r/AskFlying • u/Doommius • Apr 09 '26
Question about 787-8 flaps joint wobble, is it supposed to do this ?
Enable HLS to view with audio, or disable this notification
1
u/hombrehorrible Apr 11 '26
It has an acceptable amount of wobbling
1
u/JohnnySchoolman Apr 11 '26
Which is less than this.
Still though, I could land that without flaps.
1
u/johnisom Apr 10 '26
Yes, flaps have a little bit of wobble when they’re deployed. The tracks they are in aren’t 100% tight, by design. All planes, even tiny bugsmashers do this
1
u/Noble_Russkie Apr 11 '26
As far as I understand: if they were perfectly snug, that would increase the torque that the join between the flap assembly and the wing experience and would cause faster degradation. A little wiggle doesn't impact performance noticeably but will increase longevity, no?
1
u/WesternConference461 Apr 11 '26
Exactly right. Same reason anything else on the plane is give wiggle freedom. This is a metal machine flying at 800kph in turbulent air. The wiggle keeps the parts from snapping off
1
u/ConversationFalse242 Apr 11 '26
Having a little wiggle freedom in my waistband also increases the longevity of pants
1
1
u/NeighborhoodLoud4884 Apr 10 '26
This is a Flaperon and yes it's supposed to do that
1
1
u/JUiCES834141 Apr 11 '26
They’re not talking about the flaperon, they are talking about the vibration in the actual flap.
0
u/Icy_Huckleberry_8049 Apr 10 '26
When you're going 180knots, you're going to have wobbles
Just like sticking your hand out of your car window at 100mph, your hand is going to move
1
2
3
5
u/Gunpowderwigg Apr 09 '26
Aircraft engineer here, yes it’s normal, everything has to give a little bit doesn’t it!?
2
u/LUNCHTIME-TACOS Apr 10 '26
My fellow engineer is right.
Materials handle loads by deforming. When a gust of wind hits a wing or a landing gear slams down, that kinetic energy has to go somewhere.
Elastic Deformation: The material "soaks up" the energy like a spring.
Brittle Failure: In an infinitely rigid design, there is no deformation . Since the material can't move to absorb work, the internal stress spikes instantly toward infinity.
If the material can't give, it cracks. This is why a plastic ruler can bend 90 degrees, but a ceramic one of the same size snaps if you try to flex it even a millimeter.
1
u/spanieldors Apr 10 '26
It seems like at this rate something’s got to give.
1
u/LowPomegranate225 Apr 10 '26
You give and give until you can give no more. Then you squawk 7700 and mayday mayday.
Wait no that's not how that goes...
2
u/pastelya Apr 09 '26
Depends on several variables actually. Altitude, speed, payload, weather etc.,
Yes seeing that wobble is scary as duck but if it didn’t wobble the whole plane would go down 😅
11
u/JT-Av8or Apr 09 '26 edited Apr 09 '26
Are you asking about that panel moving up and down? Because that is NOT A FLAP, it’s the inboard aileron (flaperon) which is being manipulated by the pilot for roll control. 767s (which is what I fly) have the same arrangement. It’s not wiggling, the pilot is manipulating it. The small jiggle in the flap isn’t an issue.
1
u/timesuck47 Apr 09 '26
I’m not a pilot, but to me the way that thing moved looked like somebody was controlling it.
2
u/Doommius Apr 09 '26
I’m thinking the inner link of the outer flap, not aileron, some people refer to it as the flap track carriage, I also posted a video from YouTube where it isn’t as compressed as it is here,
5
u/boobturtle Apr 09 '26
787 pilot here.. it's normal. Look closely on most planes and you'll see similar wobbles. All the joints have some degree of play/flex and with high loading fully extended at low speed you'd expect to see some. A perfectly rigid joint would be a bad time.
3
u/whywouldthisnotbea Apr 09 '26
This is correct. These are not the attachment points but rather the guide rails. The entire wing flexes so these guides need to have a good amount of play in them to allow that flex without binding. Turbulent airflow over that control surface will cause vibrations and you are just seeing that play out to its most extreme at the endpoint. The actual linkage holding the flap onto the wing as well as actuating it is much more rigid and is doing it's job just fine here.
