Aviation Investigation Report A98H0003
1.19.8 Aircraft Engine Analysis
- 126.96.36.199 - Industrial X-Ray for Assessing Internal Component Positions
- 188.8.131.52 - Thrust Setting Determination from Engine Fuel Metering Units
- 184.108.40.206 - Determination of Engine Thrust Level Via Stator Vane Actuators
- 220.127.116.11 - Determination of Engine Thrust Level Via Bleed Valves
- 18.104.22.168 - FADEC Fault Analysis
During the investigation of the aircraft engines, the need arose to determine the position of the spool valve within the body of the thrust reverser system hydraulic control unit (HCU). The first option was to disassemble the unit. However, during disassembly there is a possibility of altering the positioning of the spool valve and, in light of the corrosion that had developed from submersion in the sea water, disassembly would not have been easily accomplished. Therefore, the unit was transported to an industrial radiograph (X-ray) facility at Canadian Forces Base Shearwater where it was X-rayed. Analysis of the radiograph film easily identified the control valve position within the HCU body. Where disassembly was required, a radiograph was taken prior to disassembly to document the internal positioning of the components for reference purposes. This technique was also used to view the locking mechanism of the thrust reverser system locking actuators.
The external examination of the FMU determined that the resting position of the sector gears differed among the three units, suggesting different fuel flows to each of the three engines at the time of impact. Physical examination of the damage to the engines was also consistent with different power settings. The FMUs were transported to the component manufacturer's facility for disassembly and examination under the control of TSB investigators. The objective was to relate the position of the FMU sector gears to the position of the fuel metering valve and fuel flow. During the examination, the position of the fuel metering valve spool relative to the metering valve sleeve was measured and compared against the manufacturer's drawings, to determine fuel flow from these measurements. This information, along with information gathered from other areas of the engines, was helpful in determining the approximate thrust levels of the engines at the time of impact.
The variable stator vane (VSV) control subsystem provides maximum compressor performance by moving the HPC inlet guide vanes and 5th, 6th, and 7th stage HPC vanes to their programmed positions in response to commands from the FADEC. During an engine start, the VSVs may be in an open position until approximately 15 per cent N2, at which time they would close. At speeds above approximately 40 per cent N2, the VSVs modulate to open with increasing N1 and N2 and are fully open at take-off and climb power. The vanes modulate with N1, N2, and Tt2.
The three VSVs were transported to the manufacturer's facility for disassembly and examination under the control of TSB investigators. Measurements were taken from the centre of the piston face to the actuator aft housing surface. This measurement was used to determine the position of the piston relative to the piston full stroke. The results of this calculation were then interpreted to provide engine thrust level. This information, in concert with other factual information gathered from the FMUs and bleed valves, helped to establish the approximate thrust levels of the SR 111 engines at the time of impact.
The three 2.5 bleed valves, one from each engine, were disassembled and examined at the manufacturer's facility under the direction of TSB investigators. Measurements were taken from the mounting surface of the housing to the end of the piston to determine the "as-received" position of the piston. This measurement value indicates the position of the piston relative to the fully-extended position, and thus reflects the percentage of its full stroke. This percentage reflects the engine thrust level in engine revolutions per minute at the corrected low pressure rotor speed. These values, used in concert with other factual information, helped to establish the approximate thrust levels at the time of impact.
From visual examination of the six 2.9 bleed valves, the determination was made regarding whether the valves were open, closed, or jammed in a position as a result of impact. This information, in concert with other factual information, helped to establish thrust levels at the time of impact.
The FADEC is a source of stored information that is particularly useful for investigating accidents in which the FDR has stopped prematurely, as it did near the end of the SR 111 flight. The information may be downloaded from the NVM at the FADEC manufacturer's facility. If the time-reference that is captured on the NVM can be accurately related to actual time, then engine faults stored in the NVM can be helpful in determining the engine status during an accident sequence. If the FADEC is powered, and only airframe faults and no engine faults are captured in the NVM, then it can be surmised that there were no deficiencies associated with the engine. Airframe faults, particularly faults related to components that provide input data to the FADEC may help establish the engine mode of control at the time of the occurrence. The stored airframe faults may help to establish the serviceability of the airframe during the accident flight. Analysis of the FADEC stored faults determined the SR 111 mode of control of the engines and also provided some altitude and airspeed reference information during the last minutes of flight.
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