C-172s flap system | TheaterFire

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tomdarch@reddit

Two I-shouldn't-need-to-know-this, in-the-weeds questions:

1) How does the control in the cockpit work to set the flaps at a given "angle"? Is it a potentiometer?

2) How does the needle that indicates the angle of the flaps work? Is it driven by the actual flap on one side or the other? Could it ever give a false reading of what angle the flaps are at? (Yes, I know I can always just look over my shoulder at the actual flaps.)

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kytulu@reddit

1) The control in the cockpit has two microswitches. As the indicator needle moves to the selected position, it actuates the switch to stop the flaps.

2) The needle is attached to the lever assembly and moves as the flaps move.

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tomdarch@reddit

Thanks!

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rowlock@reddit

Fire this DPE, honestly. He or she sounds like one of those people that looks for unreasonable technical minutiae to fail you and collect a retest fee.

You should know how the flaps behave, and basics of their operation, but asking for details that aren’t even in the POH is unreasonable.

The reason it’s not in the POH is that for the majority of cases, the manufacturer decided it was not necessary information for you the pilot to have in order to operate the aircraft safely and effectively. In which case, expecting you to know it for the private ticket is nonsense.

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Hour_Writing_9805@reddit

If it’s not in the ACS he has a good case, if it is, well it’s fair game.

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rowlock@reddit

The ACS isn’t granular enough to specify these specific mechanical systems individually, so I guess there is some room for interpretation. But even then it feels like a hell of a stretch to require knowledge from the service manual.

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Hour_Writing_9805@reddit

Could be, couldn’t be. But if they are going to their FSDO, that document and whoever makes the judgment on it will decide.

That’s part why the ACS is somewhat vague, gives them a long runway (no pun intended) to rule on decisions.

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rowlock@reddit

I think that would be ACS section CA.I.G.K1b:

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Objective: To determine the applicant exhibits satisfactory knowledge, risk management, and skills associated with safe operation of systems on the airplane provided for the flight test.

"The applicant demonstrates understanding of ... airplane systems including ... secondary flight controls"

References: FAA-H-8083-2, FAA-H-8083-3, FAA-H-8083-23, FAA-H-8083-25; POH/AFM

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Those listed reference materials are: The FAA Risk Management Handbook, The Airplane Flying Handbook, the Seaplane Ski-plane and Float-Equipped Helicopter Handbook, The Pilot's Handbook of Aeronautical Knowledge, and the POH/AFM of the plane being tested in.

So I think there's a solid argument that the maintenance manual for the test plane is outside the scope of materials being tested, since it's not listed in the references section for that part of the ACS.

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HugeAd9926@reddit

The simple answer is (and no harm in knowing it) : The right flap is connected to the left flap by a cable.

(The flap control switch on the instrument panel controls am electric motor in the right wing. This motor actuates the right wing flap)

If your battery or the flap actuator motor in the right wing fail both flaps remain in the same position.

If the cable system between the right and left wing fails (it requires regular inspection) your left wing flap likely remains stuck while you can only operate the right wing flap. This happened to me as a student on a cross-country flight with a touch and go at a destination airfield. I managed to set the right hand flap at the same extension degree as the stuck left hand flap and flew home at 80 knots (below VFE) .

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Porter_7600@reddit

I hate super technical questions like that. I understand what the DPE was trying to ask, but the question was formatted poorly for him to evaluate your knowledge on the subject.

A better way to ask that would be: "you're flying a scenic flight with a full plane within w&b out of XXX (Known local short field). You have a flap asymmetry turning base. The right flap is down the left flap is up. How does that change the flight characteristics of the airplane? How do you fix it? What else do you have to worry about? Where can you land? Calculate the Landing Performance numbers. Are you breaking the rules taking people to a different airport than departure? Would you declare an emergency? Now the divert field is IFR.

I could get half an oral out of that scenario, and better evaluate your functional knowledge, all without reengineering the airplane.

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Bergasms@reddit

Hmm, i don't have many hours and fly a low wing and only have a recreational cert so i may be a bit out of depth, but can i have a go at some of this.

You have a flap asymmetry turning base. The right flap is down the left flap is up. How does that change the flight characteristics of the airplane?

more lift from right wing, so roll to the left and yaw to the right, plane is going to be slipping now which will bleed airspeed...

How do you fix it? What else do you have to worry about?

assuming a left turn onto base (my home field has left and right circuits), your bank angle is going to be a bit dire. Right airleron to counter the right wings lift and fix the roll, left rudder to counter the now pretty bad drag on the right, power in and probably pitch down a bit to get airspeed stable then climb up, then i would try and raise the flaps to get back to a clean setup. I'd do it in that order because maybe the flaps are jammed in that state, so get plane under control first before trying to fix flaps.

