Can ailerons control a broken cable on 1/3 flaps?

[Ernie Martin]

I was hoping to get your opinion on whether there is enough authority in the ailerons to fully offset the unintended roll from a broken cable of 1/3 flaps. My guess is yes, based on the fact that the ailerons are further outboard (providing a greater moment arm) and that at their fullest they displace further down than 1/3 flaps. But it's strictly a guess and hoped others might share their opinion.

Ernie

[Learjetter]

I'll buy that at higher airspeeds. At lower airspeed, or when the outboard portion of the wing is nearing stall, like in the round-out or flare, the aileron would not have control authority to counteract the flap. Likely reasons why you normally fly control-issue approaches at higher airspeeds.

[JimC]

I don't agree that it would work at higher airspeeds but not lower. The dynamic pressure would change on both the flap and the aileron as you slowed - so the single deployed flap would be producing less of a rolling moment as you slowed.

I also don't agree that "control-issue" approaches are always flown at higher airspeeds. It depends on the issue. For example, if I have a broken elevator control cable and I'm using trim to adjust pitch, why would I come in at a higher airspeed?

As far as the original question? We have no way of knowing, but I share your same guess - yes, ailerons could counteract the roll, but you'll need a big dose of rudder to counteract the yaw as well. You might not even have enough rudder - certainly not enough for any crosswind.

[Ernie Martin]

I agree that airspeed shouldn't matter, except for perhaps the cited case of the wing ends being close to a stall. I hadn't thought about yaw, but I think the change in direction is manageable for the few seconds it will take to retract the single flap.

Thanks to both of you for your thoughts. Still hoping others might share their view. My thinking is that, if you're concerned about a flap cable break, you might want to land with 1/3 flaps in fields with longer runways -- plus remember to shove the flap lever up if you have a sudden, unintended roll excursion.

Ernie

[JimC]

Originally Posted by Ernie Martin

for the few seconds it will take to retract the single flap.

In a best case scenario, one can hope it will only be a few seconds.

Originally Posted by Ernie Martin

plus remember to shove the flap lever up if you have a sudden, unintended roll excursion.

Ernie

I imagine it will be hard to let go of the two-handed white knuckled grip I'd have on the yoke. I put my hand on the flap switch during pattern turns to landing but would my first instinct be to raise the flaps or grab the yoke tighter? It's a hard habit to break, I bet.

[Learjetter]

Originally Posted by JimC

I don't agree that it would work at higher airspeeds but not lower. The dynamic pressure would change on both the flap and the aileron as you slowed - so the single deployed flap would be producing less of a rolling moment as you slowed.

I also don't agree that "control-issue" approaches are always flown at higher airspeeds. It depends on the issue. For example, if I have a broken elevator control cable and I'm using trim to adjust pitch, why would I come in at a higher airspeed?

Jim, spot-on about the yaw.

As for the rest: I was speaking "generally"...not Skymaster-specifically. Wings are generally designed to stall at the tip first--as you increase AOA and slow, you normally lose control effectiveness in this order: aileron, elevator, rudder. As you decrease AOA and increase airspeed, you'll regain control in the opposite order. For this reason, as you slow for landing, if you have flight control issues (bound aileron, jammed elevator, iced-over/frozen rudder, etc) it's likely the POH recommends a higher airspeed to ensure the remaining flight controls are more effective. Skymaster POH may be different?

So, if there's a "stuck flap" and only one is in the wind, since it displaces more air than the smaller aileron, and is closer to the wing root, (and is therefore more effective since the wing is usually specifically bent or shaped to force a stall at the tip first) the flap will exert more rolling and yawing moment than the aileron at the same airspeed, I still contend that at lower airspeeds the flap will overpower the same displacement angle of aileron throw. How's that for a run-on sentence? (I'm on an iPhone...)

I'm willing to be wrong, as while my total time exceeds 4000 hours, my Skymaster time is less than .1% of that... Standing by for schooling...

[macaman]

The stall progresses from the tip with a swept wing but with straight wing aircraft, the root is designed to stall first. Chances are I've missed something, it is late. Check out a classic Martha King video for an example:

http://youtu.be/sKzbeWwe0wM

In theory, the wing with the flap down would have the root stall before the root on the unaffected side. There is also the thought of slipping to reduce the airflow over the flapped wing but sirens are going off in my head talking about slow and uncoordinated flight in the same breath. I like the fast approach to a long runway if it cannot be fixed as in my mind, I've already given ownership of the plane to the insurance company at that point and do whatever gets me back on the ground safely.

Great thought provoking question.

[JimC]

Originally Posted by macaman

In theory, the wing with the flap down would have the root stall before the root on the unaffected side.

The wing with the flap UP would stall first, right? What am I missing here?