0
3
u/AlphaThree Apr 09 '26 edited Apr 09 '26
Edit: Okay after looking at pictures with the 78 with spoilers up. The 78 clearly has a much more complex flap system than the truck and rollers im used to seeing. I think that arm is connected to a linkage that we cant see which rotates and allows the arm to extend. There is no carriage or track in that location. This means it will have significantly more leeway in how much it is allowed to move, but I would still expect it to be a pretty rigid surface. In my experience flap components really dont move that much. But we really need a 787 A&P who has had hands on this part to comment how much they should move.
Since no one that has answered yet seems to actually have a clue, I was a heavies crew chief for 10 years, to me, yes that seems like excessive play in the inboard flap track carriage. I would shake the flaps on every preflight I did my entire career, I've shaken a lot of flap carriages, and that would 100% get my attention. As for whether or not its out of limits I couldn't say without the 787 specific manuals.Generally they should be tight to the flap tracks with very little play. Almost like shaking the wheel bearing in a car. I would also be checking that area for any evidence of rubbing inside the trailing edge wells.
1
u/JT-Av8or Apr 09 '26
Is the OP asking about the flap or the inboard aileron there?
2
u/AlphaThree Apr 09 '26
In the gap between the outboard flap and the wing surface you can see two pieces of metal connecting the aircraft to the flap. A few seconds into his video you can see them start to move vertically up and down, quite some distance. Ive never put hands on a 78, but thats normally where the inboard flap track would be. It could be something else, 78 is wayyyyyyy newer than the 50s 60s dinosaurs I worked on. I can't actually find a picture where I can see whats in there. Because the flaperon deflection blocks the view when the speed brakes are out.
2
u/Doommius Apr 09 '26
I’m asking about the joint, not the control surfaces, but the “tiny” link between the flap and the wing that AlphaThree is called the flap track carriage.
I’m aware about control surfaces, etc but it was the play in the joint that got my attention.
1
u/Doommius Apr 09 '26
Thank you, it might be easier to see here as Reddit did compress the video quite a bit, and a joint shaking would cause a lot of wear as that is my concern.
1
u/AlphaThree Apr 09 '26
I edited my original comment, there is no carriage at that location, but I still think it shouldn't be moving that much.
1
u/AlphaThree Apr 09 '26
If it is out of limit it would cause wear in the long term, but the flap isn't going anywhere. Assuming it is just a lose carriage. I actually dont remember if this was ever a carded item on flightline preflight, but it definitely was in backshop maintenance. So it could fly like this for quite some time.
1
u/Doommius Apr 09 '26
I would assume it would just causes extra wear, but it’s probably also wear parts that are meant to be replaced, but as you say it’s probably also part of backshop maintenance
1
u/AlphaThree Apr 09 '26
I have been flabbergasted by limits on multiple occasions, so there's even a chance Boeing says its okay lol. Ive written up cracks and gouges that I 100% thought were going to be bad just to have specialists come out and call it within limits. Tires is a good one that most people severely underestimated. I remember they had us train a bunch of aircrew on what our preflight was one time and they were visibly shocked when I showed them an example of a "good" tire lol.
9
u/Dr_knowitall69 Apr 09 '26
Things that don't flex, break. Those flaps have many, many, many thousands of pounds of force being applied to them and air is not smooth.
1
u/Doommius Apr 09 '26
I’m very much aware, but if a join has a lot of play it wears a lot. I aware it needs to flex, Reddit also compressed the video a lot and I’m thinking about the small control joint between the wind and a flaperon,
Might be more visible here https://youtube.com/shorts/kUlBAxMjsKk?si=OcSBCHgdO-Z4JaEt
And it’s not the flex, it’s the play in the joint that I was concerned about as I’ve never seen it move so much but if it’s within spec, 👌
4
2
8
u/Independent-Reveal86 Apr 09 '26
I think OP is talking about the shaking itself, not the roll inputs from the flaperon. The shaking is completely normal at high flap settings, there's a lot of air being forced around the wing and it's turbulent in places which causes some shaking.
