Jabiru Wing Attachment Bolts

[DGL Fox]

Hi All,

 

This was in the latest E News from RAAus....

 

This is a Proposed Airworthiness Directive from CASA concerns the wing attachment bolts on Jabiru Aircraft models UL-C, UL-D, J160-C, LSA 55/2K, LSA 55/2J, Jabiru ST, LSA 55/3J and Jabiru ST3, all serial numbers.

 

Please pass it on to anyone you know who operates affected models.

 

The Civil Aviation Safety Authority has published the following Proposed Airworthiness Directive:

 

Avtech (Jabiru Aircraft) Series Aeroplanes http://services.casa.gov.au/airworth/airwd/schedules/ad_display.asp?sched=under&toc=jabiru

 

PAD/JABIRU/2 http://services.casa.gov.au/airworth/airwd/ADfiles/UNDER/JABIRU/PADJABIRU-002.pdf

 

Wing Attachment Bolts - Life Limit

 

Comments on this proposed AD should be forwarded to [email protected] by 20 October 2015.

 

This, and other Airworthiness Directives can be viewed at:

 

http://www.casa.gov.au/airworth/airwd/index.htm

 

https://www.casa.gov.au/airworthiness/standard-page/airworthiness-directives

 

David

 

 

[old man emu]

Spoke to my adviser who suggested that instead of an AN4- bolt, use an NAS1004- one as the NAS has a much higher shear strength. Also closely examine all brackets etc that the bolt goes through as these can loosen.

 

OME

 

 

[Nobody]

OME,

 

Given that Jabiru (and CASA) are suggesting that the bolts are OK for 2000 hours it is fair to say that the issue is one of either fatigue or corrosion. In general higher strength bolts do not have higher fatigue performance because the fatigue performance is largely a function of surface smoothness. While Having a higher strength bolt will not do any harm it is unlikely to have a significantly longer service life so they probably should be replaced at the same 2000 hour interval.

 

For information here is a good basic discussion of fatigue

 

http://www.epi-eng.com/mechanical_engineering_basics/fatigue_in_metals.htm

 

Refer to figure 3 and observe that the endurance limit for steel of 125,000MPa (AN4) and 140,000(NAS1004) is not all that different.

 

 

[old man emu]

I don't disagree with Nobody's comment. It's just a matter of spending a few dollars more for some extra protection.

 

What annoys me is that there has been no publication of pictures of the failed bolt, or of the area where it was working to fasten parts together. Given the number of these bolts which are currently in service doing this job, I wonder why only one has been reported as failing.

 

OME

 

 

[Nobody]

Here is a link to the Jabiru Service Bulletin that the CASA AD references. No pictures of the failures but does suggest that replacement with an AN5 bolt eliminates the need for the 2000 (edited: was 200 thanks OME) hour replacement but note the caution about drilling out.

 

http://jabiru.net.au/images/JSB037-1_Wing_attachment_bolts.pdf

 

 

[old man emu]

200 hour service life or 2000?

 

 

[Nobody]

200 hour service life or 2000?

whoops 2000 hours

 

 

[JEM]

Mine were replaced this week. Aircraft has just reached 2000 hrs.

 

 

[Bruce Tuncks]

Here's a cautionary story about bolt replacement: Years ago, after a U/C bolt on our Jabiru bent, the maintenance guy ( not the current one,) replaced it with a high tensile bolt from an auto shop. He was trying to do a good thing doing this, he was not just using a more conveniently available bolt.

 

Well the new bolt failed catastrophically in fatigue after about 100 landings, luckily for us it failed not on landing but as the Jabiru was being wheeled backwards out the hangar in the morning.

 

This is why they refer to "properties class" not just tensile strength. Good advice here from Nobody.

 

 

[facthunter]

Some high tensile steel are less fatigue resistant than lower rated ones. Tiger moths don't use high tensile steels at that location. A strutted hi wing is not highly loaded at that spot. The load is predominantly inwards towards the cabin. Sheer loads in fibreglass are not a simple thing. Avoid sloppy fits in a sheer situation. Nev

 

 

[Nobody]

Here's a cautionary story about bolt replacement: Years ago, after a U/C bolt on our Jabiru bent, the maintenance guy ( not the current one,) replaced it with a high tensile bolt from an auto shop. He was trying to do a good thing doing this, he was not just using a more conveniently available bolt.Well the new bolt failed catastrophically in fatigue after about 100 landings, luckily for us it failed not on landing but as the Jabiru was being wheeled backwards out the hangar in the morning.

This is why they refer to "properties class" not just tensile strength. Good advice here from Nobody.

What you possibly saw was Stress Corrosion Cracking(SCC). It is a known issue in certain materials in certain environments. In bolts greater than about 1000MPa tensile strength is it possible in an atmospheric environment. Lower strength materials are less likely to be affected. The academics tend to argue a bit on the exact mechanism however they agree that it requires, stress, a susceptible material and mildly corrosive environment. One way it has been explained to me is that a small corrosion pit forms. This causes a stress riser increasing the local stress. This higher local stress causes the material to become anodic compared to the rest of the material accelerating the corrosion. The increased corrosion causes a higher stress concentration and relatively quickly a crack progresses. There are other descriptions of the mechanisms out there too.

 

An AN bolt has a strength of about 880MPa and so would generally be below the strength susceptible to SCC. 12.9 and 10.9 grade bolts are used in the automotive industry are above the stress level where SCC may be an issue. If the bolts are used internal to an engine or gearbox then there is no corrosive environment and the bolts give good service. Also is a bolt is installed in an car with the correct type of corrosion inhibitor than then never removed for the like of the aircraft then there is a low risk of failure. Using Automotive bolts in an aircraft application where removal in installation is likely over the life of the aircraft and the bolts are not installed with the right type of goop is likely to lead to failures.

 

This is one area where higher strength in not always better.

 

 

[Bruce Tuncks]

Thanks for the info Nobody, it is the best explanation I have heard about what may have happened. I wish the bits were kept as evidence, then we could get them analysed . It sure surprised me how quickly that auto bolt failed.

 

As I said, about 100 landings, but each flight would involve a few km of taxying, and this u/c was the old sort with a fairly square cross-section which gave a lot of loading in the bolts from fore and aft bumps, so the stress cycles would have been thousands of times more than the landings.

 

The other relic I wish we kept was the flat-spotted wheel-rim. One day a student pancaked the Jabiru on from about 30 feet. The u/c didn't break, nor did the tyres burst. But the little 4" pressed steel rims got flattened at the bottom. That Jabiru u/c is pretty tough.

 

I wanted them made into a trophy for something to do with landings but they got thrown out.