Autotrim
 

Excuse my lack of knowledge, but on a recent jumpseat flight aboard a 737-200 I discovered the world of autotrim. Well I think I did anyway. During the approach the trim wheel appeared to wind itself. On landing and vacating the active rwy, the trim wheel unwound itself.

My question is how does this work? I am used to the trim in a C172 where the Pilot in command makes adjustments on the trim wheel to release the pressure required on the yoke. I guess the autotrim senses excessive movement of the yoke and automatically adjusts elevator trim tabs?

An explanation would be appreciated.

It depends if the autopilot was engaged or not. If it was then yes the autopilot trims just like the humanpilot does (only better). However if this was during manual flight (such as approach and rollout) I'd wager it was just the pilot trimming the aircraft out using the electric trim switch on the control yoke.
And trimming out a jet is a little different to trimming out a prop. When you trim your 172 you trim to reduce control column force, while leaving the control column displaced. In a large jet you trim so that the control column returns to the neutral position.

On both the jumpseat flights I had, the aircraft was flown manually using autothrottle only.

Thanks for the info.

Hmmm, HPSOV is right, I'm afraid: it is a blip-switch on the front of the control column: it cannot do it itself, unless of course the A/P is in. You'll only find autotrim on scarebuses.

...and then it's only for aircraft efficiency anyway - there's no force feedback through a sidestick to trim out in the first place!

Unless you're in Direct Law...

------------------
So what's my left hand for?

The 737-300 has a form of autotrim called speed trim which cuts in during certain conditions (low weight, aft CG, N1 above 60%, flaps not up (for the 300), AP not engaged, sensing a trim requirement etc).
I hate it because it interferes with my hand flying and usualy overtrims. And when a bloodey stab trim starts moving around of its own free will like that, theres always that tiny doubt as to wether it just might be a stab runaway this time.

Well, let's throw in the Airbus trim: There are no trim switches like you see them on other planes. There are the trim wheels on the center console, but they are only used to set initial trim for take off, then you don't touch them unless a lot of stuff goes wrong and the airplane drops out of its normal law. When it does this, it goes into alternate law...no, still autotrim. Only when it goes to direct law, a message appears to use the man pitch trim, the wheels. So you fly with the stick and trim with the wheels. If it gets real bad, you go to mechanical backup. Now a red message tells you that all you have to control pitch is the trim wheel, the stick is dead. How does the plane know how to trim itself when autotrim is active? With the stick you tell the computer where you want the airplane to go (you give it a flight vector, a vector in space, to follow). If you set the nose 3 degrees up, the flight controls deflect as instructed by the FBW to maintain the resulting flight path, not just the attitude. With the given thrust, the airplane settles into the new direction and the system trims for a 1 g condition while following this vector.
That gives us a few funny things like trimming when down when you are in a constant rate climb and the plane enters an aera of updraft. The airplane is lifted and moves above the commanded flight vector. The FBW reacts by initially lowering the nose a tiny bit to return to the original vector to maintain. A conventional airplane would have just experienced an increased rate of climb , but previously in trim , it would have stayed in trim.

Three-Twenty !!

Left hand is for all the goodies you can put on your tray table!
I can't believe that they didn't put side sticks on the 777!!
I guess the old farts who had a say in it were afraid if it!!!

"You'll only find autotrim on scarebuses."


...and just about every other modern jet aircraft. By modern, I mean in the last 40 or so years.

For example, on the 747-400:

Trim is active with the autopilot off or on. There are various modes...
"Speed Trim" is active at lower speeds, when the A/P is off. The trim runs to oppose changes in speed.
"Auto Trim" is active when the autopilot is on, trimming in response to elevator commands being given for a certain period of time by the A/P. During autoland, trim response is not delayed and responds almost instantaneously to elevator commands.

On a 400, you won't hear the trim running (no clicker), but the "pointer" in the Stab Trim indicator still moves.

Rgds.
Q.

Slasher et al,

If you trim accurately yourself, the speed trim will never sense the requirement for trim, and thus will not interfere with your flying. The system needs to sense quite a lot of out-of-trim force to operate.

If the speed trim operates whilst you are hand-flying, this is a sign that you are not trimming accurately yourself - and poor trimming is often to blame for poor accuracy in hand-flying.

The point about 'Is it a trim runaway?' is a valid one. Two points here - first, this is another Boeing 'nasty'; second, make sure you know how to locate and operate the trim cutout switches in an instant - and bear in mind you may have to do this under positive or negative 'G' depending upon the type of runaway and when it occurs.

[This message has been edited by Portly (edited 24 September 2000).]

Ok Ports. Ill give you a 737-300 TO at 41,000kg, flap 15 and max power with a TO stab setting of 4.7 units. Lets see if you can avoid any inputs from the speed trim, especialy in widespread heavy thunderstorm activity which is normal around here.

Then again it could be our 73s system is calibrated a bit oversensitive, if what you say about high out of trim force is accurate.

Yes, Slasher, I can, and here's why:

One of the conditions for Speed Trim Operation is '5 seconds following release of trim switches' (737 tech manual).

Therefore, if you have flown out-of-trim for five seconds - and that's a long time to be out of trim (try counting it out loud) - then Speed Trim will operate.

If you fly for five seconds or more with a requirement for trim, then you are not trimming properly.

I sometimes trim during the rotation, in response to an obviously wrong loadsheet trim for take-off.

Incidentally, if you often depart in the circumstances you describe, you should be a worried man/woman - you're clearly not making any money!! ;) ;) ;)