Piper crop spraying type crash in Mexico.

[Jhieminga]

Structural failure during a pull-up:
https://x.com/ondisasters/status/1698287725411987690?s=46&t=pcIEUl8R2uPorHJJ31eXTw
Pilot did not survive the crash.

See also: https://aviation-safety.net/wikibase/345178

[IFMU]

That was pretty horrible. I know nothing of Mexican aviation, but I wonder if the wing spar attach AD was complied with, or if one of the STC fixes was installed.

[ehwatezedoing]

Everybody seems clueless on the video...

[atakacs]

That wasn't crop spraying in any shape or form but some stunt for a wedding or similar occasion.
Doesn't change the end result but possibly did ask too much of the aircraft 🤔

[treadigraph]

"Gender Reveal". Oh dear...

[Jhieminga]

That wasn't crop spraying in any shape or form but some stunt for a wedding or similar
Didn’t mean to imply that it was spraying crops. I just couldn’t figure out the type from the video and thought ‘Piper crop spraying type’ to be a decent approximation. The interesting question is whether the manoeuvre was less severe, similar or more severe than during normal spraying ops.

[DuncanDoenitz]

The result is, of course, tragic and traumatic.

But if I've got this right, and I don't care how well they've cleaned it, they are using pesticide-delivery equipment to spray vapour onto the participants at a family event which includes a pregnant woman.

[Bksmithca]

The result is, of course, tragic and traumatic.

But if I've got this right, and I don't care how well they've cleaned it, they are using pesticide-delivery equipment to spray vapour onto the participants at a family event which includes a pregnant woman.
What's a little pesticide amongst friends

[chopper2004]

RIP

https://www.dailymail.co.uk/news/article-12475681/gender-reveal-plane-crash-mexico.html

[POBJOY]

Pilot is climbing at the same time as operating the hopper dump facility. Not a problem for a 'sound' machine, but these aircraft have been doing this for years when spraying, and there are numerous 'AD's' for struts, centre cluster. wing fittings, that need to be kept up. In actual fact the 'dump' reduces the actual load instantly which could mean an increase in the climb rate. In normal spraying ops the load is gradually reduced over several runs although some 'spirited' turns are frequently seen between runs. The combination of a high lift wing. plenty of power, and the ability (if wished) to instantly reduce load could easily lead to an overstress situation, and the wings are load lifters but not for abrupt manoeuvres.

[DaveReidUK]

https://www.pprune.org/accidents-close-calls/654577-piper-crop-spraying-type-crash-mexico.html

[LowObservable]

Pilot is climbing at the same time as operating the hopper dump facility. Not a problem for a 'sound' machine, but these aircraft have been doing this for years when spraying, and there are numerous 'AD's' for struts, centre cluster. wing fittings, that need to be kept up. In actual fact the 'dump' reduces the actual load instantly which could mean an increase in the climb rate. In normal spraying ops the load is gradually reduced over several runs although some 'spirited' turns are frequently seen between runs. The combination of a high lift wing. plenty of power, and the ability (if wished) to instantly reduce load could easily lead to an overstress situation, and the wings are load lifters but not for abrupt manoeuvres.
That's what I thought. My impression is that cropdusters dump in level flight and stop spraying before they turn. In this case the pitch-up starts a fraction of a second after he starts dumping water, and then the pitch rate increases for another fraction of a second before the wing starts to fold, and he's still dumping as the wing comes off.
And the party-goers mostly don't hear the wing go above their cheers, and the crump of the impact is a couple of hundred yards behind them.

[India Four Two]

Hmm, I fly Pawnee tow-planes regularly. I'm going to be looking into the AD history of the ones I fly!

[markkal]

Hmm, I fly Pawnee tow-planes regularly. I'm going to be looking into the AD history of the ones I fly!

An energetic pullup after a high speed pass (Probably at VNE) produces high loads that may well exceed the structural strenght of the longeron. Hence the failure. I would reckon this pullup would be in the 4G's + range, and may not have been the first one with this aircraft.

[POBJOY]

Pobjoy has never 'sprayed' with his PA25 (235) but the machine is a great Banner & glider tower. In fact the 'pull up' for a banner launch is not needed for spraying other than to avoid obstacles around fields. Glider launching is airframe friendly but the engine requires careful use on the descent to avoid shock cooling, and of course both operations are not carrying a hopper load which means you are well below max weight.
The top struts/fittings on a Pawnee are very important and subject to some fairly strict inspections but the basic wing is a lift junkie not an aerobatic tool. If you happen to be pulling some G and dump at the same time it is probably going to increase the G very quickly and 'possibly' the wings outboard of the struts will be taking the extra strain. A stripped out PA 25 without the spray equipment is an express lift and of course has no large fuel load, so the elevator input should be careful.

[IFMU]

It makes zero sense for the structural loads to go up when the load is dropped. The angle of attack will momentarily be the same, lift will be the same, and the aircraft will rise. As you get more vertical speed the relative wind effectively reduces angle of attack and also you have less weight.

A force-body diagram would be a good means to understand the forces.

[DuncanDoenitz]

On the face of it I would agree with IFMU, but this would seem to be contradicted by (I think) at least 2 C-130 fire-bombers which have suffered similar wing/fuselage separation immediately following payload-release. Could it be a shift of CG causing an exagerated response to pilot's pitch-up input?

[Chu Chu]

With the aircraft suddenly climbing, would the reduced angle of attack on the horizontal stabilizer due to the change in relative wind tend to pitch the nose up (if not countered)?

[Jhieminga]

Just a little theory exercise: Your G is nothing more than L/w, which means that by decreasing w, we're also increasing G if the rest stays the same. I don't know how much weight is in a full hopper (relative to total weight of the aircraft) but dumping half your weight would effectively put 2G on the airframe. If you also initiate a pull-up, you need to add that to the Gs already there from the dump. This is based on everything else staying the same though... so keep in mind that the aircraft rising, airflow changing etc will have an influence too.

