NG Stab Trim Overide Switch

[Pin Head]

I am stupid.

When and why would you use this switch.

Everything we do has a reason why.

Thanks

Pin

[ImbracableCrunk]

The override overrides the trim cutouts at the base of the control column.

It's protecting you in case of a spurious cutout and restores trim switch functionality.

[extreme P]

MAIN ELECT cutout switch - deactivates stabilizer trim switch operation

AUTOPILOT cutout switch - deactivates autopilot stabilizer trim operation

...are provided to allow the autopilot or main electric trim inputs to be disconnected from the stabilizer trim motor.

[bArt2]

@ extreme P

He was not asking about the STAB TRIM CUTOUT switches, but about the STAB TRIM OVERRIDE switch. This one is located at the aft of the center console.

Normally electric trim operation is interrupted if you apply elevator force in the other direction. There are control column actuated stabilizer trim cutout switches for this.

By switching the stab trim override switch to override, you will override this system and electric trimming is possible while applying elevator force in the opposite direction.

I can immagine using this when there is a problem with the cutout switches, however there is no reference to this switch in the QRH so maybee it is only used by maintenance.

[framer]

When? When you have a jammed flight control surface and want to use electric trim to move the horizon-stab even though the yoke may be opposing it.
Why? Because the micro-switches at the base of the control yoke prevent you applying nose up trim with nose down yoke inputs and vice versa.

[ad-astra]

On the ground during a turnaround try trimming and moving the control column forward and aft......note what happens.

Now select the STAB TRIM OVERRIDE Switch to override.

Now try trimming and moving the control column forward and aft.

All will be revealed.

Stupid definitely not. Inquisitive and keen to know your aircraft yes!

Just saw framers post which refers to "Jammed or Restricted Flight Controls" NNC 9.8

[framer]

Looks like we replied at the same time Bart2.
I agree with most of what you say however
I can immagine using this when there is a problem with the cutout switches, re is not how I picture it unfolding. If the cutout switches themselves were playing up you would diagnose an electric trim failure and just trim manually for a flap 15 landing and never touch the switch. If on the other hand you diagnosed " Jammed or restricted Flight Controls" you may well use the switch.

[repulo]

The switch's main duty is to allow for waste bag attachment.:ok:

[bArt2]

@ framer.

Indeed you have a point, and the Jammed or Restricted Flight Controls NNC does mention switching to override in case you need electric stabilizer trim.

[bArt2]

@ repulo:

That`s if you have the waste bag option installed :8

[Exup]

Hope this helps.

STAB TRIM OVERRIDE switch
NORMAL
- normally guarded to ensure normal operation of the column actuated cutout switch
OVERRIDE
- bypasses the column cutout switches (if both fail)
- restores power to the stabilizer trim switches
- when this switch is placed in override, the main electric trim will operate when commanded regardless of control column position because the column cutout switches are bypassed
- if the stab is jammed you may be up against the column cutout switch (opposite force)
- use STAB TRIM OVRD switch to enable use of main electric stab trim again
- operational test of column cutout switches and OVRD switch:
- do this at the gate during your preflight
- while trimming with electric stab trim, oppose the trim by moving the column in the opposite direction
- column cutout switches should engage, stopping the electric stab trim input
- move the stab trim OVRD switch to OVRD
- trim should resume

[repulo]

@bArt2

That option is part of the purser recruitment process.
Installing the waste bag one of the major duties..;)

[Teddy Robinson]

my background to date has been limited to other types, last of these being A320, my experience of the 73/75 are limited to random sessions with CAE (when the sim was free and the instructor was dual rated).

There have been a number of incidents/accidents where the aircraft was apparently in an auto-flight mode, trimming back to reduce speed, and at some point full power was required, the thrust line / pitch couple kicked in and resulted in a loss of available pitch down authority whilst the stab trim took time to readjust to the demand.

Out of interest, is this switch in any way related to the mitigation of this issue ?

[Derfred]

No. It's to allow stab trim operation in the case of a jammed elevator.

[olster]

Not stupid question at all; well done for asking - this switch has caused confusion historically as it is not well explained in the FCOM.

