STAB trim cutout - still able to trim??

[sbergert]

Hi

Was clicking through a Flight Controls-CBT and I think I found an error in PSX?
When setting the STAB trim cutout switches for Hydr. System 2&3 to off, the Hydraulic power of the stabilizer is totally lost and trimming should not be possible anymore I guess. In PSX, I still can with the ALTN stab trim switches ...
Tried it with the PMDG, there I get no trimming possibility anymore (besides an autoland caution when I disable one system only -> in PSX it didn't show up). In doubt, I would normally consider PSX more realistic than PMDG, but since I also expect the different behaviour...

Any ideas? Maybe I'm missing something?

[Hardy Heinlin]

Hi,

it's intentionally programmed like this. I've learned that those cutout switches affect the stab trim/rudder ratio modules (SRMs) which control the trim rate and which command the stab trim control modules (STCMs). The trim switches on the yokes are connected to the SRMs. When alternate trim is used, however, trim commands are sent directly to the STCMs and full trim rate is applied, and the trim limits are extended as well.

Does your CBT explicitely refer to this cutout function, or does it mean the shutdown of hydraulic systems 2 & 3? Of course, when hydraulic systems 2 & 3 are depressurized, the alternate stab trim will be inop as well.


Regards,

|-|ardy

When setting the STAB trim cutout switches for Hydr. System 2&3 to off, ...

These switches do not cutout any hydraulic systems. Hydraulic systems 2 & 3 remain operative when you set these switches to the CUT position. There is no OFF position either. You need to set the demand pump and EDP switches to OFF (on the overhead panel).

[sbergert]

Hi Hardy,

off course I meant cutting off the hydraulic power to the stabilizer with the two cutout switches, not the complete hydraulic system 2 & 3

Still, the alternate trim works in PSX even if both switches are in cutout...  the stab trim indication moves and I can trim.
Also, should the EICAS caution "autoland" be shown with one of the two cutout switches in cutout?

It's the Atlas Air 747 CBT (availabe on youtube, flight controls) I was watching.

[Hardy Heinlin]

https://youtu.be/Zh-vv_0i2Rs?t=996

At 16:36 there is a picture showing the trim switches on the yoke, not the alternate trim.

At 16:53 he says when 1 cutout switch is in CUT, half trim rate is applied. That's correct for the trim switches on the yoke; I say this doesn't affect the alternate trim which is always full rate because it bypasses the trim rate system.

So this CBT doesn't clearly disagree with PSX.

If someone can prove the alternate trim is incorrectly modelled in PSX, I'll modify it. But I'm not convinced yet :-)


|-|

[skelsey]

Hi Hardy,

Here's the FCOM diagram:



According to the text:

Stabilizer Trim Cutout
Hydraulic systems 2 and 3 power stabilizer trim. Two guarded Stabilizer Trim Cutout switches control stabilizer trim. With the guards closed, the switches are held in AUTO position, allowing automatic cutout of the related hydraulic system if unscheduled stabilizer trim is detected. With a Stabilizer Trim Cutout switch in CUTOUT, hydraulic power to the related trim control module is shut off.

Positioning a switch to ON overrides the automatic cutout function and supplies hydraulic power to the related control module. If automatic cutout has occurred, hydraulic power remains shut off until the respective cutout switch is placed ON.

If one actuator fails to operate, trim commanded by the flight crew reduces to half the normal scheduled rate.

It looks to me as though both the alternate and control column trim switches feed in to the STCMs, and the cutout switches shut off the hydraulic power to the trim actuator. Thus my inclination is that there should be no trim at all (normal or alternate) with the switches in CUTOUT.

However, I obviously can't vouch for what the real aeroplane does, and the FCOM diagrams may not tell the whole story!

[Hardy Heinlin]

Thank you. But this doesn't convince me either :-)

I always refer to maintenance books as they are more detailed ...


|-|ardy


P.S.: Actually, it's easy to understand: Just ask yourself why does the ALTN trim provide full trim rate when one system is cutout and the yoke trim applies half rate? Why does one cutout not affect the rate of the ALTN trim? (Rhetorical question.) So why should two cutouts affect the ALTN trim? There is more inside these modules than those simplified diragrams show.

[John H Watson]

All the Alternate Switches do is bypass the SRMs and activate separate solenoids on the STCM. But the switches do cut off hydraulics to the STCM. If you have no hydraulics, no trim.

Hardy, are you thinking of the flt control "Tail shutoff" switches on the maintenance panel? These have no effect on stab trim.