[DaveReidUK]

I don't know how much weight is in a full hopper (relative to total weight of the aircraft)

Capacity is 150 USG, so roughly 1,250 lb.

[Jhieminga]

1,250 lb is 567 kg. MTOW is 1317 kg according to Wiki. Assume a hopper that's half full, say 300 kg and a total weight around 1200 kg. Dropping that load will give you 1200/(1200-300) = 1,3 G instantaneously. If we're using MTOW and a full hopper, it goes up to 1,76 G. Not a problem on its own, but it all adds up.

[Chu Chu]

The string on a helium balloon doesn't break when you let go of it.

[DuncanDoenitz]

1,250 lb is 567 kg. MTOW is 1317 kg according to Wiki. Assume a hopper that's half full, say 300 kg and a total weight around 1200 kg. Dropping that load will give you 1200/(1200-300) = 1,3 G instantaneously. If we're using MTOW and a full hopper, it goes up to 1,76 G. Not a problem on its own, but it all adds up.
But it isn't the g that kills the aeroplane, its the bending moment of the wings; g is just a convenient means of quantifying the bending moment for a given condition, and for advising limitations. Reduce the mass to a theoretical 1lb, and it could survive 1200g. 1.3 g on a 900lb aeroplane gives the same "weight" as 1.0g on a 1200lb aeroplane.

The increased normal-acceleration may, of course, introduce a change of attitude/alpha; and therein may lie the problem.

[Jhieminga]

Judging by the way the wing folded, it will be the combined shear stress and internal bending moment at the inboard wing fittings that need to be looked at. The PA-25 has a strutted wing which could mean a local bending moment of zero at the bolts, leaving the shear stress as the remaining factor. A change in G-loading will influence this and I was just trying to illustrate that dumping a hopper full of chemicals could have contributed significantly to the total stresses in the structure, even though at first 'lightening the load' appears to be a positive thing. I think that's as far as we need to go with the theories...

[Mr Optimistic]

I think the detached wing will keep on flying so its initial motion will initially be up won't it?. If you were to model it as two flexible wings suspended by a spring at each cp with a bag of flour at the centre the wings would bend upwards at the tip. Drop the bag of flour and the bending moment at the root/joint unloads rapidly. Yes the cg would accelerate upwards but can't see the harm from that.

[hans brinker]

1,250 lb is 567 kg. MTOW is 1317 kg according to Wiki. Assume a hopper that's half full, say 300 kg and a total weight around 1200 kg. Dropping that load will give you 1200/(1200-300) = 1,3 G instantaneously. If we're using MTOW and a full hopper, it goes up to 1,76 G. Not a problem on its own, but it all adds up.

I have only flown pax to their destination, thus have sofar strived to arrived with the same payload as I departed with, so I am no expert.
Having said that, I believe the release of payload in flight would not lead to an increased G load on the aircraft. If we assume the aircraft maintains straight and level flight with no change in speed there cannot be any load change on the aircraft. In order to maintain that with a decrease in weight from dropping the load the pilot will have to reduce angle of attack, and that will reduce wing load, with a reduction of the load on the wing attachments, not increase it. If the pilot allows the aircraft to drift up because of the release of weight, he would experience that G load, but the wing attachments would not, as the lift produced by the aircraft would remain constant.
This looks like a case of the pilot pulling up too fast, and loading up the wing beyond what it could handle. That could be because the CG changes when dropping the load. And maybe the wing spar wasn't as strong as it should be.

[IFMU]

I don't know how much weight is in a full hopper (relative to total weight of the aircraft) but dumping half your weight would effectively put 2G on the airframe.
You are pulling 2G, but at half the weight. Loads would be the same at time =0+ seconds. Would go down from there.

I agree the G load due to pitch input is additive.

[physicus]

@IFMU Just do a simple thought experiment on the load factor formula: n = lift / weight.

Assume 1 for lift as it remains the same throughout the exercise. Further assume weight = 1: n = 1/1= 1. If you shed half the weight, your new weight now is 0.5. so n becomes 1/0.5 = 2. So you just doubled the load factor by sheeding half your weight.

That explains why the load factor momentarily and appreciably increases when you suddenly dump a largish amount of liquid from an airplane. that may well increase it beyond its structural limits.

And for any fellow physicists out there: yes it pains me to talk about weight. It's mass, really, we should be considering, not weight.

[ZeBedie]

The pilot may feel 2g, but the structure doesn't. Dumping from the fuselage, wing bending load is reduced, in fact.

[IFMU]

The pilot may feel 2g, but the structure doesn't. Dumping from the fuselage, wing bending load is reduced, in fact.
Exactly!

[Chu Chu]

If the mass of the fuselage is n and the force required to "accelerate" it at 1g is X, the force required to accelerate a fuselage with a mass of n/2 at 2g is also X. Of course the wings aren't massless, so reducing the fuselage mass by 1/2 would result in an acceleration of less than 2g, and reduce load on the wing spar even when the acceleration is taken into account.

[hans brinker]

@IFMU Just do a simple thought experiment on the load factor formula: n = lift / weight.

Assume 1 for lift as it remains the same throughout the exercise. Further assume weight = 1: n = 1/1= 1. If you shed half the weight, your new weight now is 0.5. so n becomes 1/0.5 = 2. So you just doubled the load factor by sheeding half your weight.

That explains why the load factor momentarily and appreciably increases when you suddenly dump a largish amount of liquid from an airplane. that may well increase it beyond its structural limits.

And for any fellow physicists out there: yes it pains me to talk about weight. It's mass, really, we should be considering, not weight.

