A Skippers Aviation de Havilland Dash 8-300, registration VH-XKJ performing a charter flight from Perth,WA to Duketon Gold Mine,WA (Australia) with 51 passengers and 4 crew, was climbing through 250 feet AGL out of runway 21 when the first officer, pilot monitoring, retracted the landing gear and at the same time popping/banging sounds were heard from the left hand engine, the aircraft slightly yawed to the left, the first officer noticed a reduced torque just below 60% on the left hand engine (PW123). The first officer reported a failure but advised there was no engine failure indication and no master caution. The captain, pilot flying, noticed the torque at 58%, all other indications were normal and decided to not shut the engine down. While climbing through 800 feet the crew retracted the flaps and declared PAN PAN. The aircraft joined a right downwind at 1300 feet, when the captain noticed the torque of the right engine was lower than expected for this phase of the flight. The crew upgraded to declaring MAYDAY. The crew advanced both throttle levers to about 80-90%, the banging sounds from the left engine ceased and both engines responded as expected. When the throttle levers were reduced again in preparation for the landing and speed reduction the banging sounds returned. The aircraft landed safely with power from both engines and taxied to the apron.
On Jul 23rd 2020 the ATSB released their final report concluding the probable causes of the serious incident were:
- Excessive erosion to the left engines high-pressure turbine blades likely resulted in the power loss.
- At the time of the occurrence, the maintenance program for the detected erosion was in accordance with the manufacturer's maintenance manual requirements.
- The aircraft experienced an uncommanded gradual reduction of torque in the right engine, a mechanical issue with the engine as the cause was considered unlikely
The ATSB described the FDR data:
The aircrafts flight data recorder (FDR) was downloaded by the operator and a copy of the relevant data provided to the ATSB. The flight data showed a sharp reduction in left engine torque as the aircraft climbed through 250 ft (see Figure 1). This was followed by a period of torque fluctuations, which aligned with the time that the flight crew reported hearing the banging sound coming from the left engine. The torque fluctuation was also coincident with minor fluctuations in the left engine inter turbine temperature (ITT), fuel flow, compressor (NL) and turbine (NH) percentages.
A slow reduction in right engine torque and ITT also occurred for the duration of the left engine power fluctuations, and until the torque increase on both engines was observed.
The ATSB described the engines:
The aircraft was fitted with two Pratt & Whitney Canada (PWC) PW123E turboprop engines. These engines, serial numbers AW0067 (left engine) and AW0065 (right engine), had accumulated 19,212 and 20,354 hours in service respectively at the time of the incident.
The operator utilised an engine condition trend monitoring (ECTM) system to track the health of the various engines throughout its fleet of aircraft. This system allowed them to track trends in engine parameters over time and respond to them as necessary. The system also provided alerts in the event that there was a deviation from the trend in any of these parameters.
The engine maintenance manual (EMM) required that borescope inspections (BSI) be conducted every 1,500 hours for monitored engines and every 1,000 hours for unmonitored engines. In this case, while monitoring their engines using the ECTM system, the operator elected to align the BSI with other maintenance items and carry out the inspections every 1,000 hours under normal conditions.
In late August 2018, the ECTM system detected a change in the trend for both engines. The status changed from Trend Normal to Notification, based on an increase in ITT and decrease in the NH. This trend shift prompted the operator to conduct an out-of-cycle BSI and perform a power assurance run (PAR). This inspection was carried out in early September and both engines were found to have leading edge and tip erosion damage to the high-pressure turbine (HPT) blades.
The damage to the left engine was more pronounced and a defect was raised in the engines maintenance log. Based on the guidance in the EMM, the left engine erosion damage required an increased inspection frequency for the BSI and PAR to every 300 hours from the previous 1,000- hour interval. In December and within the 300-hour interval, the next BSI revealed increased damage. It was judged, however, to still be within the required limits for continued operation, with the increased inspection frequency. At the time of the occurrence, the engine had accumulated a further 211 hours in service.
The ATSB analysed:
Post-flight internal inspection of the engines revealed erosion damage to the high-pressure turbine blades of both engines. Given the high operating temperature/speed and low clearances that exist within turbine engines, erosion degradation over time is expected. However, this deterioration affects the optimum airflow through the engine and reduces the overall engine efficiency. In this case, the erosion to the left engine high-pressure turbine is likely to have contributed to the power loss and banging sound experienced by the crew and the engine parameter variation recorded in the FDR data.
Skippers Aviation conducted engine condition trend monitoring on their fleet of aircraft. A change in the trend for the left engine triggered an alert, which prompted an internal borescope inspection and power assurance run to be conducted. Erosion to the high-pressure turbine was noted and an enhanced maintenance program to monitor the damage had been initiated. Technical documentation available to Skippers Aviation assisted with the detection and monitoring of the damage. However, there was no specific tip clearance limit given in the engine maintenance manual. The ATSB noted that, while this occurrence happened when the engines were under close monitoring, the enhanced maintenance program was in accordance with the engine manufacturers requirements.
The crew also reported a reduction in right engine power. It was determined that, while a possible exacerbating factor, it did not affect the crews decision to conduct the return, as the PAN call and return to Perth had been initiated before the right engine low power was noted. The subsequent engine inspection identified erosion to the high-pressure turbine. However, it was less than that of the left engine. Additionally, the flight crew reported that the right engine responded normally to the power lever increase and operated as expected for the remainder of the flight. Based on that evidence, the ATSB concluded that the decrease in right engine power was unlikely due to a mechanical issue with the engine.
Possible causes for this reduction included, a transient engine issue, technical failure of the throttle mechanism, flight crew deliberate action or flight crew distraction. However, because the FDR did not record throttle position data the reason for this reduction could not be determined.