Wings coming off

[SixPapaCharlie]

By now I am sure everyone has seen the video of the Crop Duster whose wings snapped off.

This is the 2nd incident I am aware of where dumping water resulted in the wings folding. The former being the C-130 repurposed for fire fighting.

I don't understand the physics.

Why would quickly making the plane lighter increase stress in the positive G direction.
Seems like it would do the exact opposite making the center of the mass lighter.

There is a pitch up before they snap but it doesn't look like he yanked it.

For reference, the first few seconds of this video (I understand the pilot survived):

 

[Lindberg]

I wonder if it has to do with Va decreasing with the loss of weight + hauling back on the stick.

Heavy & fast -> light & yanking

 

[Everskyward]

For reference, the first few seconds of this video

That was an... interesting... selection of clips. What was up with the pigeon?

 

[Clip4]

Increased Gs with the pitch change.

 

[aterry1067]

load factor = Lift/Weight
n=L/W

A significant loss of weight considerably increases g load. Think of the same reason the maneuvering speed decreases as weight decreases.

 

[brcase]

Ok I will try an explanation
Let’s say the plane pilot fuel weighs 1000lbs
The plane is carrying 1000lbs of water.
In level flight the plane is generating 2000 lbs of lift at 1G.
Now we drop 1000lbs of water but the speed and angle of attack stays the same and we are still generating 2000lbs of lift on an airplane that only ways 1000lbs but now the airplane is accelerating upward with a 2G load and the only control we moved was the dump lever. Now add a bit of a pull up along with and we can increase that load even more.

Now that by itself will not cause a structural failure. Unless there was structural problem with the airplane then the only way this could really happen is if it was flying above VA.

Brian

 

[kyleb]

Can't speak for the crop duster, but the C-130 had known spar cracks and was being flown under an inspection program. Obviously (to me, at least) the inspection regimen wasn't thorough enough, wasn't frequent enough, or otherwise allowed the cracks to progress to a point where the wings failed in normal use.

 

[Rich Holt]

Sen. Collins tries to explain this phenomenon during a presser. It's kinda old but still relevant.

 

[Bill]

Sen. Collins tries to explain this phenomenon during a presser. It's kinda old but still relevant.

Brilliant!

 

[bflynn]

Now that by itself will not cause a structural failure. Unless there was structural problem with the airplane then the only way this could really happen is if it was flying above VA.

I gotta think structural problem, but I don’t know the information for the plane. It’s possible they were near the flight envelope and dropping the water plus pitch up took them out of it.

 

[GeorgeC]

 

[midwestpa24]

I don't believe in either case, the final dump/maneuver was the cause of the structural failure. The C-130 for sure was a fatigue issue, and I'd guess the crop duster we will discover the same. The Arrow that lost the wing was the same, it was just climbing out from takeoff, not maneuvering at all. Airframes can handle quite a bit of stress 1 or 10 or 10,000 times. Its the last time when it finally shows its weakness, and as the pilot you probably won't which one it will be. That's why it is so important to have proper maintenance and inspections. Those cheap "Paper Annuals" can be quite expensive.

 

[wanttaja]

Read somewhere that this was *not* a duster, but a dedicated water-bomber type aircraft. If it was a duster, there's the potential for corrosion from the chemicals.

Someone posted (somewhere...) that there is also a sudden CG change as the load is dumped. Doesn't make much difference on a duster, where the load is spread out over several minutes, but the water bombers want everything gone NOW. Story was that the load was slightly aft, so the plane would pitch down. With a low-level dump run, the pilot would be tuned to twitch the stick back to compensate quickly.

Ron Wanttaja

 

[Dan Thomas]

I don't believe in either case, the final dump/maneuver was the cause of the structural failure. The C-130 for sure was a fatigue issue, and I'd guess the crop duster we will discover the same. The Arrow that lost the wing was the same, it was just climbing out from takeoff, not maneuvering at all. Airframes can handle quite a bit of stress 1 or 10 or 10,000 times. Its the last time when it finally shows its weakness, and as the pilot you probably won't which one it will be. That's why it is so important to have proper maintenance and inspections. Those cheap "Paper Annuals" can be quite expensive.

Yup. During inspections I paid attention to the wing skins. Any indication on the bottom skins, particularly near the spars, that the rivet heads had shifted, was a bad sign. Any wrinkling of the tops skins was bad, too. Same for the stabilizers. Check for any signs of cracking around strut and wing root attachments and stab attachments. Stuff being repeatedly stressed will usually leave signs that the airframe is tired out.