Ahm no. No. NO.
Imagine a balloon filled with helium with a weight attached to it floating around. Cut the string, and the weight falls down and the balloon shoots up. That is what happens when an aircrafts releases its payload in flight. Either the aircraft goes up, or the load factor is reduced. But absolutely never will the load factor increase from a decrease in payload.

[MechEngr]

Wings are a source of weight and lift. Fuselage is a source of weight. Payload is a source of weight. Ignore the horizontal stab and the negative lift in normal flight.

Lift = W_wing + W_fuselage+W_payload. If Lift remains constant after release of payload then W_wing + W_fuselage must both go up, redistributing W_payload between them by acceleration in proportion to their weights.

Since the connection between wing and fuselage also took the full W_payload, then after redistribution some of that weight will be in the W_wing and so the connection between the wing and fuselage will be less. The excess lift will not be able to put all of the weight into accelerating just the fuselage.

Imagine at the extreme a flying wing so W_fuselage = 0. The redistribution from dropping the payload is from wing to wing+; the bending in the middle goes down as the weight is more evenly distributed across the span by acceleration of the entire wing due to the now excess lift.

The connection at the wing root held until the outboard segment twisted up. It looks like the plane was pulling up after the dive to release the powder.

It looks like the forward strut failed; the most common failure mode would be buckling. It's supposed to have mid-strut braces to prevent that, but they may have been damaged previously.

Since the strut was under compression at the time maybe the outer strut fitting failed from a fatigue crack. The one on the fuselage is at such an angle I would not look there first.

See https://www.federalregister.gov/documents/2015/04/29/2015-08732/airworthiness-directives-various-aircraft-equipped-with-wing-lift-struts

(3) We are issuing this AD to detect and correct corrosion and cracking on the front and rear wing lift struts and forks, which could cause the wing lift strut to fail. This failure could result in the wing separating from the airplane.

[krismiler]

Similar to the crash of a Hercules doing firebombing a few years ago where a wing also came off. A sudden major reduction in weight and a sudden increase in wing loading during a pull up.

[Uplinker]

If a wing is providing a lift force to carry a certain weight, and that weight is removed; the stress and strain in the wing structure will be reduced. Like the string on Hans Brinker's balloon: the tension in the string, that was holding the weight up, suddenly reduces to near zero when the string is cut.

I suspect that wing failure in accidents of this nature is actually caused by a rapid unload, then re-loading of the wing, as the payload is dumped, followed by a max rate pull up. The fin of that A300 was snapped off by a rapid load - unload - load sequence applied by one of the pilots.

If you have an aircraft such as a water bomber or a crop sprayer that is subjected to rapid pull ups fairly often - in a series of alternately maximum and zero payloads, i.e loading / unloading / loading; and this is coupled with ageing and possible corrosion of the internal structure - which might not be inspected or spotted during inspections - you might have a recipe for serious structural failure?

PS; physicus, on planet Earth, the weight of an object is the attraction force produced by the action of the Earth's gravity on the mass of that object.

[physicus]

I stand corrected. Again. :)

And here I was finally hoping to have found a good example of where intuition misleads us... well, it doesn't. If the mass is dropped from the fuselage, then indeed as others have said, the load on the wing-fuselage junction will in fact decrease momentarily. If, however, the mass is dropped from the wings, then the load on the wing-fuselage junction will momentarily increase. In both scenarios the physics follows intuition.

Uplinker because gravity is not a constant but a function of where in XYZ space you are located, it is scientifically inaccurate to refer to the weight of an object without also knowing what the local conditions of the gravitational field are - hence mass being the preferred term. Interestingly, this varies by about 0.7% on the surface of the planet (owing to different rock densities in earth's crust), and of course decreases as you gain altitude and go further away from earth's surface. Other bodies like the moon and the sun play a role as well, and not an insignificant one (c.f. the tides). So, a 1kg mass on a mountain in Peru will measure about 995 grams on the same scale it reads about 1005 grams at the surface of the arctic ocean.

In aviation world, this is often evidenced by the "pragmaticists" in FAA world who for example refer to maximum take of weight (MTOW). Whereas in the "idealist" EASA lands, the same number is referred to as maximum takeoff mass (MTOM).

Just an interesting sidenote...

[Uplinker]

Do water bombers and crop sprayers contain their water or pesticide payloads in or under the wings then?

I thought they had tanks in the fuselage?

[MechEngr]

It is convenient to refer to Weight vs Lift even in accelerated frames of reference, neglecting the precise contribution from gravity acting on mass. To do otherwise seriously complicates notation in the local aircraft coordinate system.

[Jhieminga]

Ahm no. No. NO.
Imagine a balloon filled with helium with a weight attached to it floating around. Cut the string, and the weight falls down and the balloon shoots up. That is what happens when an aircrafts releases its payload in flight. Either the aircraft goes up, or the load factor is reduced. But absolutely never will the load factor increase from a decrease in payload.
That analogy doesn't work. Or if you do want to stick with this: When you cut the string, the lift on the balloon is appreciably more than its weight (I know, it should be mass...) which equates to a L/w of more than 1. If you were to be inside that balloon, you would feel that as a perceived G-force.
If a wing is providing a lift force to carry a certain weight, and that weight is removed; the stress and strain in the wing structure will be reduced.
Not exactly, it will change, but it's a bit too simplistic to call that a reduction or any kind of beneficial effect without knowing the specifics of that change. Others have commented on the W_wing and other relations. Let's look at what happens inside a wing. I can't provide the shear and moment diagrams for these wings right now but think of the wing as a simple beam, supported at the inboard end by the fixings to the fuselage (I'm ignoring the strut for now). When that wing is being used (prior to the drop), there is a force, trying to bend the beam upward, which is distributed over the entire span and we call that lift. The structure weight of the wing and any other bits (fuel inside it) are pulling the wing in the other direction. These forces do not cancel each other out as the lift force is significantly higher than the others (half the total weight) and therefore there is a distinct bending moment and other forces on the attachement fittings. From this also follows that the beam itself needs a distinct amount of rigidity and strength to cope with these forces on it. If a significant load is dropped from the fuselage, the attachment load will change rapidly. Combine this with a pull-up that will increase the lift force on said wing and the attachment fittings will see a rapid change of stresses/moment.

