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Piper crop spraying type crash in Mexico.

Accidents and Close Calls Discussion on accidents, close calls, and other unplanned aviation events, so we can learn from them, and be better pilots ourselves.

Piper crop spraying type crash in Mexico.

9th Sep 2023, 17:53

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Originally Posted by Capn Bug Smasher
Newton's second law of motion applies

Force = mass x acceleration

Force exerted by elevator is the same throughout manoeuver

Mass reduces on drop

Therefore acceleration (i.e. g-load) must increase for same force

I never realised the implication for aerial application: I have learned from this poor chap's accident.
Originally Posted by Vessbot
The question is, which of these correlates to wing stress?
Originally Posted by Capn Bug Smasher
All of them
Even if the G load increases for the pilot, that does not mean the drop will cause an upward force on the wing.... The mass has changed, force (lift) hasn't, so aircraft goes up, and pilots experience an increase in G. Wing is still producing the same lift, so no change in load on wing attachments.
I don't know why the wing failed. I think it could be either the wing brace strut being pulled out of it's attachment by the reduction in upward force on the wing while the load was dropped, followed by the wing folding up during the pull up, or the wing strut brace buckling due to the increase in force during the sharp pullup after the drop. But the releasing the load does not lead to an instant increase in positive load on the wing attachment, if anything in a decrease in positive load.

Last edited by hans brinker; 10th Sep 2023 at 00:57.
9th Sep 2023, 18:06

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Are we agreed that, if the cg of the released payload is longitudinally-coincident with that of the aeroplane, then the resultant lift/mass acceleration will be upwards along the normal axis; ie, no change to pitch angle?

A load released aft of the aircraft's cg would cause a pitch down in addition to the upward vector; fwd of the cg a pitch up.

Other things being equal, of course, if the resultant upward acceleration were along the normal axis this would alter the relative airflow vector to effectively reduce alpha on the wing, reducing lift. However, the horizontal stab would undergo an increase in negative alpha, causing the aircraft to pitch up, and at this point my tiny brain is beginning to lose the plot .........
9th Sep 2023, 19:04

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Originally Posted by hans brinker
I think it could be either the wing brace strut being pulled out of it's attachment by the reduction in upward force on the wing while the load was dropped, followed by the wing folding up during the pull up, or the wing strut brace buckling due to the increase in force during the sharp pullup after the drop.
Just so I can follow along - What do you mean by "brace strut"? The terms I'm familiar with are "lift strut" and "jury strut". The lift struts on a PA-25 carry the lifting load in compression. The jury stuts keep the lift stuts straight so they can support the compression loading.
9th Sep 2023, 19:50

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There are some excellent images showing the PA-25 strut configuration here - https://en.wikipedia.org/wiki/Piper_PA-25_Pawnee

Remember that the wing attachment to the fuselage is a pair of hinges (one bolt at each front spar fitting and one bolt at each rear spar fitting) then you can see what the struts do.
10th Sep 2023, 00:53

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Originally Posted by EXDAC
Just so I can follow along - What do you mean by "brace strut"? The terms I'm familiar with are "lift strut" and "jury strut". The lift struts on a PA-25 carry the lifting load in compression. The jury stuts keep the lift stuts straight so they can support the compression loading.
Yeah, I'm definitely no expert on the Pawnee. I was talking about the lift struts. And I knew the wing was hinge mounted to the fuselage without a solid spar wingtip to wingtip through the airplane, and that those struts carried the lift in compression. Until I saw the enlarged picture I did not know that there were 2 on each side, never mind that they had jury struts too, or what all those were called. Doesn't really matter as my dissertation was largely theoretical, and not limited to this specific model, but learned something new, so thanks.
10th Sep 2023, 01:54

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Originally Posted by hans brinker
Until I saw the enlarged picture I did not know that there were 2 on each side,
To torsional rigidity of Piper rag wings is low. Just like the high wing Pipers, the PA-25 has a one lift strut connected to the front spar and one lift strut connected to the rear spar. The lift struts control the twist of the wing as well as supporting the lifting loads.

There have been several PA-25 AD on lift struts and the fuselage lift strut attach cluster. It will be interesting to learn if this accident aircraft was compliant.
10th Sep 2023, 02:43

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Originally Posted by EXDAC
To torsional rigidity of Piper rag wings is low. Just like the high wing Pipers, the PA-25 has a one lift strut connected to the front spar and one lift strut connected to the rear spar. The lift struts control the twist of the wing as well as supporting the lifting loads.