Cheers
JHW

[Hardy Heinlin]

Strange. It's been working like this in PSX all the time. I think we discussed this in the alpha team over 7 years ago. I haven't changed it, as far as I recall ...

I'm thinking of the stab cutout, not tail shutoff.


Cheers,

|-|ardy

[John H Watson]

The stab trim cutout switches have direct control of the motor operated shutoff valve (MOV)

From one of the earlier forums...

Looking at single stab trim motor operation:

MOV (Motor operated shutoff valve): This allows hydraulic fluid to get to the other 2 valves (CONTROL/ARM) in the STCM and, ultimately, the stab trim drive motor.
If the respective thrust quadrant CUTOUT switch is in "AUTO", the control of fluid to the STCM (valve control module) is controlled by the SRMs (Stab Trim Rudder Ratio Modules). 5 seconds after a CUTOUT switch is placed to AUTO, both SRMs command each MOV to open.

I don't know how this slipped by Avi

[emerydc8]

Here's a page from the MEL.

[Avi]

I don't know how this slipped by Avi

I'm asking the same question 

[Hardy Heinlin]

OK, sufficiently convinced!

I was so used to this "feature" over the years, and nobody complained until now, that I always thought it was correct :-)

I'll modify it in update 10.1.8.


Questions:

Are any EICAS messages missing when one or two switches are in CUTOUT?

Is it correct that with ALTN trim the full trim rate is achieved even when one switch is in CUTOUT?


Thanks,

|-|ardy

[skelsey]

Is it correct that with ALTN trim the full trim rate is achieved even when one switch is in CUTOUT?

The FCOM makes no mention of this. It does say that the STCM outputs are mechanically summed and therefore if only one is operating then trim will be approximately half rate, and makes no distinction between normal and alternate trim in this regard (though it does say that alternate trim gives a wider range).

I can't see how the alternate trim could possibly drive the stab any faster with only half the hydraulic power available.

[Britjet]

I think it works like this...
Left Alternate trim switch directs #1 pressure provided it is available - i.e. Left cutout is not activated
Right Alternate trim directs #2 pressure in the same way provided right cutout is not activated.

If you have a left cutout, for example - then if you push both Alternate trim switches (which is what you would normally do) then only the right one will work so you will get half rate.

Peter.

[Hardy Heinlin]

Perhaps what I'm having in mind is not the hydraulic dependent rate but the airspeed dependent rate. Below 225 KIAS, the commanded normal rate is 2.5 times higher than it is above 225 KIAS. The commanded rate by the alternate trim is always the highest rate. Airspeed doesn't influence the alternate trim.

And after this command stage there is the second stage: the hydraulics. They may decrease the final physical rate by 50 % when half powered.


|-|ardy

[Britjet]

I'm in the sim tomorrow - I can have a fiddle about if you like..
Peter

[Hardy Heinlin]

Thanks, Peter, but I think it's not necessary; there are no doubts anymore.


Regards,

|-|ardy

[sbergert]

Glad it is solved

Concerning an EICAS-message I did only notice the amber "autoland " on the PMDG...though I don't know where they have it from...

[John H Watson]

Some of the alpha notes...

Noted today on a real aircraft, when selecting CUTOUT, the >STAB TRIM 2/3 message takes about 1 second to appear, but 7.5 seconds to disappear (when returning the switch to AUTO).

(A training manual) has some details about the STAB TRIM 2/3 messages.

The Advisory messages are latched in the SRM's (except when caused by moving the Cutout switch to CUTOUT - In this case, returning the switch to AUTO removes the message after 5 seconds).

The SRM monitors the hydraulic pressure used to release the brakes on the hydraulic motors. If a trim command is given and hydraulic pressure is not present to release the brakes, then the STAB TRIM message will be set. I don't think the Stab Trim system declares itself inoperature simply because you have turned off the hydraulics. There has to be a valid trim command, too.

If a fault causes the SRM to shut the MOV, the CUTOUT switch has to be moved away from AUTO to reset the message (5 second delay on this, also).

The wording of the text is a little ambiguous. We may have to discuss this.

The manuals say 5 second delays, but IRL 7.5 seconds.

[Hardy Heinlin]

Thanks for the quotes. I just rechecked PSX; all these details are modeled in PSX, including stab trim runaway malfunctions to test the protection logic.

(There are no notes re autoland, as far as I can see. The stab cutout keeps the autopilots operative. Pitch control remains possible without stab trim.)


|-|ardy