Jammed trim wheel today on Cessna 172

eetrojan

eetrojan

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eetrojan
So, this was a new one for me. I rented my favorite Cessna 172 to fly from John Wayne (KSNA) to Redlands (KREI) to buy beer at Hangar 24 - a righteous mission if ever there was one.

The preflight was fine, including my making sure the trim wheel was in the “take-off” position. I moved the wheel at least a bit at that point and felt nothing wrong. After taking off though, it just felt weird. The trim wheel was moving at first, but felt sticky. As I climbed toward 3,000 feet, I realized that I couldn’t trim away the yoke pressure at all because the trim wheel had become completely stuck and wouldn’t move up or down. It was stuck in a nose-up position.

I had just been transferred to SoCal, so I just flip-flopped back to tower and told them I had a stuck elevator trim and wanted to return to the airport. They immediately cleared me and asked If I needed assistance. I said not for now. I had to push-over on the yoke to descend, even with reduced power it seemed, so I made a long and wide downwind and eventually got it back on the ground.

Once I was parked, I double-check the operation and was able to push through the stickiness, but it still seems off. In particular, I found that with the wheel trimmed full-up the trim tab had a hefty angle relative to the rest of the elevator surface, but with the wheel rotated as far downward as it would go, the trim tab barely moved and the angle was small:

Cessna_172_Trim_Wheel.jpg


I would assume this isn’t normal. The range of motion should be symmetrical, right?

From now on, I think I will fully rack the trim wheel up and down as part of my preflight.

I have no beer. :(

Edit: Corrected "trim flap" to "trim tab"
 
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Interesting. Good job not panicking and getting back on the ground. No ideas right off. I wonder if the cable or a pulley is binding somewhere. Trim wheel partially jamming?

Elevator-Trim-System.jpg
 
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register@teamandras.com said:
I would assume this isn’t normal. The range of motion should be symmetrical, right?
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The numbers are, IIRC for that model, 23° tab up (nose-down trim) and 13° tab down (nose up trim). In the takeoff position the tab should be up about 10°.

The system uses chains and sprockets at the wheel and tab actuator, with cable between. If the system isn't maintained, various things can go wrong with it, some of them very seriously dangerous. Too much up travel can have the threaded rod come disengaged from the barrel nut in the actuator. A corroded rod can break and flutter could destroy the airplane. A worn thread/barrel leaves a lot of tab free play, which can also lead to flutter. Cessna allows only .025" of play per inch of tab chord. If the measurement is taken at 6" aft of the hinge line, the max play is .150". The cables tend to fray at the pulleys, and there are five pairs of pulleys. Neglected pulleys seize up and wear the cables faster. Lots of stuff to look at.

Edit: My memory was unreliable. 28°tab up and 13° tab down are the numbers for all 172s up to the P model. After that they were more like 22° up and 19° down, probably a reflection of heavier airframes, which usually get more tail-heavy. The TCDS for most any airplane has all the control surface movement specifications.
 
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Every aircraft I'm familiar with was designed with normal system authority capable of overriding maximum trim, in just such a scenario. Elevator primary control as well as trim are often not symmetrical.
 
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I dialed in about 75 percent up trim and 25 percent down trim for my RV-9A. When I want to descend, I rarely trim for it...I just reduce power and push the nose down. And when I do, it takes very little throw. Part of it is that it's fun to hand-fly the plane in this phase of flight.

One of the most common errors that DAR inspectors find on experimentals is the trim tab moving in the wrong direction for the intended trim.

Some guys fit a pullable breaker for a runaway trim servo scenario, but stick forces aren't huge at either extreme so I made no such provision.
 
SoCal RV Flyer said:
One of the most common errors that DAR inspectors find on experimentals is the trim tab moving in the wrong direction for the intended trim.
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It's not just experimentals. I have a friend who got the "fun" of going around the pattern once with a reversed elevator trim on a 182 that just came out of maintenance.

He didn't quite get it at first, and kept rolling it on and having to pull harder and harder on the yoke. Thankfully he figured it out and went the other direction after declaring and staying in the pattern instead of departing.

It had electric trim and I forget if he was using the electric or the manual and how exactly it was backward but it caught him by surprise as he tapped the electric trim for nose up initially after takeoff and it went the other way.
 
GlennAB1 said:
Every aircraft I'm familiar with was designed with normal system authority capable of overriding maximum trim, in just such a scenario. Elevator primary control as well as trim are often not symmetrical.
Click to expand...

Thanks. That was definitely true in this case. No problem overriding the trim tab position.
 
