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What happens if you don't stab trim an aircraft

Posted: Thu Sep 24, 2020 7:03 pm
by Trimeresurus
Let's say you always keep the stab trim in neutral and keep your pitch by just putting pressure on the yoke, never once using trimming(or autopilot induced trimming) during whole flight, including final approach and flare, say in a 737 Classic or NG. Would the hydraulic system have issues because it has to constantly pressurize the control surfaces, and if answer is no, why does autopilot has the use trim at all if it doesn't have the concern of hand comfort of keeping the yoke in a position with force. I assume more fuel burn to pressurize the control surfaces is the reason?

And can a 737 be flared and touched down with no trim? Would it sink too much and hit the runway too hard?

Re: What happens if you don't stab trim an aircraft

Posted: Thu Sep 24, 2020 7:10 pm
by unimproved
You can fly and land just fine with neutral trim. It just causes wear on the control surface attach points and is less efficient due to more drag.

Hydraulic flight surfaces are always powered at 3000 psi, only the flow changes when moving them.

Re: What happens if you don't stab trim an aircraft

Posted: Thu Sep 24, 2020 7:55 pm
by Trimeresurus
unimproved wrote:
You can fly and land just fine with neutral trim. It just causes wear on the control surface attach points and is less efficient due to more drag.

Hydraulic flight surfaces are always powered at 3000 psi, only the flow changes when moving them.


I guessed that too, but I somehow thought prolonged untrimmed flight would cause issues on the hydraulics, apparently I was wrong.

Are Airbuses ever trimmed by the pilot manually under normal law? Or does the ELAC automatically handle it depending on stick input and how long the aircraft has been in that pitch angle?

Re: What happens if you don't stab trim an aircraft

Posted: Fri Sep 25, 2020 12:32 am
by Starlionblue
Trimeresurus wrote:
unimproved wrote:
You can fly and land just fine with neutral trim. It just causes wear on the control surface attach points and is less efficient due to more drag.

Hydraulic flight surfaces are always powered at 3000 psi, only the flow changes when moving them.


I guessed that too, but I somehow thought prolonged untrimmed flight would cause issues on the hydraulics, apparently I was wrong.

Are Airbuses ever trimmed by the pilot manually under normal law? Or does the ELAC automatically handle it depending on stick input and how long the aircraft has been in that pitch angle?


You don't trim manually. The flight control systems handles all that automatically. You don't trim in alternate law either. Only in direct law.

On the older A330s, you have to set the trim on the ground after engine start, but on the newer ones and the A350 it is automatic.

The hydraulics are super powerful and under load the whole time. No problem holding a surface deflected for long periods. For example in A350 if you're in Backup Control (equivalent to mechanical backup), the elevator is lost. Pitch trim, using the pitch trim switches, deflects the elevators.

Re: What happens if you don't stab trim an aircraft

Posted: Fri Sep 25, 2020 12:48 am
by Redbellyguppy
On the 737 there are certain conditions where the aircraft will apply some trim for you. And, this is exclusive of anything MCAS on the Max.

If you remain at takeoff trim the whole flight... between 4-7 units in an 800 depending on weight and flap setting... the elevator control pressures will be -very very- heavy when you aren’t flying at an airspeed that requires anything close to those trim settings. A mistrimmed stab also creates a lot of drag. Consider that during approach the stab trim will be somewhere around 10 normally.

Re: What happens if you don't stab trim an aircraft

Posted: Fri Sep 25, 2020 1:58 am
by Starlionblue
I'll add that on the A330, you can trim manually via the trim wheels at any time. This freezes auto-trim while you are doing it.

On the A350, there are no trim wheels. Only trim switches. These are locked out in flight except in Direct Law and Backup Control.


Trimeresurus wrote:
Or does the ELAC automatically handle it depending on stick input and how long the aircraft has been in that pitch angle?