As pilots, we are all trained to respond to configuration changes and other effects, so bearing in mind that I have never done any flying like this, the normal reaction of the pilot on dropping a load from a hopper would be to push the stick forward, decrease AOA and thereby keeping the aircraft on its level flight path. That avoids the aeroplane moving up, although you can still get a momentary flight path deviation of course. In this case (we're veering away from the theory again, apologies) the drop and pull-up appear to be (difficult to see on this video) timed pretty close together and that's why I started wondering about the drop and the pull-up ganging up on an unsuspecting bit of metal in the airframe.

[Cedrik]

Typical prune thread.... The aircraft suffered structural failure due to faulty/defective parts. An Ag aircraft is designed to drop it's load either slowly or rapidly. Gender reveal stupidity is American, there have been a few aircraft crash doing this.

[physicus]

Blancolirio nailed it: The problem is the manoeuvring speed lowers when you shed mass. Or weight as he says. :)

He apparently used to be involved with air tanker operations and states that they're training to unload the wing slightly during the drop, and to never pull high g/bank immediately after the drop. Because your manouevring speed is now lower...

https://www.youtube.com/watch?v=wp19ch40t0c

[Pilot DAR]

In my opinion, using aircraft as "attention getters" during social events is a poor idea, and to be avoided. For obvious reasons, a failing during such "displays" ruins the event, and probably lives. Pilots should consider declining such requests, and just stand there with everyone else and enjoy a finger food.

As silly as this pilot was, I have to hope what whatever he was dispensing over a crowd had more colour than mass. Though I agree with the theme that a sudden load release changes the physics of the flight, I would be surprised to hear that this load, in this situation, was a major factor in the structural failure.

The video shows the left wingtip twisting upward (increasing angle of incidence relative to the root), then the entire left wing folding up from the fuselage. It happened so fast, that the additional momentary lift of the left wing could not induce any brief roll to the right. So I doubt that the wing spar itself failed, but more a failure of the wing structure in torsion, so that a sudden increase in lift resulting from the torsional failure, caused the left upper wing strut attach at the fuselage to fail. As those wing struts act in compression, the failure would be instantly total. I opine that overly exuberant pilot pushed a less than ideally airworthy airplane just a little too far.

[FullOppositeRudder]

I've noticed that our pilots in the AT-802s feed in increasing forward stick and down elevator during the unload of a fire drop as seen in this photo during a training exercise a few years ago. The elevator deflection is very obvious here; note also that SOP involves a small degree of flap application during the actual dump.

https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/1024x787/188a4631_acrr_257b2fed8d192710334c0163dd1debab6604ef26.jpg

[EXDAC]

In aircraft with strut braced wings the spar attachment to the fuselage is usually a simple single bolt (or pin) hinge. Without the struts in place the wing would be free to hinge up and down and could not support any lifting load. There is no spar bending load at the wing root.

[Vessbot]

@IFMU Just do a simple thought experiment on the load factor formula: n = lift / weight.

Assume 1 for lift as it remains the same throughout the exercise. Further assume weight = 1: n = 1/1= 1. If you shed half the weight, your new weight now is 0.5. so n becomes 1/0.5 = 2. So you just doubled the load factor by sheeding half your weight.
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.

That explains why the load factor momentarily and appreciably increases when you suddenly dump a largish amount of liquid from an airplane. that may well increase it beyond its structural limits.


But the structure fails at a certain load, not load factor.

[Capn Bug Smasher]

It makes zero sense for the structural loads to go up when the load is dropped. The angle of attack will momentarily be the same, lift will be the same, and the aircraft will rise. As you get more vertical speed the relative wind effectively reduces angle of attack and also you have less weight.

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.

[fdr]

That is a pretty unfortunate what to end the day.

From the start of the pitch up, the aircraft achieves an attitude change of around 20+ degrees, (needs measurement to determine the aspect change) but that happens in... ~11 frames of a 30-FPS image but he looks of it, and that is a pretty impressive pitch attitude change angle, the rate is much more impressive. The load on the aircraft is related to the resultant normal force which comes from the angle of attack that is applied. The angle of attack is analogous to the difference between the instantaneous pitch and the flight path vector. Normally we consider steady state conditions and these are not, this is a dynamic pitching case, and the usual assumption that the wing will stall at a certain angle of attack and that will limit the normal force that may be derives is not quite correct.

Unsteady effects are generally assumed to start to feature when a change in state occurs within the time that the flow takes to travel the chord of the airfoil. That has been the considered case, however, research on dynamic pitching of rotor blades shows that the effects occur outside of that assumption, and they alter the characteristic curves of CL, CD and CM vs AOA significantly. Where a wing may stall at say, 15 AOA normally, dynamic pitching will delay stall and values well above 20 AOA will still result in increasing CL, and therefore over short periods normal force from the wing. The video shows that the flight path is changing, and that rate of change comes off the pitch change to give eventually the steady state AOA. In FBW systems from the F16 through A320 and B777, there is a pitch rate ramp up that is achieved. For a fly by cable design, the elevator deflection is essentially instantaneous to the control input by the pilot, the rate is purely dependent on the drivers physiological arm motion limits. Dynamic CL, CD and CM are spectacularly different to the assumed vanilla steady state charts of characteristic CL, CD, CM to AOA, and are subject to biggly hysteresis, CL ends up looking like an italic P that over lays and extends beyond the inverted U [sine] of the CL curve; CD ends up looking like a drunk oblate spheroid, a flattened O with the case of the leans, and instead of mainly being a slightly increasing negative line until a fairly abrupt shift to positive, CM looks like a scribbled letter T, inked by someone with a bad dose of MND)
Pulling abruptly might be good for lawn mower starters, but is not great on planes. Actually, it isn't good for lawnmowers either.

