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Analysis

Summary

Electronic Engine Controller

Prior to the occurrence, the fuel metering unit fault codes were logged on the electrically erasable/programmable read-only memory of the electronic engine controller, possibly because of a loose connection at the harness or connector. It is likely that the codes were still logged on the EEPROM at the time of the event, resulting in the degraded condition of channel A.

When the in-flight shutdown occurred, the EEC was performing primarily on Channel B. When Channel B experienced the RAM parity errors and the subsequent repeated multiple resets, the EEC reverted to Channel A for primary control of the engine. As Channel A was degraded by the pre-existing FMU electrical fault codes present, neither channel was able to control the engine. The fuel-control metering valve, which is spring loaded into the closed position, then closed following signal loss, resulting in fuel starvation and engine shutdown.

In-flight engine restart

As the aircraft's indicated airspeed varied during the event from 235 knots to 270 knots, it is possible that the fluctuating airspeeds resulted in the low N2 values witnessed. The DFDR discrete signals did not include a discrete signal for the engine starter switch or the engine starter air valve, therefore their activation could not be confirmed. The faults present in the EEC would have prevented any attempt by the crew to restart the engine with the start switch, as the starter air valve would not have opened to allow bleed air for engine rotation. The engine manufacturer stated that a successful start could only have been achieved had the EEC received a "power" reset (circuit breakers pulled and reseated). Fuel switch resets would not have cleared the problem. Consequently, the flight crew's attempt or attempts at a restart could not have succeeded.

Flight Data Recorder

The Boeing 717 flight recorder installation operated so that when the aircraft taxied to a holding point and the park brake was set, the Flight Data Recorder stopped recording until the park brake was released. Essential information relating to the operation of the aircraft would not be recorded. The loss of recorded information may impede an air safety investigation and preclude an accurate determination.

 
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