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Aerobatic Fatigue Index

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Aerobatic Fatigue Index

Old 2nd Jun 2015, 09:03
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Aerobatic Fatigue Index

Hi,

I'm (still) looking into which aircraft are ideal for economical aerobatics while still having some touring ability.

One of the things I just discovered is the structural fatigue due to the high G loads. I know the Bulldog has a fatigue index, but is that a common thing for all aerobatic aircraft?

For example the Fuji FA 200 has no fatigue index here in EASA, but in Australia I found this: http://www.comlaw.gov.au/Details/F20...f-8bbf0c6d8bd7 it says to retire every Fuji FA 200 after 1700 aerobatic flying hours. Why would this hold in Australia but not in Europe?

So that got me wondering if every aerobatic aircraft could be suffering from fatigue instead of only the ones with an AD for it. Common sense tells me that it's common for every aircraft, or can this usually be dealt with in the annual inspections?
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Old 2nd Jun 2015, 09:13
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Not an expert, but fatigue affects materials differently and in theory it's possible to design steel, carbon and wooden structures that have a nearly infinite fatigue life. So some aircraft e.g. those designed for aerobatics may not need a fatigue index.
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Old 2nd Jun 2015, 09:32
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Aerobatic Fatigue Index

Depending on age, aircraft that were in military service were fitted with fatigue meters. These meters would record individual applications of particular g levels. At the end of a sortie the pilot would enter a code for the nature of that sortie in the tech log. Combining the code and the various fatigue counts would produce a fairly accurate measure of how much fatigue had been used.

Theoretically an aircraft would be dead at 100 Fatigue Index (FI). In practice this would vary and for reasons of economy would often be extended beyond this, or if fleet problems occurred then a modification might be required before reaching a particular FI.

Civilian aircraft and older ex military ones don't have a meter so the calculation of fatigue is a much blunter tool. In some regimes for example a 1 hour trip that includes 5 minutes of aerobatics would require the aerobatic fatigue multiplying factor to be applied for the whole hour.

Depending on aircraft type it might die at a particular number of hours or it might require specific component inspection/ replacement.

Of course if that component is a wing spar then practically speaking it's usually dead.

It's also not unusual for a particular type to suddenly acquire an hours restriction or aerobatics ban following a fatigue failure on a similar aircraft.

In short the point I make is that very careful research is required into the fatigue management of a particular type prior to purchase
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Old 2nd Jun 2015, 09:32
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Pirke

This is a very complicated subject and one that can only be descussed on an aircraft by aircraft basis.

For example the Bulldog is limited by the danger of fatigue cracks radiating from bolt holes in the main spar, it would be imposable to find these cracks on an annual check without removing the wings and doing NDT checks. The cost of such "on condition" maintenance is prohibitive.

On aircraft such as the PA38 I suspect that the fatigue life is more a function of where the manufacture stopped testing the airframe with a view to not wanting to support an airframe past that age.
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Old 2nd Jun 2015, 09:58
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Thanks for the quick replies. That it is a complicated subject was clear to me, given the lack of clear information I could find on the internet And I'm definitely no materials expert either... although I know that anything breaks after bending it often enough.

Although wearing parachutes is somewhat of a backup plan in case of any emergency, I do prefer to be able to trust the wings to stay attached.

How would you suggest to approach searching for an aircraft with the intention of buying it and flying aerobatics in it? As always, budget is a limiting factor, but I'd rather wait a few more years to buy something reliable than to throw it away or worse: have an accident.
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Old 2nd Jun 2015, 10:40
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Pirke
How would you suggest to approach searching for an aircraft with the intention of buying it and flying aerobatics in it? As always, budget is a limiting factor, but I'd rather wait a few more years to buy something reliable than to throw it away or worse: have an accident.
As already said most if not all civilian aerobatic aircraft do not have a fatigue index measuring device but use a method of calculation from hours flown. Therefore buying a second hand aerobatic aircraft you will never know if at some point during its life it has been overstressed where as a fatigue index meter will record those very high "G" counts, once I remember seeing an 8g recording on a Jet Provost

So ultimately if you want to feel safe about the wings staying attached is to buy new.
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Old 2nd Jun 2015, 10:43
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For example the Fuji FA 200 has no fatigue index here in EASA, but in Australia I found this: http://www.comlaw.gov.au/Details/F20...f-8bbf0c6d8bd7 it says to retire every Fuji FA 200 after 1700 aerobatic flying hours. Why would this hold in Australia but not in Europe?
Australia's old unique airworthiness certification requirements had fatigue life requirements long before they were included in FAR 23 so many types were trapped by this when the first of type was imported, not just aerobatic aircraft. Lifed items controlled by ADs.
Decathlons here in Aus have lifed components that are unique to Aus.
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Old 2nd Jun 2015, 11:48
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I think I have the oldest Fuji still flying, serial number 11. The last of the pre-production prototypes. It was built in April 1968 and went on the Australian register in May 1968. In its approximately 6700 hours, it has only done about 650 hours aerobatic time. The main problem with aerobatics these days is there is nowhere near to my base where I can do it and so don't add much aerobatic time each year.

Pirke, don't worry about the Australian restriction, it takes a lot of effort to get there.
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Old 5th Jun 2015, 14:35
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If you want economical Aeros with a good touring ability then look at the Vans aircraft, the 7 & 8 will happily do aeros 2 up and carry good loads for touring, the 4 and 6 are a bit more restricted, really just 1 up for aeros but all will cruise at 140kts plus.
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Old 9th Jun 2015, 16:55
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The Slingsby T67m has no fatigue life limitations and is a capable aerobatic mount. +6 -3 with Full inverted systems and a CS prop. It suffers from a glacial roll rate, but it it still fun to fly. As an added bonus, many of them are IFR capable with an HSI and RMI.
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