Trim effect jammed elevator emergency

[David36]

Would trim have any effect if it's used in an opposite way on a stuck elevator emergency in a convential plane (eg Cessna 172)? It is not clear for me if the trim tab really acts as a mini elevator in this scenario. There is a guy (don't remember his name) who said he did tests holding the yoke in position and using trim the opposite way and unfortunately there was no effect. But a lot of books and safety articles state that it works.

What's the truth behind this? Is this more like a flying safety myth? Or a proved flying technique?

I hope none one ever is put in such a situation to test it on his own skin.

 

[poadeleted20]

If you're talking a 172, or other aircraft with a servo tab for pitch trim, then yes, with a jammed elevator, rolling the trim in what is normally the nose-down trim direction will raise the trim tab and pitch the plane nose-up. This is a known emergency technique for which, fortunately (thanks to the dirt-simple control systems in your light singles), there is not much call to use.

Obviously, the trim tab being rather small in comparison to the elevator, you'll need more deflection of the tab to achieve the same effect, and it will probably be a bit slower action-feedback-reaction cycle. Best thing I can suggest is to take the plane up to a safe altitude, have someone in the right seat hold the yoke shaft in position against the shaft collar, using the trim wheel to control pitch, and seeing what happens.

 

[flyingmoose]

If you're talking a 172, or other aircraft with a servo tab for pitch trim, then yes, with a jammed elevator, rolling the trim in what is normally the nose-down trim direction will raise the trim tab and pitch the plane nose-up. This is a known emergency technique for which, fortunately (thanks to the dirt-simple control systems in your light singles), there is not much call to use.

Obviously, the trim tab being rather small in comparison to the elevator, you'll need more deflection of the tab to achieve the same effect, and it will probably be a bit slower action-feedback-reaction cycle. Best thing I can suggest is to take the plane up to a safe altitude, have someone in the right seat hold the yoke shaft in position against the shaft collar, using the trim wheel to control pitch, and seeing what happens.

I think you find that it does work, although it will be slow to react because of the big difference in area.

 

[David36]

Also, if the cable fails or anything in the flight control system, is there any logic to snap the elevator fully upwards or downwards? I've read some accident descriptions which states that elevator deflected upwards after cable failure. That doesn't seem plausible because as I know the airflow will streamline the elevator (assuming it will be freefloating, I don't figure out a problem where the elevator will stuck more than present position).

I'm somehow a new private pilot and became interested in subject after some discussion and heard some stories, you know, there are a lot of stories in hangar talk

 

[Fearless Tower]

read some accident descriptions which states that elevator deflected upwards after cable failure. That doesn't seem plausible because as I know the airflow will streamline the elevator (assuming it will be freefloating, I don't figure out a problem where the elevator will stuck more than present position).

That probably depends on how the control surface is designed and balanced. I suspect that it varies from airplane to airplane.

 

[dell30rb]

If the elevator is STUCK the trim tab will act as a mini elevator. BUT in reverse. You trim nose-up to go nose down.

If the elevator is not stuck, but you can't control it (broken control cable) then the trim tab will still work, but it will work as normal.

And thats assuming a 172. There are different types of trim systems

 

[tlglenn]

Best thing I can suggest is to take the plane up to a safe altitude, have someone in the right seat hold the yoke shaft in position against the shaft collar, using the trim wheel to control pitch, and seeing what happens.

And don't try using the control lock to hold the yoke in place.

 

[bobmrg]

It is certainly a proven technique...Barry Schiff wrote about it in The Proficient Pilot, chapter title is "Flight Control Failure." Every pilot should know how to handle such failures...lay hands on a good instructor and experiment.

Bob Gardner

 

[poadeleted20]

And don't try using the control lock to hold the yoke in place.

I saw your post and thought, "Well, that goes without saying," but then I thought about some of the things I've seen folks do in airplanes over the last four decades and thought, "Ummm...maybe not."

 

[Dan Thomas]

The throttle works as pitch control, too.

Dan

 

[tiger]

Question to the CFIs from someone who has never tried it. What would a standard 'procedure' for this exercise look like?