AA 587 resulted from an sequence of inputs that just had to be about as unfortunate a career move as is conceivable. While nasty, in isolation each input would not have resulted in a bad day, but the sequence resulted in a rapid increase in bending and torsion loads, alternating direction and being exacerbated by the resultant yaw which lags. Even then, one of the fundamental badnesses was the structure of the tail had a primary and secondary load path as is needed by failsafe design, but the failure of one results in an effective lever load applied to the other that exceeds its isolated load limit at the same time. The force limiting of the A300-605 in common with the OEMs practice is a throw limiter which results in an increase in sensitivity compared to a limiter that reduces the hydraulic pressure applied, but allows the full movement of the rudder pedals.

The LE translates upwards while the TE initially stays near the normal geometry, so the outboard fitting of the forward compression strut would be interesting to look at. Once it fails, the torsion as well as the rolling moment resulting from the geometry change is going to lead to a rapid bad day.

Be gently on the onset of control inputs, someone else might have been as rough as we are, most materials have a really long memory, and planet earth doesn't give way for aircraft.

Did the wing fail prematurely? I suspect... not. The PA-25 by recollection has a 4412 equivalent airfoil, which has a fairly soft stall break, dynamic pitching will lead to much higher CL than expected before the section stalls, so it is possible to get higher instantaneous loads. The forward fitting will show a compressive/buckling failure, but, there could be prior damage, a close look at the components will tell the story. That the strut gave way in the sequence is on the video, the question is why did it start to buckle. If you really need to hit hard pull ups, go do it in a Pitts or Extra, not in a normal or utility category aircraft I would suggest.

[spornrad]

The dynamic load must have been dramatic. He dropped water, within a second reducing wing load, wing relaxing / flexing down. Would not be surprised if the resonant frequency of that wing structure is about 1 / s. Just after half that period or so he increased the wing load abruptly, at the worst possible instant. AA587 comes to mind indeed.

[Vessbot]

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


The question is, which of these correlates to wing stress?

[Capn Bug Smasher]

The question is, which of these correlates to wing stress?

All of them :ok:

[Lucerne]

The result is, of course, tragic and traumatic.

But if I've got this right, and I don't care how well they've cleaned it, they are using pesticide-delivery equipment to spray vapour onto the participants at a family event which includes a pregnant woman.
The risk of any actual contamination is extremely low in that instance. The likelihood that the aircraft was well decontaminated prior to this event is very high. I would very much doubt if the unrealistic fear of agricultural chemical contamination is going to form a part of the investigation.

We have many dual purpose Air Tractors across the USA and Australia spraying in the morning and firebombing in the afternoon. Even our chemical-phobic government agencies don't blink an eye at the low potential of significant contamination of off-target areas despite the fact that it has been drawn to their attention.

Of course there are a number of Air Tractors dedicated to firebombing operations only but many aren't. Most have a recent history of both spraying and firebombing operations.

[DuncanDoenitz]

Lucerne; Thank you!

(Still not sure it's on my bucket list though).

[DaveReidUK]

Gender reveal stupidity is American, there have been a few aircraft crash doing this.

Maybe just trail a blue or pink ribbon next time ... ?

[Thrust Augmentation]

Quite low coming in with trees ahead, did the combined dump & pull-up simply turn the wing into a high drag barn door which the structure wasn't capable of supporting?

[FlexibleResponse]

An interesting analogy for those of us who have dropped bombs (and it would include other loads such as water bombers and helo external loads)…

Imagine a Lancaster bomber of a nominal weight of 45,000 lbs carrying a Tallboy bomb of 12,000 lbs for an AUW of 57,000 lbs .

The bomber is flying and trimmed in straight and level in (unaccelerated) flight at 18,000 feet and at an airspeed of 150 KIAS.

To do so, the power is set at say, 85% power and the wing AOA is generating lift force of 57,000 lbs to equal the total weight in level flight.

The crew then release the bomb over their target without changing any of the control positions or power setting. (Let’s ignore any trim changes due to changes in CG or airflow effects).

On releasing the bomb, the aircraft now weighs only 45,000 lbs…but crew maintains the power, speed and pitch attitude which are still set for a lift force of 57,000 lbs.

Will the aircraft:

A. Continue to fly straight and level at 150 KIAS;
B. Immediately descend with the bomb release;
C. Immediately climb as the lift force of 57,000 lbs far exceeds the new a/c weight of 45,000 lbs (increase in g); or
D. Enter a victory roll?

Now try the same bomb release in accelerated flight pulling up or recovering from a dive with increasing AOA and ask the same question re g force change.

[Uplinker]

The Lancaster bomber will climb after bomb release, but the forces in the wing structure will be less than they were when they were carrying the bomb.

With the bomb on board the wings are 'pushing up' with a force of 57,000lbs. After the bomb is released, the wings are still pushing up with a force of 57,000lbs, but the weight they are carrying is now 12,000lbs less, so the forces in the wing structure must be less - even though the aircraft climbs, and even though the aircrew will feel positive g.
i.e. The wings do not suddenly push up more at the point of bomb release - they are 'pushing up' the same as they were.

I have watched the video explanation about manoeuvring speed, but am confused that reducing the fuselage weight the wings are carrying somehow increases the stress in their structure? How can that be ?

