reported that she immediately notified her husband of the accident on her cell phone then notified the EMS communication specialist that the accident had occurred. The time of the call to the communication specialist was about 1000. The flight paramedic, who was sitting in the left rear seat, reported that everything seemed normal until the engine stopped making noise. The helicopter was above tree top level when the engine stopped. She braced for impact. The impact caused all the doors to open, and fuel was on the ground. She had seen sparks from the skids hitting the concrete, but she did not see any flames. Although she was able to get her feet on the ground, she could not walk and needed assistance from a first responder to get out of the helicopter. PERSONNEL INFORMATION The pilot was a 53-year-old airline transport pilot with a helicopter rating and a helicopter instrument rating. He held a second-class medical certificate issued on March 3, 2010. He had about 7,928 total helicopter flight hours, which included about 296 hours in the AS350-B2 helicopters. In the 90 days preceding the accident, he had flown 34 hours, 9 hours of which were within the last 30 days. His most recent 14 CFR Part 135 airman competency/proficiency check was successfully accomplished on August 5, 2010. AIRCRAFT INFORMATION Manufactured in 2007, the helicopter was a Eurocopter AS350-B2, serial number 4339, with a three-blade main rotor, a conventional tail rotor, and skid type landing gear, and was powered by a single 625 maximum continuous shaft horsepower Turbomeca Arriel 1D1 free turbine engine. The helicopter was equipped with an Air Methods EMS Interior and configured for medical transport of a single patient on a litter and two medical flight crewmembers. The litter was located on the left side of the helicopter and extended from the left side of the cockpit into the left side of the cabin. The helicopter's flight manual listed the maximum gross weight as 4,960 pounds. At the time of the accident, the helicopter was fueled to a 60 percent fuel load and was operating within the weight and balance limitations. A review of the helicopter's maintenance records revealed that it had 1,288 total hours at the time of the accident. The most recent maintenance inspection, performed on December 17, 2010, was part of the Air Methods Approved Aircraft Inspection Program (AAIP), which included 50-hour, 100-hour, and 600-hour airframe and engine inspections. The inspection tasks cards did not contain any actions that required inspecting the main airframe fuel filter or the Le Bozec fuel filter assembly. The Air Methods task cards referred directly to Eurocopter's task cards with no additional information. The mechanics who performed the inspections reported that they did not depress the plunger on the filter bowl of the fuel filter assembly during the inspections. Maintenance records indicated that the fuel filter cartridge was replaced during the 1,000-hour airframe inspection at a total airframe time of 1,008 hours, about 280 hours before the accident flight. The Air Methods mechanic who was assigned to the La Monte base reported that he started his employment with Air Methods on December 1, 2010. He received on-the-job training from December 1 to December 8. He performed the aircraft daily maintenance checks on the accident helicopter on December 9, 10, 13, and 14 by himself. On December 15, he performed the daily maintenance checks on the accident helicopter but with assistance from another mechanic. He reported that he had never been instructed to depress the plunger of the fuel filter assembly nor had he been instructed not to depress the plunger, since there was no guidance at the time that it was not to be depressed. He could not specifically recall if he depressed the plunger on the accident helicopter, but, if he had, it would have been on December 15 during the aircraft daily maintenance check. He stated that he would have turned on the fuel pumps while depressing the plunger. Vehicle Engine Multifunction Display (VEMD) The accident helicopter was equipped with a vehicle engine multifunction display (VEMD), which is a dual screen display that provides vehicle and engine information to the pilot during flight (known as flight mode). The VEMD also has a maintenance mode, which stores nonvolatile data for specified vehicle and engine exceedances and/or overlimits and VEMD-system-related hardware or communication failures. The VEMD also records and stores flight reports, which can be found in the maintenance pages. A flight report begins when aircraft electrical power is applied and engine generator speed (NG) reaches 60 percent. The flight report ends when the NG decreases below 50 percent with aircraft electrical power still applied. During a normal landing and shutdown, the VEMD will display the flight report for the most recent flight when NG drops below 10 percent and main rotor rpm (NR) decreases below 70 percent. Fuel Filter Bypass Operation The AS350-B2 is equipped with a Le Bozec fuel filter which contains a stainless steel filter element. The fuel filter is located forward of the firewall in the main gearbox compartment. The filter unit includes a bypass valve, a visual indicator of bypass, and a drain. An annunciator light labeled F. FILT illuminates on the Warning – Caution – Advisory Panel to indicate a clogged filter and an impending bypass. Contaminated fuel causes impurities to build on the filter cartridge. This gradually reduces the flow through the filter. Pressure then increases at the filter inlet and decreases at the filter outlet. When the pressure differential reaches 206 mb, a pressure switch illuminates the amber F. FILT annunciator to alert the pilot the bypass is likely to open. The bypass valve opens when the pressure reaches 350 mb +/- 50 mb. The engine still receives fuel, but it is contaminated fuel and unfiltered. The annunciator light remains illuminated as unfiltered fuel flows to the engine. On engine shutdown, the bypass open indicator (red pop-up button on the filter housing) is visible. The emergency procedure when