The Bell Jetranger 206B (III) helicopter was engaged in aerial firefighting operations utilising an external water bucket and staging out of a nearby national park camp ground. The pilot reported that he started flying at approximately 0750 EST after completing a pre-flight check of the helicopter which included draining the fuel sump, inspecting the fuel and confirming 106 L or 27.9 United States Gallons (USG) of total indicated fuel. At approximately 0825, while engaged in water bucket operations, he discussed his fuel status with other company pilots on a common radio frequency and noted 38 L (10 USG) of indicated fuel remaining. He finished a swath run of the fire area, dropping water and then decided to complete one more swath run before returning to refuel.
Approaching the fire line, the helicopter entered a left turn at approximately 200 ft above ground level (AGL). The pilot reported that the helicopter was buffeted by strong turbulence, which caused the helicopter to yaw left and go out of trim. He reported that the engine power then began surging and, subsequently, an engine flameout occurred. He continued the left turn, jettisoned the water and initiated a power-off autorotation to a heavily wooded area.
During the autorotation, the helicopter's main rotor blades contacted nearby trees, damaging the blades and displacing the main rotor transmission. The helicopter then came to rest on its right side, on a 45 degree slope. The pilot exited the helicopter and was picked up by another company helicopter and transported to hospital for observation. The pilot suffered minor injuries and the helicopter was substantially damaged. There was no evidence of any in-flight or post-accident fire.
The pilot reported that he had no recollection of any illuminated caution advisory lights during or prior to the event, or of the position of the switches and circuit breakers of the fuel boost pumps. He stated that he followed flight manual checklists when starting the helicopter and preparing for takeoff.
The helicopter sustained damage to the main rotor blades and controls, main rotor transmission, transmission to engine driveshaft, tail rotor driveshaft, pilot's perspex and right side forward fuselage. Examination of the engine revealed no external damage or damage to the compressor.
Fuel system examination and background
After the helicopter was repositioned upright, the fuel gauge indicated approximately 19 L (5 USG). The fuel in the fuel tank was examined and appeared to slightly cover the base of the fuel boost pumps. Examination of the airframe fuel filter revealed full fuel in the filter bowl and no contamination. Examination of the engine fuel filter revealed a small amount of fuel and no contamination. The forward fuel boost pump circuit breaker was noted as disengaged.
Unusable fuel was defined as, `Fuel that cannot be used in flight with wings level and at cruise angle of attack (or nose 3 degrees up)'. Of the total fuel on board, 4 L (1 USG) were unusable. Upon removal from the accident site, the helicopter's fuel system was drained and 23.5 L (6.2 USG) of fuel were removed. The fuel correction card annotated, `gauge indicates 38 L (10 USG) for an actual of 45.6 L (12 USG)'. The fuel correction card values were verified by adding measured amounts of fuel.
An option on that model helicopter was a FUEL LOW caution advisory light that illuminated with approximately 76 L (20 USG) of total fuel remaining. The helicopter was not equipped with that advisory light. It was equipped with two electrically operated submerged fuel boost pumps located in the fuel cell and connected in parallel to the engine's fuel supply line. Those pumps were located on the helicopter's centre-line. Both boost pumps were examined following the accident and they were determined to be serviceable.
According to the operator's operations manual, the fuel consumption rate of the Bell 206B (III) helicopter was 110 L (28.9 USG) per hour. In the 35 minutes of operating time that the pilot reported prior to the accident, approximately 64 L (16.8 USG) of fuel should have been consumed, leaving approximately 30.8 L (8.1 USG) of useable fuel remaining. That amount should have been sufficient for approximately 17 minutes of engine operating time.
The helicopter was not fitted with an optional engine auto-reignition system. Because of the low height AGL at the time of the engine surging and flameout, the pilot did not have an opportunity to attempt a manual restart of the engine.
The Rolls-Royce Allison model 250-C20B engine, serial number CAE 840551, was removed from the helicopter and transported to an engine test cell for testing. Following motoring and priming, the engine started on the first attempt with Turbine Operating Temperatures, N1 (gas generator speed) and N2 (power turbine speed) values within normal operating parameters. Engine deceleration and acceleration tests were conducted in order to simulate power changes experienced during flight. The results of the testing indicated that the engine was serviceable at the time of the accident.
Helicopter manoeuvring in turns and flight manual requirements
During coordinated turns, centrifugal force acting on the helicopter and fuel tank causes the fuel to collect evenly in the bottom of the fuel tank. It is then available for pick up by the fuel boost pumps at the boost pump inlets and through to the inlet of the main fuel supply line and to the engine. During uncoordinated turns, centrifugal force may not displace the fuel to the bottom of the tank evenly and instead fuel sloshing may take place.
The helicopter's flight manual contained a warning regarding flight with one fuel boost pump inoperative. It stated, `Due to possible fuel sloshing in unusual attitudes or out of trim conditions and one or both fuel boost pumps inoperative, the unusable fuel is ten [US] gallons'.