Seems like you want to approach it with a plan, and I wonder if there's anything to be careful to avoid (for example, messing with this at too high an airspeed - I don't think I'd want to be in the position that if my buddy in the right seat slips or lets go abruptly I have a free elevator and full nose up trim at 120kts).

 

[poadeleted20]

Question to the CFIs from someone who has never tried it. What would a standard 'procedure' for this exercise look like?

Set the plane up in level flight at or below maneuvering speed, properly trimmed, at a safe altitude (3000 AGL or so). Have your partner hold the yoke shaft from moving. Then you gently move the trim wheel and see what the response is.

Seems like you want to approach it with a plan, and I wonder if there's anything to be careful to avoid (for example, messing with this at too high an airspeed - I don't think I'd want to be in the position that if my buddy in the right seat slips or lets go abruptly I have a free elevator and full nose up trim at 120kts).

If you do it gently as suggested above, you shouldn't have that much off-trim dialed in, and if you stay at or below maneuvering speed, it shouldn't be possible to break anything.

 

[tiger]

Thanks Ron. Yes, remaining below Va seems a sound plan. I'm not going to be flying any time soon (unfortunately) but I am curious to find out just how much authority the trim/anti-servo tab on a PA28 provides.

 

[David36]

Is it going to be different on a stabilator design? Or the trim acts the same and opposite way on a jammed stabilator?

 

[poadeleted20]

Is it going to be different on a stabilator design? Or the trim acts the same and opposite way on a jammed stabilator?

If the stabilator uses a servo tab for trim, then it would work the same way. If the stabilator uses a bungee or other force input on the primary pitch control path as its means of trimming, then it would not.

 

[Dead Stick]

For what it's worth, if you're flying jets, it's something that's regularly practiced in the sim. No big deal. It's something that I do once or twice with all of my private and commercial students. Back in the day, I was flying a Stinson 108-3 Station Wagon that belonged to a friend. He had just had a local avionics shop install a new nav/com radio in it and one of the mounting brackets slipped and started to interfere with the elevator movement. I made an uneventful landing using elevator trim only.

 

[David36]

If the stabilator uses a bungee or other force input on the primary pitch control path as its means of trimming, then it would not.

Regarding this design, I figure out that either disconnected (freefloating) or jammed, the trim won't have any effect, am I right?

 

[tiger]

Back in the day, I was flying a Stinson 108-3 Station Wagon that belonged to a friend. He had just had a local avionics shop install a new nav/com radio in it and one of the mounting brackets slipped and started to interfere with the elevator movement. I made an uneventful landing using elevator trim only.

Do you mean the surface itself was jammed and you used trim in the 'wrong' direction, or you had to use trim to move the elevator in the 'normal' direction?

 

[poadeleted20]

Regarding this design, I figure out that either disconnected (freefloating) or jammed, the trim won't have any effect, am I right?

Which design? If you're talking PA28, I just checked one today -- the trim control directly drives the trim tab, so in either a jammed or disconnected stabilator situation, it should work the same as a 172 -- normal direction for free-floating, opposite for jammed.

 

[tlglenn]

Something else interesting to try if you have a plane equipped with a two-axis autopilot with altitude hold: Setup in level flight and intercept an ILS localizer. Use the autopilot to stay on the localizer. When you intercept the glideslope turn-off the altitude hold. Then use throttle and trim to stay on the glideslope.

 

[poadeleted20]

Something else interesting to try if you have a plane equipped with a two-axis autopilot with altitude hold: Setup in level flight and intercept an ILS localizer. Use the autopilot to stay on the localizer. When you intercept the glideslope turn-off the altitude hold. Then use throttle and trim to stay on the glideslope.

If you're trimmed right, you should only need throttle.

 

[Dan Thomas]

The elevator would have to be jammed pretty solidy for the trim to work backwards as an elevator. Most systems have some sponginess due to cable stretch and so on, and the elevator is likely to move a bit when the tab is moved, negating whatever you're trying to accomplish with it. Holding the yoke firmly isn't going to stop all elevator movement.

Dan

 

[poadeleted20]

The elevator would have to be jammed pretty solidy for the trim to work backwards as an elevator.

