Trim effect jammed elevator emergency

by **David36** » Wed Dec 19, 2012 4:52 pm

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.

by **Keef** » Thu Dec 20, 2012 12:29 am

Yes, it does.

If the elevator is totally jammed, the trim tab will provide some ability to adjust angle of attack. It won't be a lot, and if the elevator is stuck away from "straight and level" it's not likely the trim tab will help a lot. if the elevator is (for example) frozen by ice at S&L, then the trim tab may be of some use - if it isn't frozen, too, which it probably would be.

I don't think holding the yoke in position would provide a valid test. Control forces would mask the elevator effect of the trim tab.

by **leiafee** » Thu Dec 20, 2012 1:05 am

Always assuming you've GOT a genuine physical trim tab and not some spring-and-sellotape arrangement a-la the dear old Tommyhawk and some others! (Just relieves the control forces - doesn't have any aerodymanic effect!)

by **David36** » Thu Dec 20, 2012 7:46 am

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).

by **Keef** » Thu Dec 20, 2012 10:44 am

The answer is "it depends".

If a cable snaps or jams, it will certainly spoil your day. What the elevator will do will depend on the design, what other components are there, and the aerodynamics of the thing. You will almost certainly find that a (working) trim tab will allow you to exercise some control over the aircraft, but it will be type-dependent.

Have a read of the (counter-intuitive) bits of aerodynamic theory: for example, if you hold two pieces of paper a couple of cm apart and then blow down the gap between them, will they move apart or move together? What does this mean your elevator might do with more windspeed on one side than the other?

It's far better to make a thorough preflight, and particularly to look at the control cables where you can see them. A snapped strand means "do not fly".

by **brand1068** » Thu Dec 20, 2012 11:00 am

I know they are different - but most of my "large" models have closed loop controls on the main surfaces.

You tend to find rudder works well in one direction when one of the cables snaps (happend once)

On the evlevator however i've never had one fail and the model survive to be able to tell... Of course this could be because we fly closer to the ground so have little time to realise what the issue is before it crashes or that one way elevator is much more of an issue...

Chris

by **Paul_Sengupta** » Thu Dec 20, 2012 11:13 am

I wouldn't expect models to have elevator trim tabs.

by **David36** » Thu Dec 20, 2012 11:32 am

Keef wrote:Have a read of the (counter-intuitive) bits of aerodynamic theory: for example, if you hold two pieces of paper a couple of cm apart and then blow down the gap between them, will they move apart or move together? What does this mean your elevator might do with more windspeed on one side than the other?

Oh ya, I tried to figured out, but figure out nothing

so, it would have any effect or no?

by **Sir Morley Steven** » Thu Dec 20, 2012 11:32 am

In a PA28 the trim tab is the bit wot moves the stabilator so if that's jammed you're stuffed as trying to trim won't move anything.
On a C152 it would work.
But you will have done full and free at least twice on the ground innit.

by **David36** » Thu Dec 20, 2012 12:06 pm

Sir Morley Steven wrote:In a PA28 the trim tab is the bit wot moves the stabilator so if that's jammed you're stuffed as trying to trim won't move anything.

In this design, the trim system moves directly the stabilator, not the small tab? Why is that? The yoke what moves, the tab?

by **ZG 862** » Thu Dec 20, 2012 12:26 pm

David36 wrote:
Sir Morley Steven wrote:In a PA28 the trim tab is the bit wot moves the stabilator so if that's jammed you're stuffed as trying to trim won't move anything.

In this design, the trim system moves directly the stabilator, not the small tab? Why is that? The yoke what moves, the tab?

No tab. Just springs IIRC. Leia mentions above.

by **David36** » Thu Dec 20, 2012 12:41 pm

So, in that design, either disconnected (freefloating) or jammed, trim won't work, right? Unbelievable. Can't imagine having one of these emergency in an aircraft like that. Pretty scary.

by **greggj** » Thu Dec 20, 2012 1:13 pm

I thought that trim tab forces elevator to stay in certain position. So if the cable itself is jammed, I find it hard to believe that you could force elevator to move that way. (if it's stuck).

If cable snaps, there were accidents where pilot tried to use trim wheel to control elevation (unsuccessfully tho).

by **brand1068** » Thu Dec 20, 2012 1:37 pm

Paul_Sengupta wrote:I wouldn't expect models to have elevator trim tabs.

Not normally no, although some have fully working ones..

Chris

by **ASI** » Thu Dec 20, 2012 1:44 pm

It's worth noting that if you have a jammed elevator (with a trim tab arrangement), the trim wheel will work in the opposite sense to the placard, versus a snapped elevator cable, where it will work in the normal sense.