investigating TM mode goes to 000 EPR if AP is off

[Genista]

Hello.

 

With a fully configured PMS and all that;

If I press TM mode but autopilot is OFF, it will command 000 EPR and retard the throttles.

Is it by design/ on purpose ?

Edited Saturday at 03:37 PM by Genista
typo in title

[mph163]

Really annoying issue and being ignored by team!

[RyanAlmeida95]

From what I can find. 
For the climb, engage CMD with VNAV selected and then select TM to follow the PMS EPR.
TM is also compatible in semiautimatic mode and can be used with CWS engaged and or when using IAS or VS modes instead of VNAV.
TM at a bare minimum needs a FD and CWS/CMD on to operate VNAV or Semiautomatic performance operations to be guided by PMS. 

 

[mph163]

Wow, I stand extremely corrected 

[richboy2307]

While it is correct that TM mode requires FD+AP, we'll have another look at what it should do when conditions are not met (i.e. just disengage TM instead of commanding EPR 000/IDLE thrust).

The other thing to note is that both PMS are independent, so need their data entered separately. PMS1 data is only used by AP1 channel.

[Genista]

On 4/25/2026 at 6:18 PM, RyanAlmeida95 said:

From what I can find. 
For the climb, engage CMD with VNAV selected and then select TM to follow the PMS EPR.
TM is also compatible in semiautimatic mode and can be used with CWS engaged and or when using IAS or VS modes instead of VNAV.
TM at a bare minimum needs a FD and CWS/CMD on to operate VNAV or Semiautomatic performance operations to be guided by PMS. 

 

Thank you very much for sharing.

 

However that document does not explicitly say that TM (as you claim) would not work unless CWS/CMD is engaged; instead it just gives you the normal procedure to get the full engagement of PMS VNAV functions. Which, obviously, needs CMD on since those functions need pitch slaving too.

Not exactly the same thing!

Since AoA mode is able to command your throttles without any AP mode engaged, I am really curious why, from a systems perspective, TM would need CWS or CMD. In every other aircraft I know, including old designs like our tristar (I have DC9 and Concorde in mind), they live separately. Pitch/Yaw(YD)/Roll autopilot actuators can be engaged or not (on those other aircraft) but it won't affect what the A/T is doing because is it a different set of wiring, power supply etc.

Edited 12 hours ago by Genista

[RyanAlmeida95]

Well in this case for the Tristar, AOA, AT and TM are different functions in their own right. They are not the same as each other. TM is directly controlled by the PMS. For the PMS to be engaged you need to use CWS/CMD as per the above. 

Without the PMS engaged by using CWS/CMD then TM has no instruction.   

[Genista]

1 hour ago, RyanAlmeida95 said:

Well in this case for the Tristar, AOA, AT and TM are different functions in their own right. They are not the same as each other. TM is directly controlled by the PMS. For the PMS to be engaged you need to use CWS/CMD as per the above. 

Without the PMS engaged by using CWS/CMD then TM has no instruction.   

Much clearer thanks.

But it still doesn't indicate that engagement CWS or CMD is a requirement for it to work ! VNAV/IAS/MACH all work in conjunction with FD, regardless if CWS or CMD is engaged.

Or to rephrase. PMS can feed FD with VNAV pitch commands. Even without CWS or CMD engage. Why would TM needs it ?

(not trying to be pedantic, I just have never seen that design before and that makes me curious)

Edited 2 hours ago by Genista

[fishermanivan]

1 hour ago, Genista said:

Much clearer thanks.

But it still doesn't indicate that engagement CWS or CMD is a requirement for it to work ! VNAV/IAS/MACH all work in conjunction with FD, regardless if CWS or CMD is engaged.

Or to rephrase. PMS can feed FD with VNAV pitch commands. Even without CWS or CMD engage. Why would TM needs it ?

(not trying to be pedantic, I just have never seen that design before and that makes me curious)

Yes it does need the AP.  Says AP must be on in CMD and VNAV selected.