C172. Flap asymmetry?

[double_barrel]

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 ?

[MrAverage]

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...................

[Jhieminga]

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.

[Maoraigh1]

Is it an FAA certification requirement that aileron can maintain control with flap assymetry?

[Jhieminga]

See here: https://www.pprune.org/private-flying/165682-asymmetric-flaps.html

[double_barrel]

See here: https://www.pprune.org/private-flying/165682-asymmetric-flaps.html

Thanks! Interesting, and surprisingly reassuring! I would have expected an instant and dramatic roll that the ailerons could not counter.

[meleagertoo]

What's wrong with immediately de- selecting whatver flap input you just made?

[rarelyathome]

Possible asymmetry, particularly with electric actuators, is why I never operate the flaps with bank.

[clearedtocross]

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.

[B2N2]

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.

[TheOddOne]

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

[Pilot DAR]

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!

[rarelyathome]

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!

[double_barrel]

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.

[A and C]

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.

[Pilot DAR]

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.

[mothminor]

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 :)

[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.
If it was unsafe to operate the flaps during a turn it would say so in the "limitations" section of the POH.

[Pilot DAR]

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:http://rgl.faa.gov/icons/ecblank.gifhttp://rgl.faa.gov/icons/ecblank.gifhttp://rgl.faa.gov/icons/ecblank.gifStick---------------------------------------------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.

[zzuf]

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

[NorthLondoner]

In his early thriller 'The Most Dangerous Game' the late Gavin Lyall has a Cessna floatplane sabotaged by 'fixing the flaps' (it's when the hero, the boozy pilot of knackered DH Beaver, realises he's up against a professional). There's a vivid description of the aeroplane flipping upside down and the pilot fighting to fly out inverted, with the trim dragging him down... Given the author's flying background I've often wondered how plausible the scenario was.

[Jhieminga]

Not really relevant, but that section is not included in 14 CFR part 23 right now. The current version isn't very specific on these issues. Plenty of types will have been certificated to earlier versions of course.

[zzuf]

Not really relevant, but that section is not included in 14 CFR part 23 right now. The current version isn't very specific on these issues. Plenty of types will have been certificated to earlier versions of course.

Yes, welcome to the new certification world.
It appears to be a lot that is not included in CFR part 23, I think it would be a mistake to think that compliance will now be easier, or that the questions asked in previous FAR 23 amendments will not be asked in certification meetings.
It was comparatively easy to go through the certification standard line by line with the certification applicant, to reach an agreed certification plan. Not so sure that it will be so now.
I would not like to be an applicant trying to develop and present a consensus certification standard to the FAA.
I will bet that a flap interconnect failure will still have to be addressed if it is not shown to be extremely improbable.
Interesting days for both certification applicants and regulatory authorities.

[Jhieminga]

"It's certification Jim, but not as we know it..." ;)

There are still several paragraphs that imply (or appear to imply) the need to avoid flap asymmetry, but not in those words. So yes, things have certainly got more interesting. I'm glad not to be involved in this area of the industry.

[double_barrel]

I find it hard to believe that a c172 could be flown with one flap at 40 degrees and one fully retracted; it flies like a wet brick with both at 40deg! But I am reassured that this is low down the list of things to worry about!!

I took a particularly close look at the flaps track system when I preflighted this morning, it looks very simple and very robust.

[oggers]

The old Part 23 required this:
a. The main wing flaps and related movable surfaces, as a system must:
(1) Be synchronized by mechanical connection; or
(2) Maintain synchronization so the event of an unsafe condition has been shown to be unlikely; or
b. The airplane should be shown to have safe flight characteristics with any combination of extreme positions of individual movable surfaces (mechanically interconnected surfaces are to be considered as a single surface).

[Clare Prop]

My Boomerang aircraft were designed by Bill Whitney and have FAR 23 type certificate as well as an Australian type certificate. This about the asymmetric flap requirement is one of the things told to me by Bill Whitney himself when we were discussing the 10 year cable replacement. The aircraft has to be able to be controllable in all three axes following a total failure of one of the cables, and to be able to maintain control with one flap up and the other fully down.

Point is that many of the "thou shalt not"s taught (eg I was told NEVER to touch the mixture control below 5000 feet?!) are not backed up as limitations in the POH, some may be applied by lazy instructors as a "one size fits all" eg sideslipping with flaps down, use of carb heat in the approach, (Cessna says to put it cold AFTER landing) CSU power setting tables, use of electric fuel pumps in Cherokees, the list goes on. Question everything.