Agreed, but there are a number of ways for that to happen, including impact with an external object, or a foreign object lodged in the aft bellcrank assembly downstream of the cables. Also, there are some aircraft with pushrods rather than cables, and those have no give.

 

[David36]

Which design? If you're talking PA28, I just checked one today -- the trim control directly drives the trim tab, so in either a jammed or disconnected stabilator situation, it should work the same as a 172 -- normal direction for free-floating, opposite for jammed.

I was talking about trim spring design. Regarding it, I figure out that either disconnected (freefloating) or jammed, the trim won't have any effect, am I right?

Most systems have some sponginess due to cable stretch and so on, and the elevator is likely to move a bit when the tab is moved, negating whatever you're trying to accomplish with it.

Dan

I'm sorry but I don't uderstand your assumption. If "the elevator is likely to move a bit when the tab is moved", that would have an effect (even small) on pitch, so won't it function using trim the normal way? How is it gonna negating whatever you're trying to accomplish with it? In normal operation, trim moves the elevator and it doesn't negate it.

That's how I think about it: Let's assume your elevator or yoke is jammed, you use trim the opposite way and due to sponginess in the system the elevator moves a bit, that would have a little effect on pitch, but in the opposite way expected and that means you can use the trim normal way, even if the range might be small (because cable can't stretch a lot), right or am I missing something?

 

[poadeleted20]

I was talking about trim spring design. Regarding it, I figure out that either disconnected (freefloating) or jammed, the trim won't have any effect, am I right?

Not sure which aircraft you're talking about, but yes, if pitch trim force is created by some sort of in-line force device in the elevator control linkage rather than independent actuation of a trim control surface on the elevator/stabilator (the 172 and PA28 both having the latter), then trim will have no effect in the "jammed" case. Whether it will have any effect in the disconnect case would depend on the specifics of the system design and where the disconnect occurred.

 

[Dan Thomas]

I was talking about trim spring design. Regarding it, I figure out that either disconnected (freefloating) or jammed, the trim won't have any effect, am I right?



I'm sorry but I don't uderstand your assumption. If "the elevator is likely to move a bit when the tab is moved", that would have an effect (even small) on pitch, so won't it function using trim the normal way? How is it gonna negating whatever you're trying to accomplish with it? In normal operation, trim moves the elevator and it doesn't negate it.

That's how I think about it: Let's assume your elevator or yoke is jammed, you use trim the opposite way and due to sponginess in the system the elevator moves a bit, that would have a little effect on pitch, but in the opposite way expected and that means you can use the trim normal way, even if the range might be small (because cable can't stretch a lot), right or am I missing something?

You might get just enough elevator movement to cancel out the trim's elevator effect. For instance, if the system is jammed, you roll in nose-down pitch thinking to raise the nose using the tab, but the elevator moves down a bit due to the system's sponginess, and no pitch change happens.

Dan

 

[David36]

If the elevator moves down a bit due to the system's sponginess, I think there will be pitch change, how do you figure out that no pitch change happens if there is an elevator movement?

And even being as you say, after that little elevator movement due to cable slack or stretch, the elevator will become solidly jammed and the trim tab will move alone and this trim tab movement would have any effect. (of course, excepting the case when generally the trim tab design is too small to have any effect)

 

[poadeleted20]

There isn't that much play in the elevator control system. If there were, the plane would fly like crap.

 

[David36]

Ok, but when the elevator movement due to the play is over, the trim tab would begin to work the assumed opposite way, right or not? And if the tab is big enough to have pitch effect it would work. So the cable stretch or play shouldn't be an issue. Am I missing something?

 

[poadeleted20]

Ok, but when the elevator movement due to the play is over, the trim tab would begin to work the assumed opposite way, right or not? And if the tab is big enough to have pitch effect it would work. So the cable stretch or play shouldn't be an issue. Am I missing something?

Not as far as I can tell.

 

[danhagan]

Also, if the cable fails or anything in the flight control system, is there any logic to snap the elevator fully upwards or downwards?