It isn't just the ELACs controlling surfaces. In normal operation, each computer, both primary and secondary, is controlling designated surfaces. Many surfaces have a designated backup so they will still be controlled if the designated computer fails. For example on the A350, PRIM3 controls the pitch trim, and if that fails or is inop, there is a hierarchy of which computers take over. On the A330 it is PRIM1.

Here's the A330 architecthure.

Image

Here's the A350 architecture, showing which computer controls which surface boxed in red, and the backups indicated by yellow arrows.

Image

I don't know the A320 too well, but according to this, ELAC1 controls the stabilizer trim, backed up by SEC1 and SEC2. http://www.dutchops.com/Portfolio_Marce ... trols.html

Re: What happens if you don't stab trim an aircraft

Posted: Fri Sep 25, 2020 2:37 am
by zeke
Trimeresurus wrote:
Would the hydraulic system have issues because it has to constantly pressurize the control surfaces, and if answer is no, why does autopilot has the use trim at all if it doesn't have the concern of hand comfort of keeping the yoke in a position with force.


The reason why the autopilot is required to trim is a certification requirement. Otherwise when the autopilot is disconnected it may result in an undesired aircraft state if the pilots are not quick enough to catch it.

Re: What happens if you don't stab trim an aircraft

Posted: Fri Sep 25, 2020 9:05 am
by GogonAger
I read that it was asked about specific aircrafts but i just wanted to denote that there are fundamental differences between for example the good old small Cessna where the autopilot can only has control via the trim (but since the trim there is not moving the entire stabilizer but rather a trim tab, which acts on the control surface has a kind of indirect control of the surface) and larger jet.

The adjustable stabilizer does not only trimming but also ensures full effectiveness of the control surface over a much greater range of acceptable CG positions than on smaller planes. I do not have specific information for e.g. the B737 but at some airliners it might be possible that the control authority of the elevator alone might not be big enough or potentially completely lost, if the stabilizer is not set correctly or worst case close to stalled. So while the previous answer let me assume that it might be possbile with 737 to stay in T/O-trim the entire flight, on other jets you might come to a point where you're not able to control the aircraft with the available range of the control surface without moving the stabilizer. Especially after configuration changes or CG-shifts.

AFAIK on the Tu-154 was a big lever-kind control right in the center where you would look for the autopilot controls on other jets which was used to set an automatic stabilizer deflection linked to flap deployment. If not set according to current CG position the aircraft might become uncontrollable during approach. Trimming on the other hand was done with tabs there....
Green position for forward CG, white for medium and amber for aft CG. Amber not approved for flights with PAX. This info is partially out of a sim manual however so don't take it for granted but until now it sounds reasonable for me.


Re: What happens if you don't stab trim an aircraft

Posted: Fri Sep 25, 2020 9:19 am
by tommy1808
unimproved wrote:
Hydraulic flight surfaces are always powered at 3000 psi, only the flow changes when moving them.


or 5000 psi.

Re: What happens if you don't stab trim an aircraft

Posted: Fri Sep 25, 2020 11:45 am
by Woodreau
There is a big inertial range that is covered by an airliner - from 100 knots to 450/500 knots. A stabilizer position for a slower speed is different from a stabilizer position for higher speed.

If you flew an aircraft without trim, as you increase speed, you would be using elevator control to counteract the untrimmed stabilizer, and eventually you would run out of elevator authority to maintain pitch control.

Re: What happens if you don't stab trim an aircraft

Posted: Fri Sep 25, 2020 11:46 am
by extender
unimproved wrote:
Hydraulic flight surfaces are always powered at 3000 psi, only the flow changes when moving them.


5,000 psi on the A380. Just an FYI.

Re: What happens if you don't stab trim an aircraft

Posted: Fri Sep 25, 2020 11:54 am
by Starlionblue
extender wrote:
unimproved wrote:
Hydraulic flight surfaces are always powered at 3000 psi, only the flow changes when moving them.


5,000 psi on the A380. Just an FYI.


And A350.