ATSB research investigation on engine failures and malfunctions in light aeroplanes

In 2012, regular aviation trend monitoring identified a significant increase in the number of light aircraft engine failures or malfunctions. To formally and more fully examine both the extent of and the contributing factors behind these observations, the ATSB initiated a research investigation to assess and compare engine failures and malfunctions in light aircraft.

The investigation included single-engine aeroplanes up to 800 kg maximum take-off weight (MTOW). The weight cut-off of 800 kg encompasses the Light Sport Aircraft (LSA) group of aircraft, which are typically under 600 kg MTOW. Although some of these aeroplanes are CASA VH-registered, the majority are registered with Recreational Aviation Australia (RAAus). Aircraft registered with either body could have a certified or uncertified aircraft engine and could be factory-built or amateur-built. As such, the ATSB examined occurrences of both VH-registered and RAAus registered aeroplanes, which were reported to the ATSB and/or RAAus between 2009 and 2014 and that the ATSB had classified as engine failures or malfunctions. Engine failures or malfunctions are only reportable matters to the ATSB, under the Transport Safety Investigation Act 2003, when they happen while the aircraft is boarded for flight.

Over the 6 year study period, between 2009 and 2014, 322 engine failures or malfunctions involving light aircraft were reported to the ATSB and/or RAAus. Aircraft powered by Jabiru engines were involved in the most engine failures or malfunctions, with 130 reported over the 6 years. This represents about one in ten aircraft powered by Jabiru engines in the study set, having reported an engine failure or malfunction. Reports from Rotax powered aircraft were the next most common (87, or one in 36), followed by aircraft with Lycoming (58, or one in 35) and Continental engines (28, or one in 35). When factoring in the hours flown for each of these engine manufacturers, aircraft with Jabiru engines had more than double the rate of engine failure or malfunction than any other manufacturer in the study set—3.21 failures per 10,000 hours flown. Additionally, when the yearly rates were examined, the rates of Jabiru engine failures or malfunctions was observed to increase over the 6 year study period. The rates were further divided into registration type (VH or RAAus) which showed a very similar pattern across the four main engine manufacturers—with Jabiru powered aircraft having the highest rates for all VH-registered and most RAAus registered aircraft.

Unlike the engines of other manufacturers in this study, which showed a wide distribution of failure mechanisms, nearly half of the Jabiru engine failures or malfunctions related to a fractured component. Engine through-bolt failures were the most commonly reported failure mechanism in Jabiru powered aircraft, with 21 through-bolt fractures reported between 2009 and 2014.

Figure 15: The rates of engine failure or malfunctions for the four primary engine manufacturers in the light aeroplane set of aircraft between 2009 and 2014

Rates_of_engine_failure

Taking into account the number of aircraft registered in the study period, through-bolt failures occurred in about one in 55 Jabiru powered aircraft. Although originally designed to be replaced after 1,000 hours, 19 through-bolts failed before the 1,000 hour mark, with seven failing before 500 hours.

For the set of engines analysed in this investigation, Jabiru engines are somewhat unique in their design. Conventionally, the crankcase is bolted together with separate bolts to those used to bolt the cylinders to the crankcase. In contrast, in Jabiru engines the same through-bolts, that hold the crankcase together, also fasten the cylinders to the block.

Jabiru Aircraft Pty Ltd has implemented a number of changes to address the though-bolt failures. These include updating the type of through-bolt nut, introducing a thicker (7/16 inch diameter) through-bolt and re-designing and testing a modified 3/8 inch diameter through-bolt, which can be retrofitted to older Jabiru engines. Despite these changes, at least four reported through-bolt failures involved engines with upgraded 3/8 inch diameter through-bolt nuts. However, to date, there have been no failures reported involving the newer 7/16 inch diameter through-bolts, which are used in currently manufactured engines (present in about 20 per cent of Jabiru engines). These newer 7/16 inch through-bolts appear to have improved the reliability of Jabiru engines, although future monitoring will provide more definite evidence.

Figure 16: Schematic showing the general layout of a Jabiru four cylinder engine

general layout of a Jabiru four cylinder engine

Source: Jabiru Aircraft PTY LTD service bulletin JSB031-3

The ATSB issued a safety recommendation to Jabiru Aircraft Australia. It recommends that further safety action is taken to ensure that all owners of Jabiru engines—that have not been manufactured with new configuration 7/16 inch diameter through-bolts, or modified in accordance with Jabiru Service Bulletin JSB031-3—have access to, and are encouraged to upgrade to, the 7/16 inch diameter through-bolt configuration (or any other alternative produced to replace the existing 3/8 inch diameter through-bolt configuration). This includes newly developed through-bolts incorporating aspects to alleviate the effects of thermal expansion and damp resonant vibrations.

The ATSB also recommended that CASA continue to monitor the through-bolt failure rate of Jabiru engines, to satisfy themselves of the reliability of the 7/16 inch diameter bolts and any other alternative produced to replace the existing 3/8 inch diameter through-bolt configuration. This will determine if these modifications have sufficiently reduced the risk of an engine failure or malfunction in Jabiru-powered aircraft.

The ATSB research investigation report, Engine failures and malfunctions in light aeroplanes 2009–2014 (AR-2013-107), is available from the ATSB website at www.atsb.gov.au