[masalama]

Stab Trim Override switch is used to override the control column activated stabilizer trim cutout switches.

These cutout switches at the base of the control column cutout electric stabilizer trim when the direction of trimming is opposite to the direction of control column movement . For example, you are exerting force and pulling the control column backwards but using the electric trim to trim forward at the same time. The control column cutout switch will not allow electric stabilizer trim in this case. That's when you use the Override switch and force the electric trim to work and the best example of this is a Jammed elevator situation and the NNC directs us thus.
Hope this explains it .

masalama:ok:

[Centaurus]

On a touch and go landing, part of the drill is to first partially open the thrust levers to keep the engine spooled up to around 60% N1, select Flap 15, use the stab trim to set take off trim setting (usually about five units) and rotate at Vref.

Having conducted many touch and go landings in the real 737-200, it is common to see the PF holding back stick to prevent the nose-wheel from hitting hard as it lowers itself.

At the same time, if the PNF is hitting his own stab trim switch to reset to five units from back trim of around eight units or even more, and the PF is holding back stick to make gentle nose wheel contact with the runway, often we see the stab trim not operating because the PF is holding the stick back while the PNF is trimming forward and this opposite movement causes the stab trim to stop working until the PF releases the back pressure on his own control column.

[Derfred]

Teddy Robinson


There have been a number of incidents/accidents where the aircraft was apparently in an auto-flight mode, trimming back to reduce speed, and at some point full power was required, the thrust line / pitch couple kicked in and resulted in a loss of available pitch down authority whilst the stab trim took time to readjust to the demand.

Out of interest, is this switch in any way related to the mitigation of this issue ?

I'm sorry my initial response to your question was short and a bit rude:

No. It's to allow stab trim operation in the case of a jammed elevator.

This situation you talk about is indeed a potential occurence on the 737, but is not related to this switch. The 737 has an annunciator light "STAB OUT OF TRIM" which will illuminate during an auto go-around if the autoflight system has not had sufficient time to trim forward to allow a low altitude capture. In this situation the pilot must disconnect the autopilot and capture the altitude manually. Manual flight allows stronger control inputs than those that limit the autopilot (and faster trim IIRC).

That's according to the manuals - I've never seen it actually happen in either the simulator or the aircraft.

To clarify the jammed elevator scenario, if the elevator is jammed neutral - no problem. If the elevator is jammed aft, you will not be able to use electric trim to pitch down (or prevent further pitch up) unless you activate this switch (and vice versa if the elevator is jammed forward). This should be a memory item in my opinion because the aircraft may be unrecoverable by the time you find this in the NNC, and it's amazing how many guys in my outfit either never knew or have forgotten what this switch does!

Having said that the other pitch options available are thrust and/or manual trim.

[Pin Head]

Thanks giys

[RationalKeith]

Accuracy please.

Precise terminology is part of what is necessary to achieve that.

Obviously there are stabilizer trim command switches on each control wheel, which is at the _top_ of the control column. O

bviously there are stabilizer trim cutout switches on the aisle/stand/pedestal/whatever, behind the throttles. (‘Override’ would not be the best term for them, as they disable stabilizer electric trim.)

And of course the manual trim wheels beside the throttles, each having a handle that flips out from a detent. Each with a white stripe so help detect movement of them, offset in angle from each other to increase likelihood of seeing movement.

The question in the MAX MESS is whether or not there is a function resulting from something at the _bottom_ of each control column, that stops stabilizer trimming when the column is pulled hard back. (People refer to a ‘switch’, it could conceivably be interpretation by a computer of the force sensor that is there.) Hidden from pilots. _Perhaps_ the slow processing discovered by the FAA involves that function.

And in this PPruNE thread there is suggestion of a switch that crew can use to override an automatic over-ride performed by an FCC in certain conditions.

(Plus now there’s a claim out of Europe that the autopilot doesn’t always ‘properly disengage’ when commanded. I presume command is by a big switch on the AP control panel on the glareshield.)

Read AvWeb and Bloomberg, PPrune won’t let me post the URLs.)


It's been suggested that some of the defects found in reviewing MCAS exist on the NG as well.