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 to safety issue by Jabiru Aircraft Australia
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:
….. [the earlier February 2015 Jabiru engineering report AVDALSR105] established that the natural frequency tendencies of the 3/8” 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 3/8” 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 3/8” 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.
- Calculations showed significant reduction in preload tension resulting from a given temperature increase for the new design 3/8” through bolt compared to the standard design.
- Engine test runs were also conducted. The resonant vibration mode identified for the standard 3/8” 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 in response
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.