ATSB - O Rings - WTF??????

[walrus]

The ATSB in their report on the fatal crash of a Mooney Aircraft at Luskintyre, identified the cause of the accident as a fuel leak from the outlet of its engine driven fuel pump caused by the "failure of an O- Ring" that had apparently been undisturbed since 1993. I have no quarrel with this cause,  just the definition of the sealing method as an O- Ring Seal and the then horrifying advice that aircraft owners should start pulling things apart and replacing O- rings just in case!

 

https://www.atsb.gov.au/publications/investigation_reports/2022/aair/ao-2022-049#theinvestigation1

 

https://www.atsb.gov.au/media/news-items/2024/atsb-urges-proactive-approach-replacing-elastomer-components-after-o-ring-failure-contributed-luskintyre-flight-fire-accident

 

Firstly, while Lycoming may classify the affected part as an "O Ring" the joint in question is not remotely classifiable as an O -Ring seal application in my book because O - Rings fit in grooves -  very carefully machined grooves at that. Properly designed, such systems have very few ways of failing, (unless applied to a space shuttle) and are certainly not subjected to the obvious crushing and twisting loads imposed by Lycomings design which appears to be an SAE port of some kind (J1926)?

 

https://www.univair.com/engine-parts/lycoming/74070-lycoming-fitting-assembly/

 

.............which makes the ATSB advice look stupid and dangerous because there is a major issue with infant mortality in replacing O - Rings. They could perhaps have confined themselves to an inspection of the offending part.

[walrus]

o ring groove design......

 

https://sealingaustralia.com.au/oring-groove-design/

 

[Thruster88]

I have some doubts that the fuel pump outlet was leaking. Did the elbow or hose take a hit when the engine was ejected from the airframe. The fire seems most intense at the from of engine near the fuel control unit not at the back were the fuel pump is. The fuel drain line and elbow on the fuel pump looks to be missing presumably broken off.

 

There is a groove for the o ring on the fuel pump, open ended just like the lower seal on a cessna fuel gascolator and many others. I guess this is the oring crush type

Edited May 9 by Thruster88

[danny_galaga]

I have imagine this is not their first rodeo, and anything that has crossed your mind, will have crossed theirs too...