Where can you land? Calculate the Landing Performance numbers.

if the flap goes up and i'm landing flapless, well, same field for my plane because you can slip it like a mofo so the short runway is an option still. If i can't use slipping and i have no flap then i have to land flapless, that means more speed which means longer runway. If the flap doesn't go up and I am stuck with assymetric flap then i'm probably not diverting anywhere, i'd be able to lose plenty of airspeed due to dealing with the assymetry i'm guessing,

Are you breaking the rules taking people to a different airport than departure? Would you declare an emergency? Now the divert field is IFR.

i don't know your rules. I would be breaking them for my licence due to no nav endorsement so requirement to land at takeoff field. I would declare a pan-pan as i'm in a situation but its not immediately life threatening. A flapless landing is a normal procedure the plane can do, declaring an emergency with good cause for the safety of the flight means breaking the rules is acceptable. En-route the alternate goes IFR, hm, for me that means i cannot get there visually i guess, can i divert to another place? Is it towered and i can ask "how IFR" it is? This is an emergency where i need a longer runway, i'll accept marginal VFR. If no tower and/or just a wall of cloud then i'm not going where i can't see, return to original field and use slipping, if surrounds mean thats right out then we might have to look for a suitable long paddock.

This was an interesting thought experiment for sure.

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Porter_7600@reddit

OP was on a commercial checkride

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countextreme@reddit

"Kind of IFR, but if you ask nicely we might be able to make a SPECIAL exception for you"

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Legitimate-Watch-670@reddit

ROFL @ "HOW IFR was it?"

I'm definitely stealing this next time someone tells me it's IFR somewhere lmao!

Bonus points for if I'm VFR and ATC informs me my destination is IFR 🤣

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MoreSpoiler@reddit

If you do something and don’t like what it does, undo what you did

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TalkAboutPopMayhem@reddit

The Cessna 172S flap system is an electric, follow-up system that uses a single motor, cables, and pulleys to operate both flaps simultaneously. Unlike larger aircraft, the C172 cannot use a central spar to connect the flaps, so it relies on this more complex cable-and-pulley mechanism. Components of the flap actuator system: Flap switch: Located on the lower center of the instrument panel, this switch controls the flap position. It is often shaped like a flap and has detents for 10°, 20°, and 30° extensions. Electric flap motor: This single motor, housed within the right wing, is the power source for the entire system. Actuator transmission and jackscrew: The motor drives a transmission, which then moves an actuator tube via a jackscrew. The actuator tube's linear movement ultimately drives the cables that move the flaps. Cables and pulleys: A series of cables and pulleys transmit the force from the actuator transmission to the flaps in each wing. Bellcranks: These are pivoted levers, part of the system of linkages that convert the motion of the cables into the movement of the flaps. Flap tracks and rollers: The flaps move along tracks on the underside of the wings, with rollers guiding their extension and retraction. Limit switches: The system uses limit switches to automatically stop the motor when the flaps reach their full up or full down positions, preventing the motor from overextending the system. How the two flaps are connected: Instead of a rigid bar, the flaps on the Cessna 172S are connected through the cable-and-pulley system. This linkage ensures that when the motor is engaged, both the left and right flaps move in unison. The cables run from the central actuator mechanism out to the bellcranks and rollers attached to each flap. Because both flaps are moved by the same motor and cable system, they are mechanically linked. Any movement initiated by the pilot is transferred equally to both flaps.

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tomdarch@reddit

The limit switch part is interesting. I hadn't given any thought to how the flaps stop at the extremes (or for a given setting of the control in the cockpit.) A limit switch that either isn't activating or remains stuck could cause problems.

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Flapaflapa@reddit

we still doing the "build me an airplane" bullshit?

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_Fishcake_@reddit

Did you happen to take your checkride with a certain DPE with the initials S.A.? If you look on page 7-1 of the 172’s maintenance manual, theres a diagram of it. There’s a motor in the right wing, which turns a screw, that turns two “drive pulleys”, one in each wing, and each are connected to pushrods that physically actuate the flaps. The drive pulley in the left wing is connected to the right drive pulley using a cable called a “turnbuckle”.

Ive never been expected to dig through the maintenance manuals on any exams though…

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MoreSpoiler@reddit

If it requires the MM, it’s a BS question and I’d report it his POI

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MoreSpoiler@reddit

Sounds like DPE figured you failed before the checkride started, that’s a BS line of questioning.

