Primarily for general aviation discussion, but other aviation topics are also welcome.
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#1605481
tomshep wrote:Just over ten years old and 8000 hours. A very young Cherokee. Tragic. Looks very much like the airframe had been repeatedly overstressed. Hard to envisage any other conclusion, they are pretty tough aeroplanes. One to consider in the rent or buy debate.


You can’t make that conclusion at all. There needs to be crack initiation that leads to a fatigue failure like that- that can be from a corrosion site, mechanical damage, manufacturing defect etc Lots to consider and investigate yet I would say...
#1605485
I made no conclusion but suggested that it looked like that was the reason. The report will determine the cause, in all probability; however:
fatigue cracks in the other wing's spar cap seems to me to rule out manufacturing and I don't think, looking at the fracture face that corrosion was an issue.
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By Paul_Sengupta
FLYER Club Member  FLYER Club Member
#1607841
I've just got round to reading the second half of this thread, and throughout I had thought about posting something on metal fatigue. Then I see the preliminary report saying it was metal fatigue...

One thing about metal fatigue is that it builds up over time. It isn't black and white, it isn't something which shows over-stressing, it can be repeated stressing within limits.

In my Bulldog, I have a fatigue meter. It clicks different counters over at different levels of plus or minus g up to the limits (+6, -3). Each time I pull 1.75g, the 1.75g counter clicks up, assuming I didn't go up to the level of the next counter. These all add up over time, and the more g pulled (or pushed), the higher counters get clicked and the more fatigue is clocked up. The fatigue calculation also takes into account the number of landings. Even assuming no heavy landings, each time you land the spar goes from the wings supporting the fuselage to the fuselage supporting the wings, and vice versa for a take-off. It all adds up.

You can't just go on number of hours flown, the use to which the aircraft is put will be part of the consideration. Circuits? Steep turns? Spins? Spiral dives? Aerobatics? Straight and level cruise for hours? These will all fatigue the airframe, and critically the spar, in different ways. It isn't the case, at least as far as I understand it, that a spar used within (say) its 4.4g limit will have an infinite life. Some aircraft have "multipliers" which are supposed to be used for anything other than straight and level, but I don't know anyone who bothers with that, especially in the training environment.

With the Bulldog, as well as fatigue monitoring, there are also regular NDT eddy current inspections on the spar to check for any cracking.

Does all this mean a Bulldog's spar is weaker than that of (say) a PA28? Hell no, as the Americans say.

It's a cliche to say that some of these aluminium GA aeroplanes were never expected to be in service, let alone training service with the stresses involved, for 30 or 40 years, but this is where we are.
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By Flyin'Dutch'
FLYER Club Member  FLYER Club Member
#1607842
Paul_Sengupta wrote:It's a cliche to say that some of these aluminium GA aeroplanes were never expected to be in service, let alone training service with the stresses involved, for 30 or 40 years, but this is where we are.


The PA28 in this accident was very young both in years and hours
#1607874
[q
Flyin'Dutch' wrote:
Paul_Sengupta wrote:It's a cliche to say that some of these aluminium GA aeroplanes were never expected to be in service, let alone training service with the stresses involved, for 30 or 40 years, but this is where we are.


The PA28 in this accident was very young both in years and hours

As Paul said (my bold)
Paul_Sengupta wrote:One thing about metal fatigue is that it builds up over time. It isn't black and white, it isn't something which shows over-stressing, it can be repeated stressing within limits.

In fact I would rather say that "fatigue IS repeated stressing within limits" and it does indeed depend on the usage of the aircraft. It's the number of cycles that does it, as well as the amplitude.

In the RAF I recall one aircraft whch had huge fatigue readings following a 20 minute flight from Scampton to Finningley - I asked the pilot about this and he confirmed that the weather was absolutely dreadful - really rough and violently turbulent the whole way. I think it worked out that that flight was about 20-25% of the FI normally used in a whole year (perhaps 500 hours). :shock:
#1611954
NTSB Update

Cracks found on a second aircraft of similar vintage/usage. The cycle count to flight hour ratio in both cases is pretty high, and given they’re being used exclusively for training it’s probably reasonable to infer that a higher than usual portion of the landings might be at the high end of the load range. But to have inspected only 10 aircraft of similar usage and find another crack in exactly the same place :shock:
#1611959
In both accidents it seems that the aircraft was being operated somewhat "unusually" - in the training case, the obvious large number of cycles, and pipeline inspection would involve low level flying in low level turbulence.

Do you track cycles on your aircraft at all? I would bet most privately owned PA28s average not far from 1 cycle per logged hour.
#1718421
The NTSB final report is now available. No huge surprises, but the key finding in the probable cause was Previously established inspection criteria were insufficient to detect the fatigue crack before it grew to a critical size.

No AD published as yet, but interested if owners with cycles in the range have had their aircraft inspected or not?
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