I think this pilot simply pulled up too aggressively, and that is what over-stressed the wing. The reason he pulled up too aggressively was that he applied a larger pitch force that would have been required with the payload on board - learned as he took off and flew the aircraft towards the venue - but he forgot to moderate his control inputs after the payload had gone.
e,g, say he would need N degrees of up elevator to pitch up when heavy; he would only need, say, a half or a third of N to pitch up when empty, but he applied N, which in the now very light plane was way too much elevator input.

If he had held the stick and not moved it in pitch at payload release, the aircraft would have climbed, since the elevator was already applying a high degree of elevator pitch-up to enable the wings to carry the payload. But then on top of that he applied even more pitch up to pull up, and this combined with the already high elevator deflection, simply rotated the (now empty) aircraft much too aggressively which rapidly took the wing AoA way beyond what it was built for, (at the speed he was flying), which overstressed the wing structure, (or the wing structure was weakened due to age or corrosion).

[pilotmike]

The pilot appeared to approach the drop very low, quite fast, and possibly in a slightly decent.

Almost immediately after dropping the load, there was a dramatic pitch up, which appears to have been the cause of the failure. As others with engineering / aeronautical knowledge have said, dropping the load by itself doesn't instantaneously add any load to the wings, so it should be discounted as the direct or sole cause of the over-stressed wing. Most likely, in the adrenalin rush of the drop, the brain's messages to the hands on both the release control and on the joystick might well have become conflated to a significant extent. This might have been exacerbated by the rapidly approaching trees, and even the desire for a dramatic exit - after all, the desired effect was a dramatic display. Whether for not and pre-existing faults were present, clearly and sadly it all conspired to end this pilot's life shortly afterwards.

It is a sad fact that too many pilots, planes, helicopters and innocent bystanders have become victims of this understandable desire to give the most dramatic display at various weddings, fetes, or other gatherings of friends / family, and often to pilots who do not have the display flying skill, judgement or currency to pull of the attempted maneuvre successfully, or without putting themselves and other at considerable risk. The desire to impress is a very dangerous one, which has and will continue to cost too many lives.

[hans brinker]

An interesting analogy for those of us who have dropped bombs (and it would include other loads such as water bombers and helo external loads)…

Imagine a Lancaster bomber of a nominal weight of 45,000 lbs carrying a Tallboy bomb of 12,000 lbs for an AUW of 57,000 lbs .

The bomber is flying and trimmed in straight and level in (unaccelerated) flight at 18,000 feet and at an airspeed of 150 KIAS.

To do so, the power is set at say, 85% power and the wing AOA is generating lift force of 57,000 lbs to equal the total weight in level flight.

The crew then release the bomb over their target without changing any of the control positions or power setting. (Let’s ignore any trim changes due to changes in CG or airflow effects).

On releasing the bomb, the aircraft now weighs only 45,000 lbs…but crew maintains the power, speed and pitch attitude which are still set for a lift force of 57,000 lbs.

Will the aircraft:

A. Continue to fly straight and level at 150 KIAS;
B. Immediately descend with the bomb release;
C. Immediately climb as the lift force of 57,000 lbs far exceeds the new a/c weight of 45,000 lbs (increase in g); or
D. Enter a victory roll?

Now try the same bomb release in accelerated flight pulling up or recovering from a dive with increasing AOA and ask the same question re g force change.

The Lancaster bomber will climb after bomb release, but the forces in the wing structure will be less than they were when they were carrying the bomb.

With the bomb on board the wings are 'pushing up' with a force of 57,000lbs. After the bomb is released, the wings are still pushing up with a force of 57,000lbs, but the weight they are carrying is now 12,000lbs less, so the forces in the wing structure must be less - even though the aircraft climbs, and even though the aircrew will feel positive g.
i.e. The wings do not suddenly push up more at the point of bomb release - they are 'pushing up' the same as they were.

Agree with that.
If the pilots keep AOA constant the lift produced will remain at 57K so the aircraft will start pitching up and climbing. The pilots will feel positive G, but the wing load will remain steady (eventually the increasing pitch will lead to a decrease in speed).
If the pilots kept the altitude constant the lower pitch would lead to an increase in speed. The pilots will feel no G load, but the wings attachments will initially experience a reduction in load. (eventually the speed would stabilize at a higher point)
If the pilots keep the pitch constant it would be somewhere in-between the previous two scenarios.

[hans brinker]

Ahm no. No. NO.
Imagine a balloon filled with helium with a weight attached to it floating around. Cut the string, and the weight falls down and the balloon shoots up. That is what happens when an aircrafts releases its payload in flight. Either the aircraft goes up, or the load factor is reduced. But absolutely never will the load factor increase from a decrease in payload.


​​​​​​​That analogy doesn't work. Or if you do want to stick with this: When you cut the string, the lift on the balloon is appreciably more than its weight (I know, it should be mass...) which equates to a L/w of more than 1. If you were to be inside that balloon, you would feel that as a perceived G-force.


And that is what I said. The pilot will feel an increase in G load. But the string (=wing attachment) feels a decrease in load, because the weight stopped pulling from the other side.

[MechEngr]

Maybe just trail a blue or pink ribbon next time ... ?
Fly the pink plane or the blue one.

I went to a gender reveal once. Misunderstood the nature of the event. No one there returns my calls.

[hans brinker]

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.

The question is, which of these correlates to wing stress?

All of them :ok:

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.

[DuncanDoenitz]

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 .........

[EXDAC]

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.

[EXDAC]

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.

[hans brinker]

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.

[EXDAC]

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.

[hans brinker]

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.

[megan]

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

https://www.youtube.com/watch?v=mhOoJvTdjsQ
https://www.youtube.com/watch?v=2X5pe7bzHX4

[POBJOY]

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.

[markkal]

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.



But the structure fails at a certain load, not load factor.

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

[POBJOY]

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).