There have been several PA-25 AD on lift struts and the fuselage lift strut attach cluster. It will be interesting to learn if this accident aircraft was compliant.
That makes sense, didn't think of wing torsion. I am decent with the classic mechanics, but never got any engineering education. To me it looks like a very sharp pull up, followed by the wing folding up, fold line from the leading edge/fuselage to trailing edge/lift strut, with the wing folding backwards as well as upwards. That would be a failure of the front lift strut. We will find out if the maneuver was outside of the envelope and/or the maintenance was the problem.
10th Sep 2023, 03:26

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Applying too much "g" will do it to any airframe.

10th Sep 2023, 09:47

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There is a lot of 'CUB' technology in a PA25 with the emphasis being the ability to 'lift' an economical load for agriculture dressing. It also benefited from being a good 'strip' machine and/or even for non strips. Most of its normal ops are in a low speed mode and its load is dispersed over several runs. Non of all this makes it an ideal aerobatic or display machine, and even the strut configuration is less than ideal, as struts preferred to be 'pulled' not pushed, and a major part of their job is the torsional control of the fabric covered wing. When I collected my PA 25 (from Holland) it was in superb condition and performed as expected, but it had no G meter, and as banner towing requires a 'pull up' I fitted one as it was quite obvious that with a no payload and associated equipment other than the driver, the lifting ability of those wings was incredible and the stick loads very light. I had seen a Pawnee demonstrated at a Headcorn airshow. It flew past the crowd line slowly in typical nose down spraying mode with all the nozzles working and gave everyone a good dose of a well known 'Perfume'. !!! I can smell it now 50 years later, so it was a good display item.
10th Sep 2023, 14:45

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Originally Posted by Vessbot
You have doubled the load factor, but you have not doubled the load (i.e., lift). After the drop it remains at 1, same as before.

Are any of these aircrafts fitted with a basic acceleromater as the ones fitted in aerobatic aircrafts ?, Trainers have the standard +6G -4G units that have the possibility to wire lock and seal the reset button.
Any overloads passed certified ones for the type with a red line on both positive and negative G's limits would thus be saved for safety purposes
10th Sep 2023, 17:35

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measuring the G factor

Originally Posted by markkal
Are any of these aircrafts fitted with a basic acceleromater as the ones fitted in aerobatic aircrafts ?, Trainers have the standard +6G -4G units that have the possibility to wire lock and seal the reset button.
Any overloads passed certified ones for the type with a red line on both positive and negative G's limits would thus be saved for safety purposes
Not a standard fit out of the factory, in fact the PA 25 'panel' is very limited** although the knife edge cable cutter in front of the windscreen does rather highlight the potential low level spraying issues. I well remember my first actual 'aerial work' op out of a limited strip with an empty hopper, and associated pump and spray bars removed. The 235 HP Lycoming made short work of the strip but the low level circuit seemed to be rather endowed with turbulence that was never an issue with a C180/182. It was simple case of being in a machine that had a surplus of lift (and power) and all the small gusts were readily felt far more.
A fantastic 'cool' performer and great fun to fly, with the hopper lid big enough to fit jerry cans in for 'away days'. The original chemical 'aroma' never went away despite hosing with all manner of detergents and disinfectant. I collected the Pawnee from Holland in the winter and knew the cloud base over the other side of the Channel was likely to be an issue, but the nice chaps from the airfield fitted me up with an electric T&B which was very useful until I intersected the main railway line that ran from Kent past Headcorn and into the Redhill circuit. Headcorn asked for a flypast (there were many Pawnees based there for years) and Redhill suddenly announced the runways were closed (due snow) so we used the peri track (as in the Tiger Club days).
11th Sep 2023, 05:42

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The original chemical 'aroma' never went away despite hosing with all manner of detergents and disinfectant.
Quite a few years ago at a gliding comp, I was asked to get the QFE reading from the altimeter of one of the tugs (a Pawnee of course) to advise gliders on final glide on any change they should apply to their own altimeter setting. (long story here - I'll spare you the details). I dropped the door and poked my head inside the office to be greeted by the unmistakable 'aroma' of 2-4D Ester 80%. I was well familiar with it as the herbicide of choice for cruciform weeds in cereal crops and along with most farmers had used it myself. as the then primary attack against the most common and troublesome weeds. It must have been many years since this workhorse had been retired to glider towing, but the memories of its earlier lifetime persisted.