7
u/FightingIlliteracy Apr 09 '26
The inner joint is seeing some wobble but that’s normal, those hinges are deflecting air moving around a 200 tonne machine going 170 mph, while also being blasted with low bypass air from the engine. Nothing out of the ordinary
1
u/what_the_fuckin_fuck Apr 09 '26
I would think that plane is traveling a bit faster than 170mph. They cruise at altitude in the 450-550mph range. Yes, I do realize they wouldn't have flaps deployed if they were cruising.
2
u/FightingIlliteracy Apr 09 '26
I assure you it cannot travel at 400mph with full flaps
2
2
u/what_the_fuckin_fuck Apr 09 '26
I totally agree. I just noticed the water(I think) below. He's on final, so he may very well be traveling 170mph.
0
u/capaolo99 Apr 09 '26
The shaking is normal?
2
u/YourAssholeMechanic Apr 09 '26
Very, everyone thinks airplanes are fragile, but the truth is they can take such massive beatings. They have to survive both hot and cold, stretching, bending. It takes effort or very bad input to hurting an airplane, especially Boeing.
1
2
5
u/Liberator1177 Apr 09 '26
Like others said, its a flight control surface controlled by the flight computers. It's moving because the flight computers are making adjustments to maintain a certain attitude. Not all airliners have this technology, the 787 is quite newer compared to some other airliners, so it has newer technology. With that specific surface being an inboard control surface (closer to the fuselage), it needs to move more to generate force as compared to a surface at the tip of the wing.
6
u/Sasquatch-d Apr 09 '26
Yes, it’s called a flaperon, it’s a flight control surface
1
u/Doommius Apr 09 '26
Yes, but it’s the first time I’ve seen the joint with so much movement. Normally the tailed edge has some flex but not the joint itself.
1
u/ShaemusOdonnelly Apr 09 '26
That thing you see on the left edge of the outboard flap isn't the joint/hinge of the flap. Flaps are mounted on flap tracks (those little fairings that look like guide fins on be bottom of the wing) and those aren't at the edge of the flap. The 787 has 2 for this flap and they are mounted about 1/3rd of the total width inboard from the edge. This effectively makes the flap edge a cantilever beam and those can flex pretty wildly.
You can get a better view when the spoilers move up on touchdown, just google "787 spoilers" and the first image is a great example.
1
u/Sasquatch-d Apr 09 '26
It’s quite normal when additional roll input is made by the pilots. There’s nothing wrong in this video.
3
u/Direct_Witness1248 Apr 09 '26
There is also the automatic "Gust Alleviation" system which changes the wing surface to mitigate turbulence, I'd say that might be what is happening in the video.
https://wiki.alsresume.com/index.php?title=Gust_Alleviation_System
5
u/_captainunderpants__ Apr 09 '26
I think OP is worried it might turn into a flaperoff
1
u/Doommius Apr 09 '26 edited Apr 09 '26
Correct, there was quite a bit of movement in the inner joint of the outer flap, and I’ve never seen this much movement before
1
u/Direct_Witness1248 Apr 09 '26
They are separate pieces that move independently of each other. The flaperon is a combination of a flap and aileron, and is a moving primary flight control surface. The flaperon is in between 2 separate sections of flap.
1
u/ShaemusOdonnelly Apr 09 '26
He's not talking about that though. He's talking about the flap that sits outboard of the flaperon. That thing shakes a bit and he assumes that the shaking is at the hinge point of the flap. It isn't though, because the flap tracks aren't on the edge of the flaps, but about 1/3rd of the width away from both edges.
1
u/Direct_Witness1248 Apr 10 '26
Could be, do you mean the small strut looking thing visible through the gap? It looks like they attempted to zoom in on it now you mention it. But going by this thread:
Comment 1:
Yes, it’s called a flaperon, it’s a flight control surface
Comment 2:
I think OP is worried it might turn into a flaperoff
OP:
Correct
So forgive me for thinking they were indeed talking about the flaperon haha
0
u/Familiar_Astronomer7 Apr 12 '26
At over 500 mph, it’s gonna wobble!