the F. FILT illuminates is: Reduce engine power. - If light goes out, continue flight at reduced power. - If light remains on, land as soon as possible. There was no indication that the F FILT annunciator light had illuminated during the accident flight, and the pilot reported that he did not see any light illuminated before the engine flameout. FDC Aerofilter The helicopter was equipped with an FDC Aerofilter (Supplemental Type Certificate [STC] SR00811SE). Located on top of the helicopter aft of the main rotor mast and transmission, the after-market unit filters the outside air before it enters the engine's intake plenum. During STC certification testing, it was demonstrated that 70 percent of the surface area of the filter needed to be blocked before engine limitations were reached—about double the blockage necessary to activate the differential pressure switch and the low pressure annunciator light in the cockpit. The system has a pilot-activated engine alternate air switch that opens the alternate air doors if the low pressure annunciator light illuminates. There was no indication that the light had illuminated during the accident flight, and the pilot reported that he did not see any light illuminated before the engine flameout. The FDC low pressure annunciator light was inspected on March 31, 2010, during a 12-month calendar inspection. The next inspection was due in March 2011. The Limitation section of the Rotorcraft Flight Manual Supplement for the AS350-B2 helicopter equipped with the FDC Aerofilter provided the following Takeoff limitation: "Takeoff with LOW INLET PRESSURE annunciator light illuminated…..PROHIBITED." Engine Flameouts The engine's ignition system is only in operation during the starting sequence. Once started, combustion is continuous and self-sustaining as long as the engine is supplied with the proper fuel-to-air ratio. If the rich limit of the fuel-to-air ratio is exceeded in the combustion chamber, the flame will extinguish. This condition is referred to as a rich flameout. It generally results from very fast engine acceleration, where an overly rich mixture causes the fuel temperature to drop below the combustion temperature. It also may be caused by insufficient airflow to support combustion, which may occur as a result of a blocked engine inlet or inlet barrier. An interruption of the fuel supply can also cause an engine to flameout. Such an interruption may be due to prolonged unusual attitudes, a malfunctioning fuel control system, blocked fuel supply, air introduction into the fuel delivery system, turbulence, icing, or fuel exhaustion. The Arriel 1D1 engine is not equipped with an auto-ignition system, nor is such a system required by regulation. If a flameout occurs, the engine will not automatically restart. The helicopter's aluminum intake plenum is not equipped with a de-icing or anti-icing capability. Eurocopter identified the following general causes for a sudden loss of engine power during an un-commanded in-flight shutdown: 1. Loss of required conditions to ensure engine running: loss of fuel. 2. Loss of required conditions to ensure engine running: air supply. 3. Untimely helicopter stop or idle order due to a system malfunction and/or pilot action. 4. Engine power loss due to Arriel 1 engine failure. 5. Engine attachment degradation. Air Introduction The fuel boost pumps are equipped with check valves that incorporate bleed ports to allow pressure in the fuel lines to be bled off after engine shutdown. If the fuel system is opened, such as during filter servicing, air can be drawn into the system as gravity returns fuel to the tank through the check valve bleed port. Air can also enter the system if the Le Bozec fuel filter assembly is drained, or if there is an external leak which becomes an "air entry" with the boost pumps off (the examination of the accident helicopter revealed no external air leak). The AS350 B2 Rotorcraft Flight Manual and the maintenance manual make no reference to a daily draining procedure of the Le Bozec fuel filter assembly. METEOROLOGICAL INFORMATION About 0953, the surface weather observation at Sedalia Regional Airport, Sedalia, Missouri, located about 10 nautical miles east of the accident site, was wind from 170 degrees at 5 knots; 7 miles visibility; overcast skies at 3,200 feet; temperature 3 degrees Celsius (C); dew point -3 Celsius; and altimeter 30.08 inches of mercury. The pilot reported that snow showers were in the forecast, but the actual weather was better than the forecast. During the preflight inspection, no moisture was visible on the helicopter, and no snow or ice was present on the FDC Aerofilter. He reported that there were no snow showers or other weather issues during the flights to and from Columbia, Missouri. He reported that there were no rain or snow showers at La Monte before the accident flight. HELIPAD INFORMATION The Air Methods helipad is located at 200 West Front Street, La Monte, Missouri. Orange balls mark the wires near the 30 feet by 30 feet concrete helipad, but there are no red lights to mark the light poles, wires, and trees near the touchdown and lift-off (TLOF) area. There are no lights bordering the TLOF. A 3,000 gallon above ground fuel tank is located about 35 feet from the TLOF. A windsock is present, but it is not lighted. There is no approach and takeoff path to the helipad, which results in the helicopter needing to enter a high out-of-ground-effect hover for takeoffs and landings. There is no safety area extending beyond the final approach and takeoff area. FLIGHT RECORDERS The helicopter was not equipped with a cockpit voice recorder or flight data recorder. WRECKAGE AND IMPACT INFORMATION The accide
An interruption of fuel to the engine due to air in the fuel lines, which resulted in an engine flameout and the total loss of engine power. The reason for air in the fuel line resulting in an engine flameout could not be determined because postaccident testing did not reveal the mechanism that would lead to such a result.
Source: NTSB Aviation Accident Database
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