I'm somehow a new private pilot and became interested in subject after some discussion and heard some stories, you know, there are a lot of stories in hangar talk

I haven't checked and do not know if it applies to all GA aircraft, but I thought the elevator required TWO cables for control. Thus, if one broke you would have either up or down control (unless it was cable jam).

 

[David36]

You are right, searched about it, and usually there are two cables, but still no logic reason to snap the elevator fully upwards or downwards as I read in some less or more official reports or hangar talk.

 

[poadeleted20]

I haven't checked and do not know if it applies to all GA aircraft, but I thought the elevator required TWO cables for control. Thus, if one broke you would have either up or down control (unless it was cable jam).

Depends on the design and where the break occurs. If it's part of a bellcrank to bellcrank section, yes, you'd have one direction, but not if it's part of a loop around a pulley.

 

[David36]

What I've also read about the full disconnected free elevator scenario is that some designs might be prone to flutter, but I can't certify they were talking about some experimental designs or standard certified aircrafts cause the article I read covered also some experimental aircrafts design flaws.

 

[Dan Thomas]

There isn't that much play in the elevator control system. If there were, the plane would fly like crap.

You haven't rigged an elevator control system. On the typical Cessna I can get an inch of extra yoke movement after the elevator hits its stops, even with the cable tension set to specs. Most of that is cable stretch, and some is flex in the rest of the structure: pulley and brackets, the forward bellcrank and pushrod and so on.

There are fore and aft stops for the yoke assembly, and up and down stops at the elevator. Cessna has designed extra travel into the yoke end just to accommodate the slop in the system, and I have often found that mechanics have misrigged things by tensioning one cable instead of both when they find the tension low. They end up with the yoke hitting one of its stops before the elevator reaches its stop, and the pilot loses some authority once the air loads work against the travel.

Every system has some sponginess to it, even push-pull rod systems. There's no such thing as a totally rigid system. Thankfully, we're dealing with air here, not pavement. If we need more elevator we pull a bit more, and it moves a bit more but a little less than we moved the controls. It doesn't make the airplane fly sloppily. We just get used to the pressures we need to apply to get the control we need.

Stick the control lock in, go back to the elevator, and see how much you can flex it up and down. Shocking, I tell you.

Dan

 

[David36]

Well, if there's usually so much play, in a jammed elevator emergency scenario we could rely more on the normal trim operation way instead of the debated theoretical opposite way?

 

[poadeleted20]

You haven't rigged an elevator control system.

Actually, I have (at least, I helped the licensed mechanic who was rigging my Tiger).

 

[poadeleted20]

Well, if there's usually so much play, in a jammed elevator emergency scenario we could rely more on the normal trim operation way instead of the debated theoretical opposite way?

There isn't that much play (or, at least, there shouldn't be).

 

[David36]

In this case I come back to one of my post above:

Ok, but when the elevator movement due to the play is over, the trim tab would begin to work the assumed opposite way, right or not? And if the tab is big enough to have pitch effect it would work. So the cable stretch or play shouldn't be an issue. Am I missing something?

It seems it becomes a back and forth and little funny and confusing issue. I'm sorry guys if it gets annoying.

 

[Dan Thomas]

In this case I come back to one of my post above:



It seems it becomes a back and forth and little funny and confusing issue. I'm sorry guys if it gets annoying.

It all depends on where the jam is. If the yoke mechanism behind the panel is jammed, you have some movement available at the elevator due to cable stretch and other flex points. If the elevator itself is jammed for some reason, you have no useful movement at all, or maybe useful movement in one direction only if there's some object that has fouled the elevator bellcrank or bobweight or whatever might be back there.

The Champ/Citabria series of airplanes have a really narrow slot in the fuselage framework where the elevator's lower bellcrank passes through, and any FOD in there can stop movement in one direction. I have seen exactly that, where a gusset, poorly attached in the belly fabric formers, came loose and wedged itself under that bellcrank during aerobatics and limited the up-elevator travel. Enough was available for landing. On another occasion we found an AA battery laying in that spot, waiting to cause trouble. Someone had replaced the batteries in their headset and dropped the old ones on the floor. Those airplanes have gaps around the floor panels where stuff like that drops into the belly.

Dan