Reciprocating-engine powered low-capacity transport aircraft (8 to 10 passengers) provide an important public transport connection throughout regional Australia. In the period January 2000 to December 2005, twenty powertrain structural failures of high-power (300 to 375 brake horsepower) horizontally-opposed, reciprocating engines were associated with air safety occurrences reported to the ATSB. These occurrences ranged in severity from; in-flight engine shutdown; engine failure and forced landing; engine failure combined with in-flight fire and fracture of both upper engine mounts; to the fatal accident of a regular public transport flight following the structural failure of both engines to ditching at night. It is evident that the reliability of high-power reciprocating engines is an important requirement for the safe operation of this class of aircraft. This research investigation is a study of the factors that affect reciprocating engine reliability.
The study found that powertrain structural failure was not restricted to one engine model, one engine manufacturer, or one powertrain component. The events that initiated sequences that led to engine in-flight failure could be grouped into three categories: combustion chamber component melting; bearing breakup; and powertrain component fatigue cracking. Analysis of the factors that were associated with each category of initiating event revealed that powertrain component reliability is affected by the development of shockwaves during combustion, the response of bearings to boundary lubrication and out-of-plane alternating loads, the increase in component alternating stress magnitudes, and creation of stress-concentrating features in components during engine operation. These factors may act singly, but on many occasions it is the synergistic effect of the presence of multiple factors that result in a sequence of events ending with engine in-flight failure.
The recurrence of powertrain component structural failure events suggests that the corrective actions that are a part of the airworthiness assurance system may have been ineffective. Corrective action is dependent on accurate analysis and feedback. It is evident that analysis is affected by the complexity of reciprocating engine systems and feedback requires a broad view of the interaction of systems and a detailed view of the components of a system.
|Type:||Research and Analysis Report|
|Author(s):||Dr Arjen Romeyn, Principal Failure Analyst - Engineered Systems|
|Publication date:||22 November 2007|