Thicker 7/16 inch diameter through-bolts, fitted to newer Jabiru engines and some retro-fitted engines, have had limited service to date to confirm early indications that they reduce this risk. Retro-fitting engines with thicker through-bolts has only been recommended for aircraft involved in flight training by JSB031 issue 3. Most light aircraft in service with Jabiru engines continue to use 3/8 inch diameter engine through-bolts which, even after upgrades in accordance with Jabiru service bulletins JSB031 issues 1 and 2, remain at an elevated risk of fracturing within the service life of the bolt, leading to an engine failure or malfunction in flight.
Response by Jabiru Aircraft Pty Ltd
Jabiru Australia has recently completed an engineering study (Through bolt strain gauge test, Jabiru engineering report AVDALSR109-1, 19 November 2015) that has designed and tested a modified 3/8 inch diameter through-bolt which is believed will address the safety issue. The report states: à.. [the earlier February 2015 Jabiru engineering report AVDALSR105] established that the natural frequency tendencies of the 3/8ö through bolt were such that resonance with the engine was likely to occur and this was the probable sources of abnormal (and previously unanticipated) cyclic loads which would cause the bolts to fail. This report details further work conducted to confirm this hypothesis using an instrumented through bolt installed in a running Jabiru engine. In the course of testing conducted, the nature of loading in the through bolt has been established, vibrational resonance was detected and another aspect of the failure mechanism was uncovered; the previously unanticipated thermal load cycling. The final tests conducted were on a revised design to the 3/8ö through bolt which incorporated aspects to alleviate the effects of thermal expansion and damp resonant vibrations that were found on the standard through bolt. The revised 3/8ö through-bolt was: designed featuring a more elastic (i.e. less stiff) spring rate and rubber O-rings in the middle to damp resonate transverse vibrations. o Calculations showed significant reduction in preload tension resulting from a given temperature increase for the new design 3/8ö through bolt compared to the standard design. o Engine test runs were also conducted. The resonant vibration mode identified for the standard 3/8ö through bolt had visibly disappeared with the addition of rubber O-rings. This suggests that the addition of rubber O-rings significantly damps the otherwise damaging resonant vibrations.
ATSB comment
The ATSB recognises that Jabiru Aircraft have conducted a number of in-depth analyses of the mechanism of the through-bolt failures. Additionally, the ATSB acknowledges that Jabiru consider that both the implementation of the 7/16 inch through-bolt, and the development of a revised design 3/8 inch though-bolt, have the potential to address this safety issue across the fleet of all Jabiru engines. As noted in the internal Jabiru engineering report AVDALSR105-2, most Jabiru-powered aircraft remain at risk of a through-bolt failure. This risk exists because most Jabiru engines in use are still using older configurations of through-bolts. At the time of release of this report, about 20 per cent of engines were manufactured with the new 7/16 inch through-bolt configuration. Some older engines have been retro-fitted to accommodate the thicker through-bolts. However, the recommendation in service bulletin JSB031-3 to upgrade through-bolts to the newest available configuration of through-bolts only pertained to aircraft involved in flight training. As the use of the new 7/16 inch configuration through-bolts is relatively recent, on-going monitoring of the reliability of these through-bolts across the fleet is required. Up to 80 per cent of the Jabiru engines in service, which have the older 3/8 inch configuration through-bolts, are still at risk. Although Jabiru have designed and tested a revised 3/8 inch through-bolt which incorporates aspects to alleviate the effects of thermal expansion and damp resonant vibrations, it can only address the safety issue once these new bolts are made available to Jabiru engine owners and fitted to relevant aircraft.
The ATSB acknowledges the safety action taken by Jabiru with the release of safety bulletin JSB031-3 and the availability of dampened 3/8” through bolts, and also that there has also been a significant reduction in reported through-bolt failures since 2015. The ATSB is satisfied that these actions have reduced the risk associated with this safety issue.
The Australian Transport Safety Bureau recommends that the Civil Aviation Safety Authority continue to monitor the through-bolt failure rate of Jabiru engines to satisfy themselves of the reliability of the:
configuration (including newly developed through-bolts incorporating aspects to alleviate the effects of thermal expansion and damp resonant vibrations) to determine if these modifications have sufficiently reduced the risk of an engine failure or malfunction in Jabiru-powered aircraft.