[scifi]

Thanks Pilot DAR for suggesting 'Turn off the Master Switch'... My first reaction would have been to pull the Flap Circuit Breaker, but I realise that I don't know where it is, and would have to look around for it.
maybe we should learn where all the CBs are, and the function of each half of the twin switch master switch.

[Pilot DAR]

the list goes on. Question everything

Indeed, and make an effort to understand everything in you plane. For a Part 23 airplane there is no requirement to account for a control cable break from a handling/redundancy standpoint. Part 25 has differences in this regard. Very simply, as DB has remarked, very effective flaps, if asymmetric, could not be overcome with aileron control. In the context of a Cessna, this becomes even more extreme for those Cessnas modified with drooping ailerons connected to the flap system.

The rule about interconnection has it's roots in hydraulic flap systems. Before electric motors were common, and when the flap actuation forces were very high, a hydraulic cylinder was used to operate each of the flaps, which had the effect of making the flaps independent. Thus, a separate interconnection of the two flaps became desirable/necessary. So, in theory, that interconnection could fail, though both flaps still operate = problem. Most GA airplanes with more recent designs (all 100 series Cessnas) have flaps which are cable operated to move interconnected as both the actuation, and interconnection for both flaps. You can disconnect the Cessna flap motor, and the flaps will still move together. Thus, a failure to operate together is much more a risk than asymmetry. Indeed, due to the excellent aerodynamics of Cessna "paralift" flaps, their operating forces are happily low. Fly a manual flap Cessna, and you'll feel how nicely they move.

As I pick back through earlier versions of Part 23, I find:

Sec. 23.701

Flap interconnection.

(a) The motion of flaps on opposite sides of the plane of symmetry must be synchronized by a mechanical interconnection unless the airplane has safe flight characteristics with the flaps retracted on one side and extended on the other.

So I do see where the reference to flight with flap asymmetry has a historical origin. Though an applicant for a modification to a presently certified plane could apply to use this standard, it is doubtful that it would be permitted. The FAA "Changed Product Rule" (CPR) would be invoked by the regulator, and drive the applicant to use a later standard. A new design would be required to use the most recent certification standard.

If an element of an aircraft design is found to be unsafe, a service bulletin (SB), and perhaps an airworthiness directive (AD) will be issued to correct it (SB sometimes optional, AD mandatory in all cases). Imperfect aircraft designs may remain in service as is in the absence of an SB/AD, even though we would not do it that way ever again. If that system becomes the subject of a modification, CPR will be considered to invoke a more recent certification standard if appropriate.

When I develop certification plans (for STCs I issue), one of my duties is to go through the CPR process, and either determine (if it's within my delegation) or recommend to the regulator (if is exceeds my delegation) which amendment of the design requirements should be applied to the modification. Often, I just use the most recent amendment of the standard for convenience. The only problem with doing that may be that other aspects of the older aircraft could not pass the newest standard (seats, for example), so you'd fail before you begin.

An applicant can always propose to make an aircraft much more compliant that the requirement states. It's usually expensive and heavy, but sometimes a good idea. The challenge with dual interconnections of control systems can be that the dual systems fight each other if misrigged, and create undesired friction, or very high forces within the interconnected circuit, and break something.

I have issued approvals for modification of flap systems, though the operating philosophy was not affected, so the original design standard was still appropriate. I have no concern about Cessna flap control systems which are correctly maintained, and used.

Scifi, pulling the breaker is excellent - except that most Cessnas have the cheap CB's which you cannot pull, so you're stuck with the master being the only control you have over that circuit. I replaced the CB's in my 150 to be the type you can pull - for that very reason!

[double_barrel]

Scifi, pulling the breaker is excellent - except that most Cessnas have the cheap CB's which you cannot pull, so you're stuck with the master being the only control you have over that circuit. I replaced the CB's in my 150 to be the type you can pull - for that very reason!


Ah Ha! That explains it!!!!

I only see little flush buttons that I am assured will pop out if overloaded, but I could not see how to pull them! (On boats I'm used to cb's which are push make/push break or toggle switches with cb's incorporated.)

[Jhieminga]

In his early thriller 'The Most Dangerous Game' the late Gavin Lyall has a Cessna floatplane sabotaged by 'fixing the flaps' (it's when the hero, the boozy pilot of knackered DH Beaver, realises he's up against a professional). There's a vivid description of the aeroplane flipping upside down and the pilot fighting to fly out inverted, with the trim dragging him down... Given the author's flying background I've often wondered how plausible the scenario was.

Lyall was known for getting his facts right in his books. Apparently there were several things that he tried out himself before writing about it, but I guess this wasn't one of them! ;)