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Absence of large manual pitch trim wheels on non-Boeing jetliners.

Posted: Tue Aug 24, 2021 3:19 am
by convair880mfan
In looking at flight deck photos of MD jetliners, I do not see the large manual pitch trim wheels that are located on the sides of the center pedestal on many Boeing aircraft. How come? Do MD jetliners have manual pitch trim controls? What about Airbus and other commercial jetliners?

How do non-Boeing jetliners handle manual pitch trim changes?

Thanks in advance for any information.

Re: Absence of large manual pitch trim wheels on non-Boeing jetliners.

Posted: Tue Aug 24, 2021 3:49 am
by fr8mech
So, if you look to the left of the spoiler handle of the MD11, you’ll find 2 black and white striped handles. These are commonly referred to as the suitcase handles. Push forward, nose down…pull aft, nose up.

The early Douglas aircraft have them also. So do the MD8X aircraft, near as I can tell

Forgot to add: the handles do track normal stab movement.

Re: Absence of large manual pitch trim wheels on non-Boeing jetliners.

Posted: Tue Aug 24, 2021 4:25 am
by Aaron747
convair880mfan wrote:
In looking at flight deck photos of MD jetliners, I do not see the large manual pitch trim wheels that are located on the sides of the center pedestal on many Boeing aircraft. How come? Do MD jetliners have manual pitch trim controls? What about Airbus and other commercial jetliners?

How do non-Boeing jetliners handle manual pitch trim changes?

Thanks in advance for any information.


It bears noting that a majority of Boeing jets do not have the trim wheels, from 747 on up to 787.

Re: Absence of large manual pitch trim wheels on non-Boeing jetliners.

Posted: Tue Aug 24, 2021 4:28 am
by convair880mfan
thanks fr8mech.

Is runaway trim possible on non-Boeing aircraft? It seems like on Boeing aircraft, Boeing commercial aircraft, that you have to grab a spinning wheel and that there is a lot of cranking involved in trimming manually. Maybe I am wrong. How about other aircraft?

Re: Absence of large manual pitch trim wheels on non-Boeing jetliners.

Posted: Tue Aug 24, 2021 5:00 am
by Aaron747
convair880mfan wrote:
thanks fr8mech.

Is runaway trim possible on non-Boeing aircraft? It seems like on Boeing aircraft, Boeing commercial aircraft, that you have to grab a spinning wheel and that there is a lot of cranking involved in trimming manually. Maybe I am wrong. How about other aircraft?


Runaway trim is possible with the failure of any trim motor system, whether electric or any other mode. Such a condition has nothing to do with being Boeing or not Boeing. Unless the trim wheel has a manual link by cable to the trim mechanism (like in a C-172), it is nothing more than a way of sending commands for desired setting to the motor system.

Re: Absence of large manual pitch trim wheels on non-Boeing jetliners.

Posted: Tue Aug 24, 2021 5:03 am
by Starlionblue
There is no runaway trim procedure on the A330 or A350. There is a STAB CTL FAULT procedure on the A330 if electrical control is lost.

The A330 has (slowly) moving trim wheels. Since trim is normally automatic, you don't use them unless you're in Direct Law or Backup. (Except to set takeoff trim on the older tails that don't do this automatically.)

The A350 only has trim switches. As on the A330, you only need to manually trim in Direct Law or Backup.

convair880mfan wrote:
thanks fr8mech.


Is runaway trim possible on non-Boeing aircraft? It seems like on Boeing aircraft, Boeing commercial aircraft, that you have to grab a spinning wheel and that there is a lot of cranking involved in trimming manually. Maybe I am wrong. How about other aircraft?


AFAIK the whole rapidly spinning wheel and lots of cranking is just for older Boeing models, e.g. 707 and 737. But I could be wrong.

Re: Absence of large manual pitch trim wheels on non-Boeing jetliners.

Posted: Tue Aug 24, 2021 2:56 pm
by fr8mech
convair880mfan wrote:
thanks fr8mech.

Is runaway trim possible on non-Boeing aircraft? It seems like on Boeing aircraft, Boeing commercial aircraft, that you have to grab a spinning wheel and that there is a lot of cranking involved in trimming manually. Maybe I am wrong. How about other aircraft?


Runaway trim is possible. If the trim runs away from you on the DC8, you “split” the handles and that should stop the trimming. But, there is no direct link to the drive system. On the 8 there were alternate trim switches that drove an electric motor.

I assume the later aircraft are the same…I’ll check the AMM on the MD11 later tonight.

Incidentally, splitting the handles can also be accomplished by splitting the dual trim switches on the yokes.

Re: Absence of large manual pitch trim wheels on non-Boeing jetliners.

Posted: Tue Aug 24, 2021 9:37 pm
by Horstroad
fr8mech wrote:
But, there is no direct link to the drive system. On the 8 there were alternate trim switches that drove an electric motor.

I assume the later aircraft are the same…I’ll check the AMM on the MD11 later tonight.

The suitcase handles on the MD-11 are directly connected to the primary trim control valves via control cables and pushrods.

There are two primary trim control valves in parallel, one for hydraulic system 1, one for hydraulic system 3. Each valve controls one trim motor. Both trim motors are connected through trim brakes (which are released by hydraulic pressure) to the stabilizer drive gearbox. From there a chain runs through a differential drive fuse to each ACME screw nut of the two ACME screw actuators.

Each of the two primary trim control valves consists of two valve spools in series. One is controlled by the LH suitcase handle, the other by the RH suitcase handle.
Each Valve spool also has 2 solenoids, one for ANU, one for AND (a total of 8 solenoids, 4 on each primary trim control valve). Each solenoid has two coils, one for manual trim commands from the control wheel trim switches and one for automatic trim commands from the flight control computers.

Hydraulic fluid can pass through both valve spools only when both suitcase handles or trim switches operate together.



But there's no direct connection to the actual drive system. When there's no hydraulic pressure you can not trim. There's no electric or mechanical backup.

Re: Absence of large manual pitch trim wheels on non-Boeing jetliners.

Posted: Wed Aug 25, 2021 8:33 pm
by bigb
A lot of modern aircraft have gone away from the trim wheel. See also the CRJ, E-JETs, CSeries/A220. These aircraft will have a stab trim cutout procedure in the event that a runaway trim is suspected.

Re: Absence of large manual pitch trim wheels on non-Boeing jetliners.

Posted: Fri Aug 27, 2021 12:24 am
by SteelChair
There's no trim on most of the sidestick aircraft because there is no "feel," no feedback. It's just a stick that drives actuators thru computers.

My understanding is that the trim switch in the A220 is basically a target speed control.