SoCal RV Flyer said:
I dialed in about 75 percent up trim and 25 percent down trim for my RV-9A. When I want to descend, I rarely trim for it...I just reduce power and push the nose down. And when I do, it takes very little throw. Part of it is that it's fun to hand-fly the plane in this phase of flight.

One of the most common errors that DAR inspectors find on experimentals is the trim tab moving in the wrong direction for the intended trim.

Some guys fit a pullable breaker for a runaway trim servo scenario, but stick forces aren't huge at either extreme so I made no such provision.
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I installed a "push to enable switch" on my elevator trim. It is very fast and powerful when above 125 kts.
 
Thanks for the right up. I always check to make sure it works and in the right direction, but just through enough range to verify that. Think I'll start going through more range.
 
Just a small word of caution: If it's any worse than what was described here, ensure ATC knows how stuck elevator trim affects your ability to fly the airplane. Many controllers won't know what elevator trim is to begin with, nor what the effects of stuck trim are. In some instances this can be considered a serious flight control problem, and that's all you'd really need to say to communicate the problem in an understandable way.
 
IME trimming nose down is typically always fewer turns than nose up when starting at the "takeoff" placard.
 
I'll defer to any mechanics out there, but the observation you made about the relative angle of the tab in the up/down position may be related to the fact that the elevator was in the full down position (in the photo)... I think there is an active relationship between the position of each//I say that because one of my instructors once showed me that if you pull the elevator to the full up and move the trim tab until its flush with the elevator, you have set it to the correct takeoff position. (Skyhawk has a rear window to allow a view of that)
 
FlySince9 said:
I'll defer to any mechanics out there, but the observation you made about the relative angle of the tab in the up/down position may be related to the fact that the elevator was in the full down position (in the photo)... I think there is an active relationship between the position of each//I say that because one of my instructors once showed me that if you pull the elevator to the full up and move the trim tab until its flush with the elevator, you have set it to the correct takeoff position. (Skyhawk has a rear window to allow a view of that)
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Isn't it a whole lot easier to use the TO mark on the trim wheel?
 
FlySince9 said:
I'll defer to any mechanics out there, but the observation you made about the relative angle of the tab in the up/down position may be related to the fact that the elevator was in the full down position (in the photo)... I think there is an active relationship between the position of each//I say that because one of my instructors once showed me that if you pull the elevator to the full up and move the trim tab until its flush with the elevator, you have set it to the correct takeoff position. (Skyhawk has a rear window to allow a view of that)
Click to expand...
Yea, i've notice when I preflight the angle the trim tab makes with the elevator changes as you raise and lower the elevator. Is that what you are referring to?
 
FlySince9 said:
I'll defer to any mechanics out there, but the observation you made about the relative angle of the tab in the up/down position may be related to the fact that the elevator was in the full down position (in the photo)... I think there is an active relationship between the position of each//I say that because one of my instructors once showed me that if you pull the elevator to the full up and move the trim tab until its flush with the elevator, you have set it to the correct takeoff position. (Skyhawk has a rear window to allow a view of that)
Click to expand...

Thanks. I'll check that next time.
 
FlySince9 said:
I'll defer to any mechanics out there, but the observation you made about the relative angle of the tab in the up/down position may be related to the fact that the elevator was in the full down position (in the photo)... I think there is an active relationship between the position of each//I say that because one of my instructors once showed me that if you pull the elevator to the full up and move the trim tab until its flush with the elevator, you have set it to the correct takeoff position. (Skyhawk has a rear window to allow a view of that)
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Let's clear that up.

1. The tab doesn't move much with elevator movement.

2. The service manual for the specific model defines the proper trim tab rigging. In some older 172s' for instance, the tab is close to streamlined when the trim is set to the takeoff positions. In other models, like the L or M, the tab will be up around 8 or 10 degrees in the takeoff position. This is because the tab travels are to be set first, then the indicator is to be set so that it travels to the same distance from each end of the indicator slot, and that will place the tab up somewhat in the TO position For an instructor to tell you to see that it's streamlined for takeoff is wrong, because it applies a standard for one model to all the models, and too many mechanics do the same thing and misrig it. Further, other airplanes will have the tab in a different spot for takeoff, such as the 210, where it's up 10 degrees.

3. The 172 has extra down trim (tab up) because the downwash off the flaps raises the nose so that little nose-up is needed, and lots of nose-down if loaded aft.
 
luvflyin said:
Thanks for the right up. I always check to make sure it works and in the right direction, but just through enough range to verify that. Think I'll start going through more range.
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As a student I get a lot out of this forum. Though I have a high respect for our checklists, some things seem like overkill. Not anymore.
Every flight I have been doing the checklist, which does in fact include moving the trim all the way through its range forward and backwards, then set to takeoff position. When I've been a little anxious to get flying it felt kinda stupid. So I read this and think "hmm..not so pointless after all".

