Trim effect jammed elevator emergency

[David36]

Would trim have any effect if it's used in an opposite way on a stuck elevator vemergency 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.

[westhawk]

Actually testing the effect of trim tab position on tail down force being exerted in flight would require that the elevator be firmly held in a fixed position. Doing this by holding onto the yoke is problematic and probably won't yield a valid test result since the pressure required to hold the elevator fixed would be changing with the actual trim tab position and would probably result in a small amount of elevator movement in spite of one's best efforts. I tried it once following a similar discussion and didn't believe I was actually holding the elevator in a truly fixed position. Nevertheless my initial impression was that even if I were able to hold the elevator fixed, the effect of changing the trim tab position would have only a small effect upon the resulting pitch attitude of the airplane. It's my impression that only a small amount of influence upon the pitch attitude of the airplane would be possible using trim tab alone with a jammed elevator. Perhaps together with shifting weight within the cabin, changing configuration and power, using the trim wheel might be one factor in providing enough pitch authority to keep the airplane under some modicum of control.

Looking at it from another perspective the surface area of the trim tab compared to the surface area of the elevator is likely in rough proportion to the amount of tail down force change which the trim tab is capable of producing compared to that of the elevator. The effectiveness of the trim tab in changing the neutral force position of the elevator relies upon the distance aft of the elevator hinge line and the amount of aerodynamic force generated by the trim tab deflection. (arm X weight = moment) The aerodynamic force generated by the diminutive trim tab surface area with the elevator being held in a fixed position would be tiny compared to that of the much greater surface area elevator capable of being deflected.

Still it would be interesting to go up and flight test this assertion and put the question to rest once and for all! It would be necessary I believe to construct a clamping device which could be installed on the yoke to positively lock the elevator in a fixed position so the experiment could be carried out with some level of assurance as to it's validity. Of course the locking device would have to be quickly and positively removable even under high load conditions. Who would go to the trouble and expense to find out?

So I'm just going to operate under the presumption that while the trim tab might contribute somewhat to controllability with a jammed elevator, the other measures mentioned above are also available in the event they are required.

Now on to considering a broken cable or other control system malfunction...

Happy flying,

westhawk

[ksjc]

Perhaps your "books" refer to jet aircraft trim systems which typically don't use tabs but screw-jacks that lower and raise the entire horizontal stab to trim. The plane could be flown this way albeit in a fairly narrow pitch margin.

FYI- Tabs create drag...not good for a jet.

[westhawk]

FYI- Tabs create drag...not good for a jet.

Would trim have any effect if it's used in an opposite way on a stuck elevator vemergency in a convential plane (eg Cessna 172)? (my bold)

I think the OP's question wasn't really about jets. Still, I used to fly Hawkers (trim drag probably not a major consideration on this type!) and they have trim tabs in all three flight axis. Not at all uncommon with older designs and those not utilizing hydraulically powered flight controls. Tabs have other disadvantages as well, but so do movable stabilizers. There must be a good reason why "jammed stabilizer" procedures exist. And there was AS 261, the Westwind in Scranton, PA and perhaps others as well.

Pilots have been discussing whether an airplane such as a 172 could be controlled in pitch with elevator trim tab in case of a jammed elevator since long before I took my first lesson in 1983. I thought David36 asked a good question.

One more thing about the Hawkers: They do two things extraordinarily well, descend and slow down! Steady reliable beasts they are too. :ok:

[David36]

As I said, I'm interested in a conventional small plane design (elevator, trim tab e.g. C172) and the "books" were not for trimmable horizontal stabilizer design or orther designs.

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).

[Desert185]

Running elevator trim in the opposite direction with an operable elevator yields a bit more pitch authority, but adds considerable pressure to the control wheel/yolk/stick. Using this technique with a jammed elevator would probably contribute to some pitch effectiveness given the above.

[All-The-Nines]

Excuse my ignorance, but with a basic trim system would you not need to trim in the opposite direction if you had a jammed elevator? As it is the deflection of the trim tab in the airflow which in turn moves the elevator in the opposite direction?

For example, you are going down the runway ready to rotate and realise your elevator control locks are still in. Your natural reaction would tell you to move the trim wheel down, because in the air in a normal scenario this would cause the nose to rise. However, in this instance the trim tab moves down on the elevator but the elevator does not move and so you've managed to create a lesser tailplane down-force and thus your aircraft is now more nose heavy with no chance of rotating.

Instead, if you trim forwards which you'd expect to be the wrong reaction, the trim tab is deflected up in the airflow causing a greater tailplane downforce with the nose now wanting to rise.

[westhawk]

Certainly no ignorance on your part ATN. You're exactly right that that with a jammed elevator nose down trim creates more tail downforce. :ok: Nose down trim input deflects the trailing edge of the tab upwards.

[Lightning Mate]

Tabs create drag...not good for a jet.

Not only that - they simply wouldn't work because of irreversible powered controls.

[David36]

What I've also read about the full disconnected free control surface scenario is that a freefloating surface is prone to flutter and this having really bad consequences. Also, trim tab cable failure could lead to deadly flutter issue. What do you think? I tend to disagree with this, thinking that a freefloating surface is loaded and streamlined due to airflow and not inherently "banging" up and down in the flow, what do you think?

[JammedStab]

I know it is a slightly different topic but of simila interest is this recent article by Barry Schiff.....

"Last month in this column (“Proficient Pilot: Look, Ma, No Rudder!” August 2012 AOPA Pilot), I described an embarrassing situation in 1975 during which I had made several flights in a factory-new Aerostar 601 without realizing that the rudder had never been connected to its cables. Although nothing serious came of it, the potential had existed for tragedy. Fortunately, problems associated with the primary flight controls are rare—but not so rare that they can be ignored.

