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C172. Flap asymmetry?

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C172. Flap asymmetry?

Old 1st Apr 2019, 07:38
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C172. Flap asymmetry?

Preflighting my little C172, and wiggling the linkages to the flaps, I began to wonder what would happen if I selected flaps and only one side extended.

I guess that if one side jams, then both are jammed. No drama. But if one side's linkage fails is it possible that the other will extend normally and leave one retracted? Or will a failed linkage cause that side to float half deployed ?

Either way, I guess that the consequence would be instant loss of control? Would there be time to detect the problem and somehow fix it by matching both sides? Does it happen ?
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Old 1st Apr 2019, 07:55
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I believe it's happened, more than once. I had the same thought and figured, depending on which stage one side was jammed on, if opposite aileron and/or rudder were not enough to counter the extra lift on the jammed side, there wouldn't be much else you could do. You could perhaps counter the extra drag (and subsequent yaw) by opening the opposite door (and holding it open an amount with the third hand) but, you might end up with too much drag. All a bit too much for my O level brain really and I hope I never have to find out if it would work...................
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Old 1st Apr 2019, 09:09
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Just a thought, but the open door would only introduce drag and won't correct or change the lift imbalance due to asymmetric flap deployment. I wouldn't pin any hopes on that option. You wouldn't be countering the drag, you would introduce more drag in an attempt to restore a balance in yaw. It's not the yaw that's the problem though, it's the rolling moment.

A failed linkage would cause that side to move to full up, unless the linkage would jam the motion at some point, which isn't likely. The aircraft would show an increasing tendency to roll away from the side with the deployed flap. There isn't much you could do at such a point, except correct with aileron and if possible, retract the flaps to a position where the rolling moment is minimal, provided you've got time and altitude. Check your POH though... I'm just speculating based on theory.

Fortunately, the linkage is loaded in compression and it needs a massive overload to fail. So unless you've got a bent or damaged linkage, the chances of this happening are pretty slim.
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Old 1st Apr 2019, 09:59
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Is it an FAA certification requirement that aileron can maintain control with flap assymetry?
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Old 1st Apr 2019, 11:36
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See here: Asymmetric flaps
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Old 1st Apr 2019, 12:04
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Originally Posted by Jhieminga
Thanks! Interesting, and surprisingly reassuring! I would have expected an instant and dramatic roll that the ailerons could not counter.
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Old 1st Apr 2019, 12:53
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What's wrong with immediately de- selecting whatver flap input you just made?
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Old 1st Apr 2019, 15:33
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Possible asymmetry, particularly with electric actuators, is why I never operate the flaps with bank.
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Old 1st Apr 2019, 16:09
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Happenend to me once, in a C182, during type rating instruction. On a go-around, the left flap did not retract. Plane rolled to the right, but at slow speed, I could just level it with full left aileron and some rudder. The instructor, seeing my predicament, took a look back and identified the problem. Then we hit the flap selector down and the right flap extended. The plane was light, so we could climb, make a full flaps circuit and land normally. Cause: A screwdriver left in the wing after maintenance got stuck in the flap gear. Had some guardian angels that day. And had some animated discussion with the maintenance guy after the hands stopped shaking.
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Old 1st Apr 2019, 17:00
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This is why you (briefly) keep your hand on the switch when selecting flap setting changes.
That and I still don’t like flap changes in a turn in a light GA aircraft.
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Old 1st Apr 2019, 18:37
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[QUOTEPossible asymmetry, particularly with electric actuators, is why I never operate the flaps with bank. ][/QUOTE]

I did a checkout once in a 172 at Stapleford with the legendary Eric Thurston (a privilege to have flown with him). Anyhow, as I was turning final, I thought some more flap was required, so started extending them. 'ROLL WINGS LEVEL, NOW!' said Eric. 'OK, now go ahead and make your flap selection'. During the de-brief, which took longer than the flight, he gently explained that you'd never sort out flap asymmetry in a turn, so always select flap in level flight. That was 25 years ago, and I still teach my students this, whatever the type. So, yes, keep the hand on the flap lever, even for those with detents you can select. We had a 152 once which gave uncommanded flap selections because the microswitches were out of alignment - a bit embarrassing but with the 152 you can climb away with max (only 30deg) flap. You had to reach behind the switch and fiddle with it to get the selection you wanted...

