If a control cable were to snap in flight...

SixPapaCharlie

SixPapaCharlie

May the force be with you
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First Officer 6PC
Lets say an Aileron or elevator, or rudder, or elevon, or flapperon, or ruddervator, or other cable snaps while you are chugging along...

Would the control surface now free from your white knuckle death grip:
A. Start flapping wildly?
B. Be held neutral by the air flow?

If B, once you slow to land, would the control surface begin to drop causing loss of control?
 
Let go of the controls and see what happens....


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I had the pin between the flap motor and bellcrank for the flaps fall out in flight right after takeoff in my Lancair.
On takeoff I typically use a little flaps. Shortly after getting airborne, I noticed the plane wallow a bit as if it wasn't generating the lift I typically see. Upon gaining a little altitude, I noticed the flaps were trailing in reflex position as if I had retracted them. I landed without touching the flap motor and intentionally a little fast. On the ground the flaps were flapping in the breeze so to speak; totally disconnected from the flap motor.

So, in my case the answer was B: held neutral by the air flow.
Slowing to land they were still held neutral. Upon turning off the runway they slowly settled to the down position as gravity overpowered the force of airflow.
 
Speed, air loads and lack of control stiffness = aileron flutter.

 
SixPapaCharlie said:
Lets say an Aileron or elevator, or rudder, or elevon, or flapperon, or ruddervator, or other cable snaps while you are chugging along...

Would the control surface now free from your white knuckle death grip:
A. Start flapping wildly?
B. Be held neutral by the air flow?

If B, once you slow to land, would the control surface begin to drop causing loss of control?
Click to expand...

I had a similar issue happen to me in a C-182. The trim is a stabilator. Shortly after takeoff with a load of jumpers, the ball bearing that holds the trim in position popped loose, never to be seen again. This allowed the trim to float freely. Fortunately, the plane kept climbing just fine, and I let the jumpers out over the field. Actually got them to altitude.

Coming back down was a bit more of an issue. Every time I moved the elevator, the stabilator would move back to equilibrium. This made for a descent that was more like a roller coaster ride. Once I got onto a long final, I chopped the power and held the trim wheel with my right hand while using my left hand to move the yoke as needed. Since the trim wheel was on the floor, I was bent over quite a bit and could barely see above the glare shield. It made for an interesting landing.

BTW... That stoopid ball bearing, and the spring it's attached to, cost a pretty penny.
 
SixPapaCharlie said:
If B, once you slow to land, would the control surface begin to drop causing loss of control?
Click to expand...
You've got a plane. You like to make videos. Only one way to find out.

41PXWQMFW7L.jpg

Or you could look planes like the DC9. IIRC the the elevator of the DC9 isn't connected to any sort of direct control mechanism. Just flops in the breeze. Its position is set by moving trim tabs at the rear. So when the pilot moves the control column forward or backward, the trim tab moves and thus sets the position where the elevator rides in the wind.

TAFV08P07_19.jpg
 
Velocity173 said:
Yep. He knew that there were houses in his area so he tried his best to get it towards the water. Once realizing his inputs were useless, he punched out.
Click to expand...

Figured it was something like that.
 
I don’t believe light GA designs rely on control cable tension to prevent flutter. So if you want to simulate loss of a cable, just let go of the yoke and rudders. That’s what ya get. Sometimes it’s possible to lose control on only one side btw. Then try and fly using the trim surfaces which have separate cables, or turn using only rudder, etc. good exercise. You’ll be surprised how doable it is. Landing is hard though....


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For some reason, I don't think anyone has answered Briany's "real" question because people have lived to tell about it.
 
Cables don't fail if they're properly inspected at prescribed intervals. That's what legally-mandated inspections are for: to find bad things before they become lethal. But a lot of airplanes seem to get pretty casual inspections, based on the stuff I've found: corroded, frayed, worn cables, seized cable pulleys, worn attaching hardware and hinges, systems far out of rig. You get what you pay for.
 