A walkaround $200 annual will find none of that. Cropsprayers are hardworking airplanes and should get even more attention than usual.

I don't believe the G loads increase with the dump. That wing is lifting all that weight, and removing the weight removes the load factor on the wings. Va relates to accelerated stalling under load factor, and this is not an accelerated stall situation, either loaded or unloaded. Those wing spars suffer compression loads on the top, and tensile loads on the bottom; compression loads can compress the metal over time so that when the load is removed, it cracks and then collapses.

Edit: BOTH wings let go, implying that the spar center section failed. That center section is inside the fuselage, where leakage from the hopper could get at it and cause corrosion. There are plenty of corrosive compounds in those chemicals, especially in fertilizer.

 

[Dan Thomas]

...but the water bombers want everything gone NOW.

I worked on them. They had options to time and meter the release to draw a line across the ground with the retardant, or they could dump it all at once, but it still took some time. The gates are only so big. They had to have that option in case of engine failure on climbout.

 

[eman1200]

I think it's because they broke*

*NTSB, feel free to use that in your report if you like.

 

[Unit74]

The real point of this vid is 6PC leaving for work at the 2min mark. It appears he is very blessed, and very "active" for sure.

 

[steingar]

Saw a video of a Mooney wing folding. Of course he was pulling hard out of a 3000 foot/minute dive.

 

[Warren Dunes]

I think it's because they broke*
*NTSB, feel free to use that in your report if you like.

Naw... It was because he failed to file a VFR Flight Plan.

 

[CJones]

Read somewhere that this was *not* a duster, but a dedicated water-bomber type aircraft. If it was a duster, there's the potential for corrosion from the chemicals.

Someone posted (somewhere...) that there is also a sudden CG change as the load is dumped. Doesn't make much difference on a duster, where the load is spread out over several minutes, but the water bombers want everything gone NOW. Story was that the load was slightly aft, so the plane would pitch down. With a low-level dump run, the pilot would be tuned to twitch the stick back to compensate quickly.

Ron Wanttaja

The plane in this particular video appears to be a spray plane. You can see the spray booms, air-powered water pump, and cowl-mounted light bar installed. The spray planes have the same dump gate to use for emergency as the fire drop guys, though.



Total armchair quarterbacking here, but he appears to be coming in 'hot', too - way faster than he would be if actually spraying. Doesn't look like a turbine, so I'm spitballing here and guessing that working speed is somewhere around 120-130mph maybe? It also looks like the left wing rolls up before the right wing. Makes me wonder if he was trying to do a hard roll to the right while also pulling up. I remember at a formation clinic WAY back where Doug Rozendaal mentioned how rolling while the airframe is already 'loaded' with force is a big no-no. Basically pull OR roll, but never both at the same time.

 

[Matthew Johnson]

load factor = Lift/Weight
n=L/W

A significant loss of weight considerably increases g load. Think of the same reason the maneuvering speed decreases as weight decreases.

But the spar only knows/cares about stress, not g loading: 2k lbf is the same it it whether the airplane is at 1g or 2g.

 

[TonyG]

But the spar only knows/cares about stress, not g loading: 2k lbf is the same it it whether the airplane is at 1g or 2g.

My take: you're both right (at least when the dumped load is in the fuselage). Background: I've got an ag endorsement, but I'm also a credentialed rocket scientist (Ph.D. Astrophysics) and engineer (Aero and Electrical both) - which means I know almost nothing about a lot of poop. Or maybe a lot about a little.

When you do an emergency dump in real life (especially the first time) there's suddenly a lot going on. Per aterry1067, the airplane loads up immediately. Per Matt Johnson, "so what- the spar sees the same thing, before and after". Both true. But there's potentially more going on immediately afterwards.... some airplanes may be at the edge of control stability. If you've ever flown aerobatics in an RV-4 or similar with a barely in-bounds aft cg, you may have seen this: above 3-4 g's, you have to push FORWARD (not stick movement, no kidding pressure) to keep it from loading up further. If something similar is at play here, the loading could run away before it could be suppressed. Plus, this is a really significant transient/impulse - I understand the design and certification rules to be a little silent on all the crap they can cause.

Having done these dumps, my guess is there's some combination of an instability and PIO at play. (You also end up slamming your helmet into the cage above you on the first one as you overcompensate for the increased g).