TOO
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Old 1st Apr 2019, 21:24
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Is it an FAA certification requirement that aileron can maintain control with flap assymetry?
No, that is not a requirement, and few certified planes I have flown would be able to do this at all, much less within the pilot's strength and endurance.

Yes, all certified planes have demonstrated a "balked landing climb", which will be full flaps, full power, and a decent climb rate. If you must climb away with full flaps out, allow for a shallow climb, and have no fear, the plane will do it. If you have an engine failure doing this, it's going to get scary fast, so do retract the flaps at the earliest suitable opportunity.

The Cessna singles flap system is very robust in terms of asymmetry. The only real risks are those of a pushrod, or track failure. Yes, a pushrod failure could see a sudden retraction, and your only action would be to fully retract the remaining flap. You'd want to have some altitude, and be comfortable in unusual attitudes. I have never heard of a Cessna flap pushrod failure. I have had a broken flap track. There was no asymmetry, the flaps just jammed at 20, and would not retract. I was doing a water touch and go in a Cessna 180, so I was not going to stay, I flew the go, found I could not retract the flaps, decided to not fiddle with them at all, and flew it home that way. The right flap was hanging awkwardly as I taxiied in. When doing your walk around, with the flaps fully extended, do have a look at the track attachments for anything odd. In a Cessna, I am not alarmed by selection of flaps in a turn.

Certain Piper twins are reputed risky for flap asymmetry (I've been warned flying Navajo and Twin Comanche, perhaps there are other models). This is because one flap is driven, which then drives the other flap. The interconnecting drive can fail. So, on those types, small extensions at a time, and if asymmetry develops, easy does it trying to correct things - no further extension, and caution about large selections.

As for a nuisance lateral imbalance, fuel may become the greater annoyance, so have an understanding and plan for that. I was once right seat in a Piper Cheyenne, and a fueling error gave us a 400 pound imbalance at the tip - that was a problem!
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Old 1st Apr 2019, 22:26
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Originally Posted by TheOddOne
Possible asymmetry, particularly with electric actuators, is why I never operate the flaps with bank. ]
I did a checkout once in a 172 at Stapleford with the legendary Eric Thurston (a privilege to have flown with him). Anyhow, as I was turning final, I thought some more flap was required, so started extending them. 'ROLL WINGS LEVEL, NOW!' said Eric. 'OK, now go ahead and make your flap selection'. During the de-brief, which took longer than the flight, he gently explained that you'd never sort out flap asymmetry in a turn, so always select flap in level flight. That was 25 years ago, and I still teach my students this, whatever the type. So, yes, keep the hand on the flap lever, even for those with detents you can select. We had a 152 once which gave uncommanded flap selections because the microswitches were out of alignment - a bit embarrassing but with the 152 you can climb away with max (only 30deg) flap. You had to reach behind the switch and fiddle with it to get the selection you wanted...

TOO
Glad to be in such august company. My students have it instilled from Ex 4.2!

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Old 2nd Apr 2019, 05:22
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Thanks all. Very interesting and very useful to think these things through. I will get into the habit of holding onto the flaps lever for a second or two and mentally checking that all seems ok before going back to the throttle. Scary however to picture that situation while on a touch and go when you would not feel anything abnormal until you lifted off. No instructor has mentioned to me not operating flaps in a turn, that's something to consider as well; although to be honest, I don't think I would want it absolutely fixed in my head as a rule.
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Old 2nd Apr 2019, 07:00
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Flap asymmetric malfunction very unlikely on the basic Cessna aircraft and would only be caused by gross failure of the maintenance of the aircraft. The only failure that is likely to be undetectable at maintenance inspection is an electrical failure of the flap motor

