Aviation Investigation Report A98H0003

1.12.9  Examination of the Engines

  1. 1.12.9.1 - General
  2. 1.12.9.2 - Engine 1
  3. 1.12.9.3 - Engine 2
  4. 1.12.9.4 - Engine 3

1.12.9.1  General

The three engines were examined to determine their operational status just prior to impact. Each engine is capable of producing a maximum thrust of 62 000 lb at sea level. No engine mechanical failures were discovered that could have prevented any of the three engines from operating normally.

The electronic circuit boards from the Engine 1 FADEC were recovered; however, the electrically erasable programmable read-only memory (EEPROM) chip containing the NVM had been stripped from the circuit board. As a result, no recorded FADEC information for Engine 1 was retrieved.

The FADECs from engines 2 and 3 were recovered, and information was extracted from the NVM in each of the units. These FADECs recorded 10 identical faults for the occurrence flight. Engine 2 FADEC recorded an additional 10 faults, 7 of which were duplicates of the previous 10 faults (see Section 1.12.9.3). All of the faults recorded on the FADEC NVM occurred after the aircraft had descended to approximately 10 000 feet. All of these faults were related to the loss of inputs from both of the aircraft's FCCs and ADCs, and the loss of 115 V AC and 28 V DC electrical power to the FADECs.

The disruption of 115 V AC electrical power resulted in the loss of inlet probe heat to the associated engine. Inlet probe heat is required to allow the FADEC to control engine speed in the primary EPR mode; therefore, with the loss of inlet probe heat, the engine control mode would have reverted from the primary EPR mode to the soft reversionary N1 mode. The FADEC then establishes a down-trimmed N1 schedule to maintain the same thrust level as prior to the reversion. A SELECT switch is provided on the FADEC control panel, which is part of the overhead panel, to allow pilots to remove the downtrim from the N1 schedule; the engines then operate in the hard reversionary N1 mode. The engine will respond to throttle inputs in either reversionary N1 mode.

Faults are recorded within the FADEC in 20-minute accumulated increments and are not synchronized with aircraft FDR time. This makes it difficult to correlate any fault written to the FADEC memory with aircraft UTC time. Therefore, it was not possible to precisely correlate the FADEC faults with FDR recorded information.

1.12.9.2  Engine 1 (STI1-74)

Impact damage to Engine 1 was consistent with an engine operating at a high rotational speed and producing high power at the time of impact. Impact marks were left on the throttle shaft and throttle quadrant when the throttle fractured free of the pivot shaft. When the impact marks were aligned, the position of the throttle lever was determined to be consistent with a thrust level of approximately 45 960 lb in the EPR mode of control, or 54 195 lb in the hard reversionary N1 mode of engine control. It is unknown whether the pilots had selected the hard reversionary N1 schedule mode of engine control.

The metering valve in the fuel metering unit (FMU) was at or near the maximum fuel-flow position, equating to a fuel flow of between 26 900 and 28 400 lb per hour. This is consistent with a high-power setting.

1.12.9.3  Engine 2 (STI1-75)

The impact damage to Engine 2 was consistent with an engine that was rotating at a windmill speed and not producing power at the time of impact. Impact marks were left on the throttle shaft and throttle quadrant when the throttle fractured free of its pivot shaft. There were two impact marks on the throttle quadrant. When the impact mark on the throttle shaft was aligned with one of the marks on the throttle quadrant, the throttle was in a position similar to that of Engine 1. When the second mark on the quadrant was aligned, the throttle was in approximately the idle position. The damage associated with this second mark on the throttle quadrant was assessed to be the result of post-crash movement. Therefore, it is concluded that the throttle lever was in the forward position at the time of impact, even though the engine was not producing power.

Of the 10 additional fault entries recorded on the Engine 2 FADEC, 3 were related to the loss of TRA inputs to the FADEC. The TRA wiring for Engine 2 is routed from the centre pedestal, above the cockpit ceiling, and then aft through an area of known fire damage in the forward cabin drop-ceiling. The loss of TRA inputs would cause Engine 2 to revert to a fixed thrust mode, and to maintain power at the last validated throttle angle. As the FDR had stopped recording, the EPR values were not available. The last recorded thrust settings obtained from the FADEC before the engine was shut down were approximately 72 per cent N2, or approach idle.

FADEC information indicates that Engine 2 was shut down at an altitude of about 1 800 feet[60]  and airspeed of 227 knots TAS. The rewriting of existing faults in the FADEC NVM can normally only occur if the engine is shut down by the selection of the FUEL switch. In the case of SR 111, the possibility of the electrical circuitry to the switch being compromised by fire damage was also considered. The wiring in the aircraft is such that selection of the FUEL switch causes two electrical circuits to become grounded, and one electrical circuit to become powered. As two of the three associated wires were in wire runs completely outside of the fire-damaged area, it is considered improbable that fire damage to the wires could have occurred coincidentally to produce this precise electrical configuration. Therefore, it was determined that the pilots purposely shut the engine down by activating the FUEL switch.

1.12.9.4  Engine 3 (STI1-76)

Impact damage to Engine 3 was consistent with an engine that was producing power above flight idle, but not at full power. Because the engine had lost the pitot heat input signal to the FADEC, the engine would have reverted to the down-trimmed N1 mode. Impact marks were left on the throttle shaft and throttle quadrant when the throttle fractured free of its pivot shaft. When the impact marks were aligned, the position of the throttle lever was determined to be consistent with a thrust level of approximately 40 315 lb in the EPR mode. Because the engine had lost the pitot heat input signal to the FADEC, the engine would have reverted to the soft reversionary N1 mode. Therefore, based on only the throttle position indications, the engine thrust levels would have appeared to be at a relatively high power setting.

However, the FMU provides different information. The metering valve in the FMU was determined to be at an intermediate position equating to a fuel flow of between 3 180 and 3 420 lb per hour. This fuel flow is consistent with a power setting just above flight idle. The information derived from examining the FMU provided a more accurate representation of engine status than was provided by the throttle position impact marks. Also, the physical damage to the engine is consistent with a low-to-medium power setting as indicated by the FMU setting; the analysis of the associated engine components further indicates that the engine was operating at or slightly above flight idle at the time of impact.


[60]    According to the engine manufacturer, the accuracy tolerance of the altitude readings provided by the full-authority digital electronic control (FADEC) is ± 470 feet.

Previous | Next

Date modified :
2012-07-27