Dan Thomas said:
Never assume something like that. https://www.google.com/url?sa=t&rct..._tension.pdf&usg=AOvVaw1GwE5oE_xiyGdZqLgiFrNz

Engineers take into account a lot of stuff, and cable tensions figure into control suface stability. I spent enough years adjusting cable tensions according to the manuals...
Click to expand...

I’m not planning on cutting a cable to be a test pilot. And of course cable tension is important. But my guess is in most designs at most air speeds you’re gonna be ok. If tension was critical, given all the ga aircraft I’ve flown with lousy rigging, there would be a lot of crashes by now...

How many Ga designs are mass balanced I wonder... good article...




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the big question is if the surfaces fail together or independently. If you have both Flaps stuck in in the same position you're in better shape than one failed down and one failed up. Same with each of the other pairs.
 
Ravioli said:
the big question is if the surfaces fail together or independently. If you have both Flaps stuck in in the same position you're in better shape than one failed down and one failed up. Same with each of the other pairs.
Click to expand...

Split flaps are bad but you can often un-split with the good side. There is a big difference between a stuck surface and one that is just free and uncontrolled, id think those are less of an issue, flutter aside...


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Dan Thomas said:
Never assume something like that. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=2ahUKEwir56r9wdnfAhUI6IMKHZ3hBB0QFjABegQIBhAE&url=http://acversailles.free.fr/documentation/08~Documentation_Generale_M_Suire/Cellule/Commandes/Cables_de_commande/Controles/Control_cable_tension.pdf&usg=AOvVaw1GwE5oE_xiyGdZqLgiFrNz

Engineers take into account a lot of stuff, and cable tensions figure into control suface stability. I spent enough years adjusting cable tensions according to the manuals...
Click to expand...

Even if cable tension was part of the flutter equation, given that flutter can only happen above Vne, I would not expect to see flutter at moderately low speed even if cable tension was lost.
 
sarangan said:
Even if cable tension was part of the flutter equation, given that flutter can only happen above Vne, I would not expect to see flutter at moderately low speed even if cable tension was lost.
Click to expand...

FAR 23.629

Flutter.

(a) It must be shown by the methods of paragraph (b) and either paragraph (c) or (d) of this section, that the airplane is free from flutter, control reversal, and divergence for any condition of operation within the limit V-n envelope and at all speeds up to the speed specified for the selected method. In addition--


  • (1) Adequate tolerances must be established for quantities which affect flutter, including speed, damping, mass balance, and control system stiffness; and

    (2) The natural frequencies of main structural components must be determined by vibration tests or other approved methods.
Assumptions can kill. I would not presume to predict that flutter wouldn't be an issue if any cable broke. It might not flutter. If it did, my opinion wouldn't matter.
 
If a control cable were to snap in flight...
Click to expand...

....I would call my cousin Roy.....'cause he ain't never saw no airplane crash like the one we gonna have...
 
My CFI took me through a complete lap in the pattern the other day and I never touched the yoke. With an 8 knot crosswind we brought it all the way into the flare with trim and rudders until he finally let me grab the yoke and land it. It was in a 182 too! Gave me a lot of confidence if the situation ever occurs where i lose the elevator.


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SixPapaCharlie said:
Lets say an Aileron or elevator, or rudder, or elevon, or flapperon, or ruddervator, or other cable snaps while you are chugging along...

Would the control surface now free from your white knuckle death grip:
A. Start flapping wildly?
B. Be held neutral by the air flow?

If B, once you slow to land, would the control surface begin to drop causing loss of control?
Click to expand...
Voisin aircraft, in the early days of WWI, didn't have a "balanced" aileron system. Moving the control stick pulled bellcranks on the bottom of the ailerons only...nothing pulled them up. If the plane was sitting motionless, both ailerons just hung down.
39112263535_cc46ab9e79_b.jpg

So...for low, slow airplanes at least...my vote would be "B".

Ron Wanttaja
 
SixPapaCharlie said:
Lets say an Aileron or elevator, or rudder, or elevon, or flapperon, or ruddervator, or other cable snaps while you are chugging along...