I honestly think the questions about higher speed, rudder for yaw, etc are academic. Based on Robert Rice's description of his incident, and other reports I've read, it seems you have two options with asymmetrical flaps in any Cessna:

1) get the unbroken flap up as quickly as possible and continue to a higher-speed, no flaps landing

2) crash.

The thought that you'd be able to maintain effective control for more than a few seconds, even with only 1/3 flap down, contradicts the evidence.

Let's say the ailerons could counteract the rolling moment of one flap at 1/3 (I think they can) - but now imagine the situation you're in. You've used all of your aileron authority and a lot of your rudder authority. You're struggling to keep it upright, let alone fly a straight line. You're an instant test pilot in an unfamiliar aircraft only a few seconds away from the ground. Your life expectancy is low, to say the least.

And the thought that they'll break at 1/3 deployment is also misleading - the pressure on the flaps (and therefore tension on the cable) is vastly increased at full deployment. They're just not going to break at 1/3 if you've had them at full any time in the last few flights.

I suggest two things if this problem makes you lose sleep:

1) Leave your hand on the flap switch for a bit any time you go to full flaps. The cables will break when stress is added. Real-world incidents in 336/337s (and other Cessnas*) show this problem is survivable if you get the flaps up IMMEDIATELY.

2) If you still can't sleep, REPLACE YOUR FLAP CABLES.


*
http://www.pprune.org/archive/index.php/t-260242.html
http://www.faa.gov/aircraft/safety/a...7_04_Alert.pdf
http://www.aero-news.net/index.cfm?d...9-48d422ca215a

[Ernie Martin]

Great info. Thanks all for contributing. I realize 1/3 flaps has lighter loads on the cables and the risk of a break is lessened, but it's good to know if the ailerons have enough authority to counteract. Key point is to keep your hand on the flap lever for a few seconds after lowering them -- not so much so your hand is closer, but more as a reminder of what you need to do instantly if a cable breaks. I also agree that if a cable is going to break, it's likely to do so when you first lower the flaps, not several minutes later when you are closer to the ground.

Ernie

[JimC]

Originally Posted by Ernie Martin

I also agree that if a cable is going to break, it's likely to do so when you first lower the flaps, not several minutes later when you are closer to the ground.

Ernie

Although lowering the flaps to full immediately upon reaching Vfe will generate a very high load, it *may* be higher during pattern turns (esp base->final, which is often steeper than dwnd->base) if your airspeed is higher than normal. Reading the accident and incident reports, breaks always seem to come at first deployment of full flaps (more likely) or base->final (less likely, but it happens.)

My first instructor in a 150 taught me to never deploy full flaps during the base-> final turn. If your inside flap cable breaks (it has less load than the outside flap, but who knows which cable is frayed?) you'll be inverted before you can react. Deploy before the turn, or after, but not during.

[bjherron]

Just curious, how many flap cable failures have been reported? Were any of them due to the flaps being extended above recommended range?

I read about Rob's flap cable failure in his 336, but didn't he also have a STOL kit installed? Wouldn't that add more stress to the cables and induce other variables?

I am curious how many reports there are of actual failures.

Brian

[JimC]

I've read of many failures (Maybe 10? - the problem affects most Cessna single models as well as the 336/337) but I have yet to read one where the airspeed at time of deployment is recorded or mentioned.

Here's an interesting link - it includes two descriptions of pilots successfully flying Cessna singles with asymmetric flap deployments. So it seems the answer to the OP's question is "Yes, it can be done." It also includes examples of flap failure on *retraction,* which I hadn't heard of earlier.

http://cessna170.com/forums/viewtopic.php?f=2&t=6608

[JimC]

This question interested me - I did a bit more investigating. I found plenty of anecdotal evidence that many pilots have survived a full asymmetrical flap deployment in many kinds of light aircraft. This makes sense - you have two ailerons counteracting one flap, and the center of pressure of each aileron is farther outboard by a factor of more than 2 (x3, if the flaps and ailerons are of equal span.) Every crash I read about came from a problem that occurred close to the ground. The only crash of a large airplane I found was AA 191 -the DC-10 at O'Hare that lost an engine. That flight was under control until it dropped below the slat-retracted stall speed.

A few people on the internet thought FAR 23 required that the airplane be controllable in an asymmetric condition - I could not find that explicitly, but I did find the FAR that says the wing must handle the load (and not snap in half.) FAR 23.701 implies that a possible asymmetric flap failure that results in an uncontrollable condition will pass certification if the failure is "extremely improbable."

[Ernie Martin]

The just-released Special Airworthiness Information Bulletin CE-13-27 reports of a broken flap cable on approach where the pilot was unable to retract the other flap, and had to control the aircraft using full opposite aileron. Point being that retracting flaps may not help if a cable snaps. For lengthy runways, you may wish to consider a new normal of only 1/3 flaps if you are worried about this issue.

Ernie

[hharney]

400 series Cessna's

Here's the file referenced above