Some years ago, a private pilot getting checked out in a Cessna 182 was asked by his flight instructor to roll into a steep turn. While the pilot was maintaining altitude during the turn by applying back-pressure to the control wheel, the wheel fell limp in his hand. Pulling back had no effect because of a failed elevator cable.

Given that dramatic situation, what would you have done to restore and maintain control of aircraft attitude?

A typical response would be something such as, “Well, I’d use elevator trim to control pitch.”

That’s a valid answer until you take the question one step further and ask, “How would you land?”

Those same pilots probably would say something such as, “I’d control my attitude and descent profile with trim and power.”

The answer seems logical but implies that they likely have never tried it. Flaring and touching down using only power and trim is far more challenging than you might imagine. Yes, you can use power and trim to make a stabilized approach toward the touchdown zone of a runway, but only to a certain point. Then comes the big surprise. As the airplane descends into ground effect (at a height above touchdown equal to the wingspan of the airplane), the nose will begin to pitch downward. Unless a pilot anticipates the direction and magnitude of this pitching moment, he likely will land nosewheel first and eventually break something. It takes a deft and timely combination of trim and power to make a safe landing without an elevator. This nose-down pitching moment is the result of changing airflow that occurs as the influence of ground effect increases. Similarly, an airplane tends to pitch nose-up as it leaves ground effect after takeoff, although a pilot adjusting climb attitude after liftoff tends to camouflage this effect.

There is, however, a safer and easier way to land an airplane following the failure of an elevator cable.

Although you cannot raise the elevator by applying back-pressure to the control wheel (in this case), this does not mean that you cannot lower the elevator. There are two cables connecting the control wheel (or stick) to the elevator that control pitch; one cable raises the elevator, and the other lowers it.

A clever way to restore pitch control, therefore, would be to apply substantial nose-up trim, much more than you would ordinarily need. You could then apply nose-down elevator to prevent the nose from rising excessively. From then on, you could modulate forward pressure on the control wheel to maintain the desired attitude. In other words, you would ease forward pressure to allow the nose to rise and push a bit more mightily to lower the nose. The point is that you can maintain any reasonable pitch attitude by varying forward pressure on the control wheel.

Flaring for landing could be made by gradually releasing forward pressure on the yoke.

If the cable used to lower the elevator were to fail, you could restore pitch control by applying substantial nose-down trim and modulating back-pressure on the control wheel to maintain the desired attitude. In other words, you would release some back-pressure to allow the nose to go down and pull harder to raise the nose.

Unless both elevator cables were to fail or become disconnected (a virtual impossibility), a pilot experiencing a single-cable failure has the capability to make a safe landing as long as he has previously thought about how he would handle such an emergency. A pilot confronted with this problem should consider making a no-flap landing. Extending flaps creates strong pitching moments that can be difficult to arrest when only limited pitch authority is available. Landing with flaps up also places the airplane in a relatively nose-high attitude that reduces the amount of pitch authority needed to flare. Finally, try especially hard to avoid a go-around. The pitching moments at such a time can be particularly difficult to overcome with limited pitch control.

The nature of this emergency and the increased landing distance associated with a no-flap landing suggest that the pilot should attempt landing at an airport with a long runway.

Although one can practice landing using the described procedure, I urge that no one do so unless under the watchful eye of a competent instructor who has himself performed this maneuver."

[Big Pistons Forever]

The Cessna POH provides direction on how to handle elevator control failure. I would suggest that procedure should be the one to be followed.

[David36]

What about the flutter issue? I found many light sports and I think also some standard FAR 23 certified don't have mass balanced ailerons. (American Champion series: Scout, Citabria etc.).

Do you think that in these aircrafts, for example, in an aileron cable failure the freefloating surface will violently flutter?

[Tarq57]

For example, you are going down the runway ready to rotate and realise your elevator control locks are still in. Your natural reaction would tell you to move the trim wheel down, because in the air in a normal scenario this would cause the nose to rise.

I don't know about your natural reaction, but mine would be to close the taps and slam on the brakes, no matter what lay ahead.

[David36]

I agree, it is obvious that pilot error and engine failures are some of the most common crash causes.
No one is immune to pilot error, but I do not want to get more deep into details about the most common reasons for engine failure leading to fatalities, I think you know what I mean. The airplane flies very good with the engine stopped just as the glider was doing when I took my first flying lesson, of course the L/D ratio is lower.
All you need is some free surface on ground if you are too low. If you don't have that, you are in big trouble. However, you know these aspects, don't want to detail them anymore, what I want to point out is that I figure out there is a gross difference between an inlfight breakup, loosing a wing vs loosing engine power.

[westhawk]

For example, you are going down the runway ready to rotate and realise your elevator control locks are still in.

I didn't catch this sentence the first time around. I actually witnessed this accident (http://www.ntsb.gov/aviationquery/brief.aspx?ev_id=20011120X02271&key=1)and was one of the first few people on scene. Too late, nothing could be done. When the fire department arrived a minute later they doused the fire quickly but the occupants had never had a chance.

westhawk

[Big Pistons Forever]

For example, you are going down the runway ready to rotate and realise your elevator control locks are still in. Your natural reaction would tell you to move the trim wheel down, because in the air in a normal scenario this would cause the nose to rise.



I would suggest the natural action would be to remove the control lock really really quickly.:ugh:

This scenario has got to be about the most preventable incident/accident scenario possible and speaks to such a total and complete lack of airmanship checklist discipline and common sense that it has IMO nothing to do with a true jammed elevator situation.

My personal experience is that a certain segment of student/low hour pilots fixate on weird and wonderful emergency scenarios to the detriment of attaining and maintaining their skills at avoiding the problems that are actually likely to give them a problem in the real world.......