Engine failure during take-off involving Bombardier Dash 8.

On 11 November 2019, at about 1510 Central Standard Time, a Bombardier DHC-8-315 aircraft, registered VH-ZZE and operated by Surveillance Australia, was about to start the take-off roll from Darwin Airport, Northern Territory, on a surveillance flight. There were four crew on board.

The aircraft was on the departure runway with the brakes on. Power was applied to both engines, but when take-off power was reached, and prior to the release of the brakes, the crew heard a loud bang. The take-off was aborted, and air traffic control advised the crew of smoke from the right engine. After reviewing the engine instrumentation, the crew shut down the engine and returned the aircraft to the maintenance hangar.

A subsequent inspection of the runway identified metal fragments behind the aircraft’s take?off position. An external inspection of the right engine revealed significant damage to the power turbine (PT) assembly.

FINDINGS.

The PT shaft of the aircraft's right engine fractured due to fatigue cracking, resulting in secondary damage and engine failure. The fatigue cracking initiated at corrosion pitting, which was probably associated with prolonged low?altitude operation in a marine environment.

The PT shaft originally installed in the engine was replaced during its first overhaul in 2011 due to excessive corrosion pitting. However, the finding of corrosion was not escalated by the maintenance organisation to Pratt & Whitney Canada (P&WC), possibly due to the informal reporting process at the time (this process was replaced in 2018 with formal guidance and criteria for reporting such findings).

The ATSB investigation also identified that the PT shaft in Pratt & Whitney Canada PW100 series engines operating in certain marine environments is susceptible to corrosion pitting, which can grow undetected between scheduled inspections, increasing the risk of shaft fracture and engine failure.

What has been done as a result.

Pratt & Whitney Canada advised the ATSB that it had commenced a review of historical overhaul experience of the PT shaft in an effort to identify which engines and operators are potentially exposed to an increased risk of PT shaft corrosion.

In addition, P&WC has proposed a range of safety action to address the safety issue concerning corrosion-related fracture of PT shafts in PW100 series engines that should complement its formalised reporting. This includes considering a borescope inspection of the PT shaft between overhauls during hot section inspections (HSI) with defined corrosion inspection criteria. A method to remove contaminants from inside the shaft during service is also being investigated. Additional mitigating action for engines within the PW100 engine fleet that have completed an HSI but are potentially exposed to the risk of PT shaft corrosion, is also being assessed.

While the proposed actions should address the safety issue, no timeline for their implementation was provided. As such, the ATSB has issued a safety recommendation to P&WC to support the proposed action.

Engine failure

Examination of the power turbine (PT) shaft by the engine manufacturer, Pratt & Whitney Canada (P&WC), identified conclusively that it fractured due to fatigue cracking that originated from corrosion pitting. Excluding the low pressure impeller leading edge erosion and diffuser corrosion, the observed damage to other engine components, including the second PT shaft fracture location, was consistent with secondary damage due to the initial fatigue fracture.

Although the gas core of the engine continued to operate after the PT shaft fracture, no power could be transmitted to turn the section of the PT shaft still connected to the gearbox and propeller. As a result, no engine power was available, and the crew shut down the engine.

Power turbine shaft corrosion

The corrosion products identified at the PT shaft fracture origin indicated that the corrosion was probably caused by salt-laden air. Since air from the compressor entered the PT shaft during normal engine operation, corrosion causing contaminants (for example, salt) and moisture within the ambient air probably built up on the inside of the shaft, allowing corrosion to occur.

The engine had been operated at low altitude over the sea throughout its life, with PT shaft and air system corrosion evident at its first overhaul and subsequent engine examination. These observations supported P&WC’s conclusion that the PT shaft corrosion was caused by the engine’s operational environment.

As a result of the operator utilising an engine on-condition maintenance program, the engine operated significantly beyond the hard-time overhaul of 8,000 hours, with no additional maintenance of the PT shaft. Coupled with the absence of a cleaning process to remove contaminants from the shaft during inactivity, meant an increased operational time and calendar time between PT shaft inspections, both of which probably contributed to corrosion formation and growth.

Since post wash engine runs were being performed and the engine had not undergone any preservation, it was unlikely that any fluid or moisture would have remained in contact with the shaft surface for any extended period while the engine was inactive. There were also no indications that the shaft’s protective aluminium coating had been incorrectly applied. Therefore, it was unlikely that any of these factors contributed to the observed PT shaft corrosion.

Corrosion management

The information obtained by the ATSB during this investigation indicated three instances of corrosion forming within the PT shaft of PW100 series engines that grew undetected between the scheduled overhaul inspections. All three occurred within the operator’s engine fleet which have been used primarily in a marine environment, with one resulting in a fracture of the shaft.

The PT shaft fracture due to marine environment exposure demonstrated that the existing protective coating and maintenance requirements were not sufficient to prevent progression to corrosion?related fatigue fracture. Although corrosion pitting may not always lead to a fracture before overhaul, it provides sites for fatigue crack initiation, presenting an increased risk of premature fatigue fracture.

Given the power turbine shaft design and function is the same across all PW100 series engines, and there are other PW100 series operators conducting maritime surveillance activities, there is potential for this risk exposure to extend to other operators within the global fleet.

While P&WC indicated that based on its experience the PT shaft coating provided good corrosion protection, corrosion pitting outside of repair limits might not necessarily have been reported by maintenance organisations during overhaul. Reporting such findings was probably less likely prior to the introduction of the additional guidance through the CAT 5 process in 2018. Therefore, P&WC may have had limited visibility on engines exposed to an increased risk of PT shaft corrosion.

Although engine AT0010 exhibited ‘deep corrosion pitting’ of the PT shaft outside of repair limits at the engine’s first overhaul, the finding was not escalated by the maintenance organisation to P&WC for further assessment, possibly due to the informal reporting process in place at the time. Additionally, it does not appear that the location, extent, size, and depth of the corrosion was recorded. Furthermore, there was no recorded information available regarding an assessment of the finding. Therefore, there was insufficient information to determine whether that specific finding should or should not have been escalated.

Nevertheless, the circumstances of this occurrence illustrate the importance of specific guidance and criteria on how to assess corrosion. In this instance, this would probably have provided P&WC with greater visibility of engines in the global fleet that are exposed to an increased risk of corrosion-related failure.