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Chemtrailcreator@reddit

In basic terms…. the flap motor is in the right wing. The linkage, that bar that pushes the right flap out, is attached to a bell crank inside the wing that the flap motor actuates.

On that same bell crank attached to the right flap linkage and the motor, there is a cable that runs across the cabin to a similar bell crank in the left wing next to the left flap. That bell crank is attached to the linkage bar that moves the left flap.

So essentially, motor moves the right flap which at the same time pulls the cable that actuates the left flap simultaneously. They are basically mechanically linked to each other to move at the same time in whichever direction. Motor fails, both flaps stop. Left flap jams, it jams the right flap too. Right flap(the one with the motor attached) jams, the left flap won’t move either.

The only way one can get an asymmetric flap deployment is if that cable breaks connecting the two. In that case the right flap (the one with the motor) will move and the left one won’t

None of this will info is in the POH, the POH gives a very vague explanation. It’s in the maintenance manual.

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JT-Av8or@reddit

It might have been far more of an issue and he just wrote up that as the failure because otherwise it would be too long. When I was an evaluator I’d do that. It’s like a hundred “eh…. I guess that’s kinda borderline but not a fail”

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whywouldthisnotbea@reddit

Yup. OP call the FSDO and ask them if it was reasonable to have failed for this reason. They are going to pretty quickly be asking for this asshat's information.

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VanDenBroeck@reddit

I doubt that it was the only reason they failed but if it was, yep FSDO needs to rattle the DPE’s cage.

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LordCrayCrayCray@reddit

It sounds like the DPE wanted to hear about whether or not asymmetric flap extensions a possibility, and what possible failure modes exist.

What happens if one flap is jammed and can’t move. What drives the flap (electricity), and realization that loss of electrical power would cause them to not move. Ether or not the candidate thinks that there are one or two motors and has considered if it is a risk.

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VileInventor@reddit

Yeah I mean I knew this at the commercial level but it’s because I was studying like i was a CFI and i was digging in the weeds for info. A dpe ACTUALLY asking and failing on that is kinda crazy to me.

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cinemashow@reddit

Wow. Went down a rabbit hole because I didn’t know how they work either. And I was never asked about them. Found this diagram. Hope it’s clear:

https://www.manualslib.com/manual/1637920/Cessna-172-Series-1996.html?page=456#manual

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RaiseTheDed@reddit

Honestly IMO that's a bit in the weeds. You're not building an airplane on a commercial checkride. Not an A&P checkride.

To actually know that you'd have to look in the service manual (you can find PDFs online), it has a good diagram.

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TheSkyFlier@reddit

Hell, you don’t even need to know that to pass O&Ps. If you can point to which control surface is the flaps you can get your A&P.

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Malcolm2theRescue@reddit

This sort of crap is like teaching Vladimir Horowitz how to build a piano. We don’t build the piano, we play it. You put the lever down, the flaps come down or they don’t. Go to abnormal checklist. Or one comes down and the other doesn’t. Now you have a real emergency and this information is useless. Now both arms, hands and your brain are busy fighting to control the airplane. No checklist.

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TheOldBeef@reddit

If you actually failed only for that reason... the DPE is a moron. Knowing that level of minutiae does absolutely nothing to make you a better pilot.

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Skynet_lives@reddit

This is very in the weeds and not in any reference source that a pilot would be expected to read. The POH is about as deep as they should go.

My guess is that you flubbed this question so hard that it was the straw that broken the camels back. Can’t imagine any DPE wanting to know more then, electric flap system that’s mechanically linked in the structure of the aircraft. Protected by a 10amp circuit breaker. Since that pretty much all that’s in the POH.

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usmcmech@reddit

There is a detailed schematic and description in the POH

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RaiseTheDed@reddit

It isn't actually that descriptive in the POH that I can see. There's a diagram, and a very small paragraph.

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Cherokee260@reddit

We don’t even have that shown in the owner’s manual.

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rFlyingTower@reddit

This is a copy of the original post body for posterity:

Can anyone explain to me the system behind the flaps? I understand that it is electronically actuated by the flap motor which pushes the flaps to the desired degree.

That’s the answer I gave on my commercial DE and I failed the oral because I couldn’t explain how the right flap was connected to the left flap.

After more research I’m seeing some stuff talk about cables and bell cranks. But I’m not sure exactly how it works and I can’t find any resources to help my understanding of it.

I want to be able to clearly explain it and I just need help to do so. If anyone can offer any help, my retest is tmrw and I would really appreciate it

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