But it is kind of hard to see when in the pilots seat. I have to check if I really look I can see the position, or else guess I have to add for myself to put it forward before doing the preflight check?
 
LongRoadBob said:
As a student I get a lot out of this forum. Though I have a high respect for our checklists, some things seem like overkill. Not anymore.
Every flight I have been doing the checklist, which does in fact include moving the trim all the way through its range forward and backwards, then set to takeoff position. When I've been a little anxious to get flying it felt kinda stupid. So I read this and think "hmm..not so pointless after all".

But it is kind of hard to see when in the pilots seat. I have to check if I really look I can see the position, or else guess I have to add for myself to put it forward before doing the preflight check?
Click to expand...

That's what I do. One of the first things I do is check the rudder and elevator for correct movement. I do it while I'm standing outside sticking one foot in to press the rudder pedals and I can see back there. That of course won't work on all planes.
 
LongRoadBob said:
But it is kind of hard to see when in the pilots seat. I have to check if I really look I can see the position, or else guess I have to add for myself to put it forward before doing the preflight check?
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What aircraft? In many you can just pull the elevator up and then you can see the trim tab just fine by looking out the back. Others, not so much. Also have to know if doing that if it's an anti-servo tab since that'll change the position of it, if it is.

On most Cessnas I can just look out the back window and pull on the elevator during the flight control check.

On the Seminole, it's many feet in the air. The best one can do is while standing on the wing, reach inside and behind you while facing rearward before you board and pull the elevator up from that position. You can just see the top of the elevator and see that the anti-servo tab moves upward higher than the rest of the assembly. The bottom you look up at during the walk around from below.

As far as moving the trim the entire range for every flight, that seems a bit overkill. Moving it and seeing that it moves in the correct direction and also that it's not stuck in some wild position is plenty enough to know that you can overpower it, as long as it's somewhere near neutral or slightly nose up.

Most light aircraft trims are not designed to be strong enough to completely overpower the pilot. The electric trim is intermittent on one of the airplanes I fly and it's mostly just an annoyance that you push the button and it goes nowhere. You still have to push or pull to fly whatever pressure you were trying to trim away, and reach for the slow manual trim wheel and start cranking. And then it'll start working again during the flight and work perfectly on the ground. Mechanics can't find it.
 
Denverpilot, it's a Piper warrior II. got a dent in my forehead from going "duh...of course" on your suggestion to raise the elevator in order to see the trim position. I definitely see some of the elevator when I check full deflection so that ought to work.

also realized I have to be sure which direction, thought about it and checked another post here which gives it, so I know it isn't backwards.

Thanks!
 
LongRoadBob said:
Denverpilot, it's a Piper warrior II. got a dent in my forehead from going "duh...of course" on your suggestion to raise the elevator in order to see the trim position. I definitely see some of the elevator when I check full deflection so that ought to work.

also realized I have to be sure which direction, thought about it and checked another post here which gives it, so I know it isn't backwards.

Thanks!
Click to expand...

Tips and tricks man, all sorts of tips and tricks you pick up over the years. Happy to share. The checklists are great but they don't say stuff like, "Hey while you have that elevator pulled into your lap, take a peek back there at the trim tab." That's just stuff you pick up along the way.

Old timers taught me these things, I just pass them along. Some think all my white hair makes me an old timer too, but I'll probably grow a lot more white hair instructing. Haha.
 
denverpilot said:
Tips and tricks man, all sorts of tips and tricks you pick up over the years. Happy to share. The checklists are great but they don't say stuff like, "Hey while you have that elevator pulled into your lap, take a peek back there at the trim tab." That's just stuff you pick up along the way.

Old timers taught me these things, I just pass them along. Some think all my white hair makes me an old timer too, but I'll probably grow a lot more white hair instructing. Haha.
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Nate,

You're not much older than me, so if you're an old timer, i guess that might make me one too
 
register@teamandras.com said:
I had to push-over on the yoke to descend, even with reduced power it seemed, so I made a long and wide downwind and eventually got it back on the ground.
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Keep in mind that if you have to keep forward pressure on the yoke due to trim position, slowing the airplane will relieve that pressure. The reverse holds true for holding back pressure, of course.
 
MauleSkinner said:
Keep in mind that if you have to keep forward pressure on the yoke due to trim position, slowing the airplane will relieve that pressure. The reverse holds true for holding back pressure, of course.
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And on many airplanes, even full nose up trim makes for a good approach speed, power off. On a 172, it will give you best glide when clean and power idle.
 
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