[FullOppositeRudder]

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!".

[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'.

[ChrisVJ]

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.

[Lucerne]

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.

[EXDAC]

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?

[sgs233a]

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.

[Jhieminga]

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.
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.

[CISTRS]

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.

[krismiler]

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.

[megan]

https://www.youtube.com/watch?v=wp19ch40t0c

[MechEngr]

That video is a bit off. The curve as weight decreases scales rather than shifts to the left by the square root of the ratio of the actual weight over the maximum weight. This is because the dynamic pressure, and therefore the resulting lift at a particular airspeed and AoA, varies as the square of the airspeed. Cut the weight in half and, for a given AoA, the plane will need 70.7% of the original speed (sqrt(1/2)) to develop matching lift.

Not sure about this case as I don't know how much water was on board, but if it was 1000# of water and max wgt of 2900#, then the factor for reaching stall AoA for a particular load factor will be scaled left by 0.8, so the maneuvering speed, by that calculation should be 80% of the fully loaded case.

I feel like I am missing why the maximum load factor doesn't go up as the aircraft is lighter. Sure, by 8 or 9 Gs the pilot is the limiting participant, but design for fatigue life results in higher attachment capacities. It also doesn't seem clear why where the payload is stored makes no difference to the maximum load factor. Putting that container on a wingtip would certainly change the characteristics in a way the simple calc fails to capture.

[IFMU]

I feel like I am missing why the maximum load factor doesn't go up as the aircraft is lighter.
Pretty hard to calculate allowed G loading on the fly. Plus if the actual loads are lower from being under gross, that is adding margin to the life of the aircraft.

My aircraft is rated 4.4G at gross, and 6G at aerobatic gross. What if I'm 20 lbs under aerobatic gross? Can I pull 6.2G? What aircraft manufacturer wants to give a pilot that leeway?

[Pilot DAR]

I feel like I am missing why the maximum load factor doesn't go up as the aircraft is lighter.

Mathematically, it does. But, there will not be any approved data to support operations into that corner of the envelope. The limiting G factors are prescribed by the design standards and regulation, not convenience to the pilot for pushing it.

If a pilot chooses to pull really close to the G limit on a decades old airplane, I hope that they know its airworthiness intimately!

[Uplinker]

Out of interest, do small aircraft like this and others have C and D checks, as airliners do?

[EXDAC]

Out of interest, do small aircraft like this and others have C and D checks, as airliners do?

No, they don't, at least not in USA. Typically, for non commercial use, an annual inspection is required. In commercial use a 100 hour inspection may be required if 100 hours reached before the next annual inspection is due.

Airworthiness Directives may impose addition inspection requirements.

[megan]

I feel like I am missing why the maximum load factor doesn't go up as the aircraft is lighterBecause such things as the battery container, engine mounts etc are designed to support the unchanging weight of the respective items to the certified "g" limit, going beyond the certified limit will over stress those respective mounts. Don't think there are many pilots who would appreciate their engine falling off..

[markkal]

Because such things as the battery container, engine mounts etc are designed to support the unchanging weight of the respective items to the certified "g" limit, going beyond the certified limit will over stress those respective mounts. Don't think there are many pilots who would appreciate their engine falling off..

The G'load neither up or down when the A/C is lighter or heavier, It's the stall speed that is affected. A heavier aircraft will stall at a higher velocity than a lighter one for the same amount of G's applied.

[ZeBedie]

Is there a CofG change when the hopper is dumped?

[IFMU]

Is there a CofG change when the hopper is dumped?
The hopper on a Pawnee is on the CG.

[FlexibleResponse]

An interesting analogy for those of us who have dropped bombs (and it would include other loads such as water bombers and helo external loads)…

Imagine a Lancaster bomber of a nominal weight of 45,000 lbs carrying a Tallboy bomb of 12,000 lbs for an AUW of 57,000 lbs .

The bomber is flying and trimmed in straight and level in (unaccelerated) flight at 18,000 feet and at an airspeed of 150 KIAS.

To do so, the power is set at say, 85% power and the wing AOA is generating lift force of 57,000 lbs to equal the total weight in level flight.

The crew then release the bomb over their target without changing any of the control positions or power setting. (Let’s ignore any trim changes due to changes in CG or airflow effects).

On releasing the bomb, the aircraft now weighs only 45,000 lbs…but crew maintains the power, speed and pitch attitude which are still set for a lift force of 57,000 lbs.

Will the aircraft:

A. Continue to fly straight and level at 150 KIAS;
B. Immediately descend with the bomb release;
C. Immediately climb as the lift force of 57,000 lbs far exceeds the new a/c weight of 45,000 lbs (increase in g); or
D. Enter a victory roll?

Now try the same bomb release in accelerated flight pulling up or recovering from a dive with increasing AOA and ask the same question re g force change.


The answer to the accident has been revealed in some of the posts but it is still possibly not clear to all of us.

@blancolirio in his video (above posted by @megan) referred to and explained the phenomena quite well using the terms of change of maneuvering speed with weight change.

Expanding on his line of reasoning, Lift required for level flight is equal to Weight, sometimes expressed as L = Nz x W (where N = g and z = vertical plane and W = weight).

So a Pawnee flying at say 2900 lbs at 124 mph Max weight man speed) in level flight is creating 2900 lbs of lift at the current AOA (L = 1g x 2900).

If the weight of the Pawnee is reduced by 1200 lbs of hopper emergency jettison and the AOA remains constant, the aircraft wings are still generating 2900 lbs Lift, but the Weight has reduced to 1700 lbs.

ie, 2900 = Nz x 1700

That translates to Nz = 2900/1700 which gives Nz = 1.7g

However, the pilot pulls back the stick to almost the stall AOA to achieve the max g of 4.5g at 124mph (the max man speed for 2900 lbs) which is expressed as L = 4.5g x 2900, which gives L = 13.050.

