The Saab SF340B aircraft was being operated on a scheduled passenger flight from Dubbo, NSW to Sydney. The crew reported that, during the descent into Sydney and while passing flight level 110, they selected the auto-coarsen system ON. After passing through 7,000ft AMSL, the aircraft suddenly yawed to the left and the left propeller feathered. The crew reported that they immediately noticed illumination of master warning and caution annunciations consistent with a left engine failure. A scan of the engine instruments confirmed that the left engine appeared to have shut down, with the engine torque and fuel flow indications at zero. At the time of the apparent engine shutdown the aircraft was under autopilot control. The crew reported that they were not operating any engine controls and that the power levers were positioned below the 64 degree position. The crew reported that the aircraft's auto-ignition system, which was armed when the ignition system was selected to NORM at engine start, did not operate throughout the incident.
The left engine was secured, and the crew informed the flight attendant and passengers of the situation. The crew declared a PAN to air traffic control, reporting an engine shutdown, and a local standby was declared. The crew elected not to attempt an engine restart due to the aircraft's proximity to the airport, and an uneventful one-engine inoperative landing was conducted.
Immediately following the incident, the operator's maintenance personnel carried out operational checks of the left engine. During those checks the engine and its auto-ignition system were found to operate normally.
An investigation into the incident, carried out by the operator, resulted in the removal of the left engine's hydromechanical unit (HMU) and the digital electronic control unit (DECU) for examination. The aircraft's fuel system was also extensively inspected with no defects or anomalies detected. The aircraft was then returned to service and the problem had not recurred in the 15 months following the incident.
The operator forwarded the removed HMU and the DECU to the component repair vendor. That examination found no problems with either component that would have contributed to the incident and both components were returned to the operator as serviceable items. The DECU was subsequently fitted to another aircraft in the operator's fleet. A short period after fitment, the unit was again removed following engine `torque fluctuation' problems on that aircraft and the component was again returned to the component repair vendor for examination. During that examination, a fault with `stressed' solder joints on a central processing unit board within the DECU was discovered and corrected. Several internal microcircuits were also replaced.
The aircraft was equipped with an engine ignition system that, when selected to NORM, automatically provided `flameout protection', or auto-ignition. The DECU sensed the rate of engine deceleration, comparing it to an internally programmed `rate of change' or `flameout schedule', and at a predetermined figure energised the ignition system for 7.5 seconds. This was indicated by illumination of a white ignition light on the flight status panel. The ignition was shut off when the gas generator speed decreased below 62% to prevent a `sub-idle relight'. The pilot in command reported that on the previous sector, the auto-ignition light had illuminated for a period of about 4 seconds during the descent, for no apparent reason.
The auto-coarsen system was designed to automatically feather a propeller in order to achieve a rapid reduction in propeller windmilling drag in event of an engine failure during takeoff, approach and go-around. The auto-coarsen system operated in a high or low mode depending on whether the power levers were above 64 degrees (high) or below 64 degrees (low). The active mode was indicated by the illumination of an annunciator positioned on the flight status panel. There were no markings on the power lever control quadrant to indicate the 64 degree position to the crew. The auto-coarsen system monitored a number of parameters including power lever angle and when in high mode, engine torque. The engine torque signal was provided by the DECU. During an auto-coarsen event, where an engine failure is not the reason for the event, the engine can continue to operate at low power with corresponding fuel flow, inter-turbine temperature (ITT), oil pressure and temperature indications.
The engine manufacturer reported to the ATSB that they had received several field reports that identified where an internal failure within the DECU had resulted in an intermittent loss of the torque signal output from an engine. The engine manufacturer further reported that, when allied with conditions of power lever position and engine parameters that are pre-programmed into the auto-coarsen computer, an intermittent loss of torque signal could result in the auto-coarsen computer mistakenly detecting an engine failure, triggering an inadvertent propeller auto-coarsen event.
The engine manufacturer reported that they had recently become aware that their standard acceptance test procedures (ATP), performed on DECU's following maintenance, had not always been successful in isolating intermittent loss of torque signal faults. They advised that they had improved the ATP procedures for DECU's that had been identified as `loss of torque signal' units. The occurrence DECU had not undergone the improved ATP inspection when it had been returned for examination following the occurrence.
The standard operating procedures to be followed by flight crews were detailed in the operator's Aircraft Operations Manual. These procedures contained the flight checklists to be followed in normal, abnormal and emergency situations. The aircraft manufacturer issued Revision 32 of the Saab 340B Aircraft Operations Manual in February 2001. That revision modified the normal checklist, deferring selection of auto-coarsen from the transition checklist to the landing checklist in order to minimise the time with auto-coarsen on, thus reducing the probability for an inadvertent auto-coarsen event. At the time of the occurrence the operator's checklists specified that auto-coarsen be selected to ON at FL110, as part of the transition check. The reason the operator's checklist was not revised to reflect the manufacturer's data was not available due to subsequent organisational changes.
The Flight Data Recorder (FDR) information from the aircraft was analysed by the ATSB to assess the operating parameters of the engine during the incident. That analysis revealed that approximately 37 minutes after take-off, with the aircraft in cruise flight, the flight data recorder had begun to record invalid information. This resulted in no useful data being available from the FDR for the remainder of the incident flight, a period of about 24 minutes. An examination of the aircraft's flight recorder system carried out by the operator during post incident system checks found no reason for the malfunction.