Hi, I was asking also on Cessna forum, but there I had no luck, so I turned here, if someone experienced same issue.

I have 1968 FR172E, and when I extend landing flaps at 40, flaps are shaking a bit and it will shake also with tail and I can feel vibrations in plane, mainly on controls. It is not any strong shake, like flutter, but it is annoying, I can use flaps at 20 and it is OK, but at 30 it is worse, and on 40 it is bad. I haven't seen it on any other Cessna, my friend has 1970 F172H model and it is also shaking, but only very small, like normal vibration, also on club 172S it is only light vibration.

I will replace flap rollers for McFarlane, because of wear, flap tracks has some wear, but it is still in limit, but I thought, that if flaps are extended, airflow will push the flaps and lock them against extension mechanism, but it is obviously not this case.

I gave to say I am mechanic, I also asked few other mechanics, what can cause this, but noone ever faced same problem. Here in Europe is less Cessnas, than in USA, and backcountry planes have many modification, so maybe I will be lucky and someone will turn me on the correct path.

First task would be replace the rollers. Then if persists and tracks are worn, replace the tracks.

That said, my 170 had significantly worn tracks and rollers, but didn’t vibrate as you describe.

Have you investigated the flap actuator system to expose problems there? Do flaps deploy symetrically?

MTV

Yes, Flaps adeploys symetrically, flap actuator system was cleaned and lubricated in annual. It is older system, as long as switch is held in position, flaps are moving. Anyway it was OK after maintenance, it started just one day, without prior warnings.

It looks like it is only one Cessna on the world with this problem. Flap tracks are worn, but still in the limit. Limit is 0.6035 inches, but my tracks are on 0,570 inches (worst place). I will replace the flap rollers, but I think it would not help.

One thing to check for is broken flap brackets. Retract your flaps and go to the back of the wing. Push up on the trailing edge of the flap and look at the top of the wing right where each flap hinge is. It should not move. If you see the upper wing skin flex upwards at the hinge, you may have a broken flap bracket. Flap brackets break when people extend their flaps at speeds in excess of the flap extension speeds. You have to split the trailing edge of the wing to replace them.

Wayne

What airspeed are you deploying 30-40?

I would lower to flaps 40 then push up on each flap, perhaps by holding the other flap down, and try to find if there is a soft area in the structure.

Might want to get a cable tension meter up in there and see if the tension is loose?

Inspect pulley brackets well under simulated flight loads. Might be hard to see sheared rivets on one end of a bracket.

You might have an issue in your horizontal stabilizer or elevator structure that is being triggered by the flap position and flight loads? Check that your elevators are firmly riveted to the bellcrank brackets, grab one elevator at each end and move each side in opposite directions should not be any movement.

Aft wing spar attach? Rock the wings so hard the wheel comes off the ground with an eye and ear open.

Rocket


elevator trim tab end play within tolerance?

Tomorow I will check everything you mentioned. I open flaps to 30-40 at 80 MPH (old speedmeter) which is approach speed by POH, definitely in white arc. Stabilizer and elevators are ok, it has been disassembled few months ago, also trim tab and it's mechanism. Good idea with that test with retracted and extended flaps, tension has been checked few months back. Left flap has bigger freeplay, than right, but flap tracks are in limit.

There should be plastic bushings on the outward flap roller bolt keeping the flap from slopping side to side. Check that those are in good shape. I have them on outboard and inboard roller bolts on my 182.

$8.44 for a set of 4 from McFarlane. Part No. 1220114-1

I'm not sure if this is a contrubutor but cant hurt to replace them.

I checked them yesterday and they are there, they are part of service bulletin. Anyway I am going to order McFarlane improved rollers, and they are part of kit FLP-KT-1U.

I did some rigging, and right flap needed to be rigged a bit, but I did not do a flight test yet, but I will keep you updated, I believe it could help someone when facing same problem. The bad part is I am first to solve it.