But, he reduces the weight of the Pawnee during the pullup with the emergency jettison to 1700 lbs (13,050 = Nz x 1700).

That translates as Nz = 13,050/1700 which gives Nz = 7.67g

Which clearly is an overstress far in excess of the aircraft ultimate g limits

Now I am not that expert with math so I would kindly ask the Flight Test Engineers on the forum to cross-check my reasoning and figures for accuracy.

[Jhieminga]

I'm no FTE, but your calculations appear to be correct. Even if he didn't pull the full 4.5g he was still adding a significant amount of g to the 1.7 incurred by the load drop leading to the disastrous outcome. It also illustrates, in a sad way, how Va (manouvring speed) is not a fixed number but changes due to a change in aircraft mass.

[RichardJones]

Once again, we are reminded of this. If you bend a piece of metal, long enough and oftern enough it is going to fail. The trick is to find the impending fail point, before it gets you

Also jettisoning a load, at that rate, puts a lot of strain on the airfram. Sadly it is able to bring any fail points to the fore

[Cedrik]

Normally dumping even when pulling up does not overstress the aircraft. Do all the maths you like, doesn't make it so. Talk to an Ag pilot, they actually know. Take the standard waterbombing aircraft now, an 802 is stressed to lower G than a Pawnee, the job is to dump water on fires. The Pawnee referred to was deficient with maintenance which caused catastrophic airframe failure. It is a silly thing to do gender reveals in any way let alone an aircraft.

[RichardJones]

Normally dumping even when pulling up does not overstress the aircraft. Do all the maths you like, doesn't make it so. Talk to an Ag pilot, they actually know. Take the standard waterbombing aircraft now, an 802 is stressed to lower G than a Pawnee, the job is to dump water on fires. The Pawnee referred to was deficient with maintenance which caused catastrophic airframe failure. It is a silly thing to do gender reveals in any way let alone an aircraft.

Well I did 10 years ag flying, on 6 different types. Started 50 + years ago, when just turned 21 years old.
We did practice jettions, using water, every so oftern for possible fire fighting use. We applied solids also, straight out the bottom of the hopper. There is a noticable "rumbling" type vibration if you like. You could feel it through the airframe and stick.
Let me tell you, there is quite a lot of aerodynamic bufferitng, not to mention trim changes. My gut told me it was hard on an aircraft. Disturbed airflow from beneath the elevator/tailplane.
Also chemical is highly corrosive of course. That may have had a part to play. The pullup at the at the point of jettison, whether commanded or uncommanded would have put a lot of stress on that spar, in this accident. The result is there for all to see.
I was asked to do the odd demo. I always refused. The reason being, if I screwed up, I didn't want it to be witnessed by a lot of people. There is no dignity in that.
Air show pilots, have a lot of guts IMHO.

[Peter Fanelli]

What's a little pesticide amongst friends

Good point, and silence from the chemtrail people.

[FlexibleResponse]

We did practice jettions, using water, every so oftern for possible fire fighting use. We applied solids also, straight out the bottom of the hopper. There is a noticable "rumbling" if you like. You could feel it through the airframe and stick.
Let me tell you, there is quite a lot of aerodynamic bufferitng, not to mention trim changes. My gut told me it was hard on an aircraft. Disturbed airflow from beneath the elevator.
.... The pullup at the at the point of jettison, whether commanded or commanded would have put a lot of stress on that spar, in this accident. The result is there for all to see.

These comments from Richard Jones are very interesting, especially his gut telling him that the drop was hard on the aircraft.
Is there something going on that we have not factored in to load dumping?
I wonder if the high density of the water flowing from the hopper under the wing would increase the forces applied around the wing root
attachment structure and strut attachments as a consequence just from the high density water flow and/or buffeting around the wing and tailplane in itself?


https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/635x589/rveue_aa5b68b5ccb52fc7c21157e66047358b87ac332d.jpg

Image lifted from:
https://aviation.stackexchange.com/questions/90733/g-load-during-payload-drop

[RichardJones]

These comments from Richard Jones are very interesting, especially his gut telling him that the drop was hard on the aircraft.
Is there something going on that we have not factored in to load dumping?
I wonder if the high density of the water flowing from the hopper under the wing would increase the forces applied around the wing root
attachment structure and strut attachments as a consequence just from the high density water flow and/or buffeting around the wing and tailplane in itself?


https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/635x589/rveue_aa5b68b5ccb52fc7c21157e66047358b87ac332d.jpg

Image lifted from:
https://aviation.stackexchange.com/questions/90733/g-load-during-payload-drop

Thankyou for taking the time to read and consider my observations etc.
That's quite an accolade, coming from an academic, for a dumb farmboy, who left school at 15 with nothing.
None of us know all the answers. No one.

[RichardJones]

https://youtu.be/uG9baGRCOXE?si=fRQsRz5bm1IWfPtE

[Cedrik]

Yes it does increase the load on an aircraft but.......... maintenance or lack of it was the factor in all three of those wing off accidents. There are over 1000 firebomber 802's flying. As I said they are not stressed for as much G as a normal ag aircraft, they drop 3 ton a time. There have been zero incidents of 802's loosing wings. Every ag pilot dumps loads for training and practice in every aircraft from Pawnee through to the 802 from solids to liquids. There are no supplements any any of the ag aircraft manuals about not dumping loads, it's part of the brief.
I have been flying ag since 1980 and flown most machinery including the 8 on firebombing. Design faults have been cause in the early 60's of a few accidents, ones since have been a lack of maintenance or negligence by the operator and is what ag pilots have to put up with to be employed. I still think gender reveals are a stupid act of any sort.

[RichardJones]

There is strenght in numbers. Wings I mean.

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