CASA's response to ATSB safety recommendation
CASA accepts this recommendation. CASA will address this issue and take appropriate action to continue to monitor the through-bolt failure rate of Jabiru engines through Airworthiness Direction processes and Service Difficulty Reporting. As this is likely to be a protracted process, CASA is not in a position to specify a specific completion date at this time.
The ATSB welcomes the CASA’s acceptance of the safety recommendation. The ATSB will continue to monitor the progress of the CASA’s oversight of the through-bolt failure rate of Jabiru engines, for both the 7/16 inch diameter bolts and modified 3/8 inch diameter bolts.
Final response from CASA
CASA accepted this recommendation and provided a response indicating that CASA will take appropriate action to monitor the through-bolt failure rate of Jabiru engines through Airworthiness Direction processes and Service Difficulty Reporting.
CASA has now determined (June 2019) that the continuation of the safety measures in instrument CASA 39/19 (and its predecessors) is warranted for a further period of 3 years. During this period, CASA will continue to monitor the instances of loss-of-engine-power events in relation to the aircraft, to ensure the safety measures in the instrument remain effective. CASA believes that this instrument sufficiently reduced the risk of an engine failure or malfunction in Jabiru-powered aircraft and will continue to monitor for another 3- year period to determine reliability of the 7/16-inch diameter bolts.
The Australian Transport Safety Bureau recommends that Jabiru Aircraft Australia takes further safety action 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:
Jabiru has developed a modified through bolt which includes an elastomer element to dampen lateral vibrations. Lateral vibrations of the through bolts is a phenomena which has been confirmed by testing of an instrumented engine. Jabiru Aircraft is making dampened through bolts available as part of the continual service of the Jabiru engine fleet and encourages owners and operators to have them installed. JSB031-3 also recommends upgrading other engines to the most current through bolt configuration.
Jabiru continues to encourage owners and operators to implement the published Jabiru operation and maintenance procedures and service letters and service bulletins by conducting regular Jabiru engine workshops that are very popular with LAME, L2 and L1 maintainers. The number of workshops run has been increased to two per month and these are resulting in more appreciation of the requirement for maintenance.
The ATSB welcomes Jabiru Aircraft’s intension to make the newly modified 3/8 inch through-bolts available to Jabiru engine owners. The ATSB also notes that the service bulletin does recommend upgrading engines to the most current through-bolt configuration, but limits this recommendation to aircraft used for flight training, and that the thicker 7/16 inch diameter through-bolts require engine modifications. The ATSB will monitor this safety recommendation until the 3/8 inch diameter through-bolt with dampening design is available for owners.
Jabiru issued JSB031-3 in January 2015. In short, JSB031-3 is applicable to GEN2 engines and mandated for those operating in flying schools. The service bulletin requires the replacement of 3/8” through bolts each 500 hours or an upgrade to the 7/16” through bolts. The service bulletin was mandated only for flying schools as the evidence pointed to the particular usage pattern of flying schools being a significant contributor to failures. Since the release of that service bulletin, there have been three occurrences of through-bolt failures noted in the ATSB database. Given the nature of these failures, it is highly likely that if they do occur, that they will be explicitly noted, and so it is with reasonable confidence these are noted as the only through bolt failures in the period 2015-22 (eight years). This compares to 21 failures in the period 2009-14 (six years).
Jabiru can also advise that the 3/8” through-bolts with dampeners were released to production in December 2015.
In reviewing the service record since January 2015, Jabiru’s conclusion is that the combination of availability of the 7/16” through bolts, the dampened 3/8” bolts and the management of risk via JSB031-3 has successfully and significantly reduced the risks identified in AR-2013-107-SR-055 to an acceptable level.
Jabiru service bulletin JSB031-3, which is applicable to Gen 2 engines and mandated for those operating in flying schools, requires the replacement of 3/8” through bolts each 500 hours or an upgrade to 7/16” through bolts. Furthermore, Jabiru have confirmed that the 3/8” through-bolts with dampeners were released to production in December 2015. Safey action taken by the manufacturer appears to have adequately addressed this issue as there has been a significant reduction in reported through bolt failures since 2015. This is reflected in the post safety action risk assessment which has assessed the residual risks as being acceptable.