The incident quoted above about a C152 giving uncommanded flap movment is unlikely to be a microswitch misalignment as such but a Malfunction of the flap position feedback cable that mis-positions the microswitch block to cause the uncomanded movment.
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Old 2nd Apr 2019, 11:55
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DB, if you're flying an electric flap Cessna, relax about the flaps. I commend your interest and awareness, that's great, but ultimately, the flaps of a Cessna aren't going to get you, if, during the walk around check you were happy with them. If they will not extend at all (99%) likely an electrical fault, expect a longer landing roll (you should be practicing that anyway). If they operate uncommanded (referenced microswitch failure) either let them go where they're going, they'll stop on their own. Or, if you find that they are errantly extending during a go around (which is very unlikely if you've selected them up), turn the master switch off, and they will stop moving. Get in stable flight at altitude, turn the master on, and deal with it there. The electric flaps don't move fast enough to put you out of control (intended design feature). 'Worst is that you fly half a circuit full flaps, not a problem in a Cessna. If you are flying a manual flap Cessna, lucky you! (they're great planes). Those flaps can be moved fast enough to surprise you, and yes, a sloppy pawl, and sudden retraction is a skill demanding event. Very unlikely you're flying a manual flap rental Cessna.

To each their own, but I'm content to operate Cessna flaps at any appropriate phase of flight. In teaching emergency canyon turns, extending the flaps during a climbing turn is an element of the maneuver. I've had flap problems in Cessnas, but in 42 years of flying them, asymmetry and uncommanded operation were never problems for me.

For other types, the risks are different. The best way to understand your risk, is to understand the plane! Most flight manuals have a section at the back (should be section 7), in which these systems are described enough for you to understand how they work, and design features which could pose a risk.
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Old 2nd Apr 2019, 19:01
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If you operate a control and the aeroplane does not do what you want (or expect) it to, best return the control to it`s original position
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Old 3rd Apr 2019, 01:50
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I am told by an aircraft engineer and designer that for Australian certification, which is based on FAA, an aircraft must be able to demonstrate that the ailerons can maintain control with one flap up and one fully down.
If it was unsafe to operate the flaps during a turn it would say so in the "limitations" section of the POH.
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Old 3rd Apr 2019, 03:23
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I am told by an aircraft engineer and designer that for Australian certification, which is based on FAA, an aircraft must be able to demonstrate that the ailerons can maintain control with one flap up and one fully down.
Not correct. The FAA design requirement closest to this is:

Sec. 23.143

General.

(a) The airplane must be safely controllable and maneuverable during --
(1) Takeoff;
(2) Climb;
(3) Level flight;
(4) Dive; and
(5) Landing (power on and power off).
(b) It must be possible to make a smooth transition from one flight condition to another (including turns and slips) without exceptional piloting skill, alertness, or strength, and without danger of exceeding the limit load factor, under any probable operating condition (including, for multiengine airplanes, those conditions normally encountered in the sudden failure of any engine).
(c) If marginal conditions exist with regard to required pilot strength, the "strength of pilots" limits must be shown by quantitative tests. In no case may the limits exceed those prescribed in the following table:
Values in pounds of force as applied to the control wheel or rudder pedalsPitchRollYaw
(a) For temporary application:Stick---------------------------------------------
6030--------------------------
Wheel (applied to rim)-------------------
7560--------------------------
Rudder Pedal-------------------------------
----------------------------------------------------150
(b) For prolonged application.
10520
No mention of asymmetric flaps.

At least one type I have flight tested specifies adjusting a flap to trim out a roll force. The amount of flap displacement to really affect roll trim is about 1/8" at one flap trailing edge. Can you imagine the roll trim effect of one up and one down? If the flaps are effective, any significant asymmetry will overpower a pilot.

If it was unsafe to operate the flaps during a turn it would say so in the "limitations" section of the POH.
Very true.
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Old 3rd Apr 2019, 07:07
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Originally Posted by Clare Prop
I am told by an aircraft engineer and designer that for Australian certification, which is based on FAA, an aircraft must be able to demonstrate that the ailerons can maintain control with one flap up and one fully down.
Either yes or no, depending on how compliance with FAR 23.701 is achieved.
https://www.risingup.com/fars/info/part23-701-FAR.shtml

Last edited by zzuf; 3rd Apr 2019 at 08:47.
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