Bottom line: Honestly dunno, but there's a lot of semi-violent sh&t going on in the airplane in a really short period of time, and that's seldom good.

--Tony

 

[MauleSkinner]

Having done these dumps…there's a lot of semi-violent sh&t going on in the airplane

Even if the words are a full paragraph apart, they join together in my brain.

 

[nauga]

When you do an emergency dump in real life (especially the first time) there's suddenly a lot going on. Per aterry1067, the airplane loads up immediately. Per Matt Johnson, "so what- the spar sees the same thing, before and after". Both true. But there's potentially more going on immediately afterwards.... some airplanes may be at the edge of control stability...

H/T to @Matthew Johnson for pointing out the load vs. acceleration discrepancy, and you bring up some good points; however, there are a couple of other things that come into play: The load *distribution* changes. Note that the typical failure mode is at the root, where the attachment points are on a non-strutted wing. Losing a bunch of weight from the fuse will result in a reduction in wing root bending moment. Losing weight in the wing will cause an increase in the bending moment (upward). Another factor at play is that unless the load being dropped is at the CG there will be a trim change regardless of (in)stability, causing a nose-up or down pitching moment depending on the placement. And @Lindberg mentions the decrease in Va and "hauling back on the stick" - in reality when it may not take "hauling" to overstress, depending on how fast you are and what happens to the bending moment at the root. "Pickle, pause, *pull*" is real, and the pull does not necessarily need to be all that hard , especially when compounded by other factors like corrosion and fatigue.

Nauga,
who has seen stubby little wings bend a lot.

 

[Warren Dunes]

Which means I know almost nothing about a lot of poop. Or maybe a lot about a little...

"The process of modern education is to learn more-and-more, about less-and-less, until eventually you know absolutely-all-there-is-to-know about nothing-at-all." ---My high school physics teacher.

 

[TonyG]

Even if the words are a full paragraph apart, they join together in my brain.

Doh!

Losing a bunch of weight from the fuse will result in a reduction in wing root bending moment. .

That's absolutely right, but the reduction in bending moment comes after the return to unaccelerated/steady state flight. All the hoppers that I'm aware of are in the fuselage, so @Matthew Johnson is right about the immediate response (short of the trim change you mention). That trim change could be enough to increase the moment enough to fail a corroded spar. Most hoppers are as close to the CG as they can get them, but....

Either way, a lot going on in a short time. Add corrosion, and things could go south fast.

My old man flew F-100's in Vietnam. They'd launch from strip alert for ground support with a variety of stores at different stations - on that long swept wing. Sometimes they'd need to release the inboard stores first, and would be in an unapproved configuration after the release (I recall him saying it was bad because of both aft CG and a loading that would induce some weird wing twisting.) Hard on the airplanes, almost killed him once.

--Tony

 

[Tarheelpilot]

Just as a point of reference the large turbine ag planes, specifically the ones designed for dual use on ag and fire, have published limitations for max dump speeds. The recommended procedures for fire drops involve things like flaps and speeds on the low side of the envelope.

Smaller aircraft such as the one in this thread do not have those limits published. I doubt the engineers thought it was necessary. Primary uses of dumps: engine failure, overloaded takeoff, cleaning and dry product application. None of those involve everything leaving now at a high speed.

many times ag pilots ding the wing removal switch they are showing off and find themselves uncomfortably outside of the envelope.

 

[gkainz]

I think there was a P2V Neptune a few years back that had a wing failure. Grounded that fleet (permanently), IIRC?
Then wasn’t there a spotter plane that also had a wing fail a couple of years ago?

 

[kyleb]

I think there was a P2V Neptune a few years back that had a wing failure. Grounded that fleet (permanently), IIRC?
Then wasn’t there a spotter plane that also had a wing fail a couple of years ago?

You may be thinking the PB4Y Privateer.

And yes, a spotter came apart too. Can't remember what that was.

 

[gkainz]

You may be thinking the PB4Y Privateer.

And yes, a spotter came apart too. Can't remember what that was.

You’re right - A friend of mine retired from the Navy as an AD (engine mech) and worked at Hawkins and Powers in Greybull, WY on them until they shut down.

http://www.ruudleeuw.com/greybull.htm


I saw a P2V Neptune fire bomber somewhere not that long ago … conflating memories!

https://neptuneaviation.com/firefighting/

Maybe I was recalling this as well.

https://www.aopa.org/news-and-media...30/neptune-p2v-fire-bomber-retires-to-montana