I commented on this to one of the tuggies - "Hell yes!" he said "they all smell like that!".
11th Sep 2023, 18:27

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Seconded Pobjoy. I've flown a few Pawnees (its one of my favourite types) and that aroma was always there. Great aircraft though. I wrote about someone crashing my favourite PA-25 in 'Requiem for a Pawnee'.
11th Sep 2023, 22:03

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My aircraft had struts (in tension.) I was surprised at the apparent lightness of the spar/fuselage attachment until I realised that it was mostly in compression and that two thirds of the fuselage load was carried through the spars. Not only that but, because of the strut angles the load on the strut/wing/fuselage fittings was twice or more the actual fuselage weight. Add in up to 3G and those fittings really were the critical ones. (Of course they are all critical but they were the ones I worried about most!)

The same, but in compression would apply to the Pawnee struts.
12th Sep 2023, 01:34

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Originally Posted by FullOppositeRudder
Quite a few years ago at a gliding comp, I was asked to get the QFE reading from the altimeter of one of the tugs (a Pawnee of course) to advise gliders on final glide on any change they should apply to their own altimeter setting. (long story here - I'll spare you the details). I dropped the door and poked my head inside the office to be greeted by the unmistakable 'aroma' of 2-4D Ester 80%. I was well familiar with it as the herbicide of choice for cruciform weeds in cereal crops and along with most farmers had used it myself. as the then primary attack against the most common and troublesome weeds. It must have been many years since this workhorse had been retired to glider towing, but the memories of its earlier lifetime persisted.

I commented on this to one of the tuggies - "Hell yes!" he said "they all smell like that!".
It is now many decades since aircraft have been approved to spray 24-D Ester. I'm actually glad about that as it does save us some nightmares wondering what volitised drift may have occurred after application. However, I do use it at home and must admit a kindred feeling during the Spring when it's time to attack those broadleaf weeds.
12th Sep 2023, 02:08

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I think I have flown 4 different PA-25, all glider tugs. I don't remember any of them smelling of chemicals. Maybe the smell dissipates faster in the hot and mostly dry SW USA where I flew them.

Light stick forces in a PA-25?? Not any I flew but I was spending most of my flight hours racing standard class gliders so I suppose it's all relative.

Back to the accident - Has anyone seen a preliminary report yet?
12th Sep 2023, 06:41

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Originally Posted by DaveUnwin
Seconded Pobjoy. I've flown a few Pawnees (its one of my favourite types) and that aroma was always there. Great aircraft though. I wrote about someone crashing my favourite PA-25 in 'Requiem for a Pawnee'.
Not sure the exact chemical, but the Pawnee glider tug I flew years ago hadn't sprayed in decades, had the hopper removed, and still had a very strong, very distinct chemical smell.... that was a great little airplane.

Of note to this unfortunate accident, there are a number of structural ADs for the Pawnee... the main spar cluster being one of them. The one I flew had severe corrosion discovered in an aft fuselage cluster after I'd already flown it for a season...required a bunch of structure to be cut out and new structure welded in.

The structure of ag planes in particular gets exposed to some very nasty, very corrosive chemicals..... and some of the more complex bits offer places for chemicals to collect and start to cause issues.... The newer ag planes, like modern Air Tractors have done a lot to address this, with very easily removable fuselage panels and extremely good corrosion proofing and sealing of structural stuff.
12th Sep 2023, 10:29

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Originally Posted by hans brinker
Wing is still producing the same lift, so no change in load on wing attachments.
There is a distinct difference between load (force) and stress. I was talking of internal stress. The stresses on the wing attachments may change significantly due to a change in total aeroplane mass.
Originally Posted by hans brinker
But the releasing the load does not lead to an instant increase in positive load on the wing attachment, if anything in a decrease in positive load.
You need to distinguish between total loading on the entire aeroplane, which has a defined positive and negative direction, and internal stresses for which the coordinate system may be completely different. You're trying to equate total loading to internal stresses and that doesn't work. In the same vein, the analogy you used with the string on the balloon has no relation to what happens inside a wing structure.

I think we'll have to leave the engineering track for now. There is enough information and speculation, as well as very useful information, in this thread and until we can learn more from a preliminary report, or the final one, we will most likely continue to run in circles.
12th Sep 2023, 11:16

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The tone of questioning reminds me of one of my first flying instructors questions...

Q
What would you do if I fell out of the aircraft?

A
Re-trim immediately.
12th Sep 2023, 23:45

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Anyone who’s been a standing passenger on a bus knows what it’s like when the driver accelerates and then brakes. A similar affect is applied to the wings with a sudden decrease in weight and a rapid increase in load factor. Repeat it often enough with ageing metal, possibly affected by corrosion or chemicals, and eventually something will fail.