The forward fuel boost pump circuit breaker was removed and tested. Aircraft system 28 DC voltage was applied and varying amperes were introduced to the circuit breaker in an attempt to determine its serviceability. The circuit breaker operated normally up to its rated 10 ampere rating with no anomalies noted.
Pilot's shoulder harness
The pilot's left shoulder harness had broken and separated at a point just forward of and below the pilot's shoulder. The manufacturer's date stamped on the harness belt was March 1973. The pilot's seat belt had an inspection tag attached with the inspection date 11/99 annotated. Details of the inspection were not annotated in the helicopter's documentation.
Pilot shoulder harness testing
The pilot's left shoulder harness was sent to an independent belt and harness testing and repair organisation for testing. The webbing was identified as MIL-T-50368 Type IV, 2 inch Nylon Webbing, rated at 2,000 pounds strength. The rated assembly strength of the harness assembly was 1,500 pounds. Testing revealed that the webbing failed at a value of 391 pounds, or less than 20 percent of the original strength of the material. Factors contributing to the loss of original strength were ageing related to ultraviolet light exposure, abrasion damage and contamination by turbine oil.
Seat belt and shoulder harness standards
Australian Civil Aviation Order (CAO) Part 108, Section 108.42, contained specifications for aircraft safety belts (seat belts), harnesses (shoulder harnesses) and inertia reels manufactured in Australia. Contained within CAO Part 108, Section 108.42 was a reference to Technical Standard Order (TSO)-C22g, which specified minimum performance standards of aircraft seat belts when new.
CAO Part 108, Section 108.42, also contained a reference to British Air Registration Board (ARB)/Civil Aviation Authority (CAA) Specification Number four (issue two). That document specified minimum performance standards for British manufactured aircraft shoulder harnesses when new and was not applicable to equipment manufactured in the United States of America (USA) such as the separated harness in this occurrence.
The Australian Civil Aviation Safety Authority (CASA) Airworthiness Directive, AD/RES/29 amendment 1, mandated identification of aircraft seat belts to indicate that the part was an item approved by the manufacturer. Part of that directive mandated appropriate identification to demonstrate compliance with CAO Part 108, Section 108.42.
The helicopter was manufactured in 1979 in the US under the requirements of the US Federal Aviation Administration (FAA), Federal Aviation Regulations. FAA Technical Standard Order (TSO)-C22g (amended 1993) outlined minimum performance standards for aircraft seat belts when new. TSO-C114 (initial issue 1987) outlined minimum performance standards for aircraft torso restraints (shoulder harnesses) when new. Prior to 1987, there were no requirements for shoulder harnesses. Only TSO-C22 was in effect when the occurrence helicopter was manufactured.
At the time of the occurrence, there were no requirements to confirm compliance to applicable design standards of shoulder or seat belt harnesses while in service, or to specifically identify shoulder harnesses.
Aerial firefighting operations are typically conducted in heavily forested, remote areas with water sources readily accessible to the helicopters for rapid refilling and turn-around times. Pilots experience high workloads and are susceptible to task saturation during firefighting external load flights and may fly multiple flights during a short period of time. Pilots may rely on the helicopter fuel gauge more so than during, for example, a cross-country flight where fuel consumption may be calculated using fuel burn versus time. Helicopter fuel gauges are subject to variations in roll and pitch attitudes, which make an accurate indication difficult.
The investigation found no evidence to suggest that the helicopter was not serviceable at the time of the accident. If the forward boost pump circuit breaker had been disengaged during the flight, the fuel pressure/load meter gauge should have indicated a low reading and the associated BOOST PUMP caution advisory light should have illuminated. The investigation could not conclusively determine how, or when, the forward boost pump circuit breaker became disengaged.
The pilot's shoulder harness failed because of deterioration of the material of the harness. The harness had remained in service even though the material had degraded, as there were no existing requirements to determine the serviceability of the harness following installation.
The investigation determined that the fuel supply to the helicopter's engine was interrupted, resulting in engine surging and subsequent flameout.
As a result of this occurrence, the Australian Transport Safety Bureau issues the following safety recommendation:
The Australian Transport Safety Bureau recommends that the Civil Aviation Safety Authority review the adequacy of the continuing airworthiness requirements for seat belts and shoulder harnesses to ensure that they maintain applicable design standards throughout their service life, when installed in Australian registered aircraft and helicopters.
|Date:||24 September 2001||Investigation status:||Completed|
|Time:||0825 hours EST|
|Location:||5 km W Kurrajong Heights|
|State:||New South Wales||Occurrence type:||Fuel starvation|
|Release date:||24 September 2002||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Accident|
|Highest injury level:||Minor|
|Aircraft manufacturer||Bell Helicopter Co|
|Type of operation||Aerial Work|
|Damage to aircraft||Substantial|
|Departure point||Bilpin Area, NSW|
|Departure time||0750 hours EST|
|Destination||Bilpin Area, NSW|
|Role||Class of licence||Hours on type||Hours total|