Would the control surface now free from your white knuckle death grip:
A. Start flapping wildly?
B. Be held neutral by the air flow?

If B, once you slow to land, would the control surface begin to drop causing loss of control?
Click to expand...
I was about at 300 feet on final in a Cessna 140 when my left flap bracket failed and the left flap blew up. At full flaps with one up and one down you do not have enough aileron control to counter the roll effect. Fortunately the old 140 had stick flaps and I was able to immediately dump the flaps. Electric flaps and I would likely have been toast!
 
At my last annual we re rigged the plane. The control cable for the ailerons is a continuous loop. Depending on where it breaks, I think I could effect a bank in one direction or the other.
 
Dan Thomas said:
(1) Adequate tolerances must be established for quantities which affect flutter, including speed, damping, mass balance, and control system stiffness;

[...]

I would not presume to predict that flutter wouldn't be an issue if any cable broke.
Click to expand...
I don't presume it but the requirement you posted does say the design must account for less-than-perfect stiffness (emphasis added). Personally I'm usually more concerned about trim tabs fluttering (and possibly driving the surfaces) when disconnected than I am with control surfaces themselves disconnected.

Nauga,
avoiding the buzz
 
nauga said:
I don't presume it but the requirement you posted does say the design must account for less-than-perfect stiffness (emphasis added). Personally I'm usually more concerned about trim tabs fluttering (and possibly driving the surfaces) when disconnected than I am with control surfaces themselves disconnected.

Nauga,
avoiding the buzz
Click to expand...
Agree, I know of a Cessna 340 test pilot that was killed because of a bad fiber lock nut. Lost the bolt in the elevator trim tab, tab fluttered and jammed full down trim. Airplane did an inverted 1/2 loop, inverted stall and resulting flat spin. He had a chute on but did not get out in time.
 
FWIW (and by way of example), failure of hydraulically-actuated surfaces on larger airplanes are usually (always, IME) analyzed considering locked (stuck) surfaces or fail-to-float, meaning free-floating as with a broken cable in a mechanical system. ETA: The Maryland F-117 crash being a notable exception :(

Nauga,
who thinks not everything should be free
 
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Dan Thomas said:
Cables don't fail if they're properly inspected at prescribed intervals. That's what legally-mandated inspections are for: to find bad things before they become lethal. But a lot of airplanes seem to get pretty casual inspections, based on the stuff I've found: corroded, frayed, worn cables, seized cable pulleys, worn attaching hardware and hinges, systems far out of rig. You get what you pay for.
Click to expand...
They do fail when a mechanic puts in new cable and failed to use the proper go/nogo measurements that the swedge was secure. Aileron cable released, all you got now is rudder and elevator. Ailerons went in trail, no issues below 80mph, both drooped when stopped. Landed out in the bushes, what ever was straight ahead, flat tire in the process.
 
I was out flying rc planes with my buddy, trying to get a little aerial footage, and we got a little too close. End result: his prop went into my T-28's left wing, and took the outer quarter of the wing off, along with the aileron (foam slices easily!). No problem at all controlling the plane with the remaining aileron...even did a couple of rolls this way, which were slow and sloppy, but still possible.

Now, if an aileron came loose and jammed at full deflection, that's another story.
 
nauga said:
FWIW (and by way of example), failure of hydraulically-actuated surfaces on larger airplanes are usually (always, IME) analyzed considering locked (stuck) surfaces or fail-to-float, meaning free-floating as with a broken cable in a mechanical system. ETA: The Maryland F-117 crash being a notable exception :(
Click to expand...
The United 232 Sioux City crash comes to mind as an example of the control surfaces staying where they were when the failure happened.
 
Velocity173 said:
Yep. He knew that there were houses in his area so he tried his best to get it towards the water. Once realizing his inputs were useless, he punched out.
Click to expand...

That, but also, that plane has a very sudden deceleration, it slows almost to a stop instantly. I wouldn't be surprised if the pilot momentarily blacked out.
 
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