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ACARS not available

Started by acme, Thu, 25 Dec 2014 17:28


Whenever I try to start ACARS via MENU on the CDU I get the message : "ACARS-IN USE" "SELECT ANOTHER SUBSYSTEM"

What do I have to do to use ACARS ?

Please help.



Hello ACME,

For ACARS I suggest to use the center CDU (on 'communications' panel), which has dedicated ACARS (and I presume why you get the message about ACARS being in use). Well, that's what I use for the time being (whilst getting further knowledge of PSX).



Hi Andrej,
thank you very much for your help. Now I got ACARS running.

I did not even know that there was a third FMC (CDU).


Jeroen Hoppenbrouwers

Although most subsystems, such as the FMS, have explicit support for more than one MCDU at the same time and independently, other, simpler subsystems don't have this support. ACARS is one of them. If one MCDU is logged into the ACARS subsystem, no other MCDU can use it.

Logging out can be accomplished by selecting another subsystem. Just backing out to the MENU is not sufficient; notice that the MENU says "< ACT >" near the subsystem prompt that is currently active, i.e., logged on.

To support three MCDUs, the subsystem must have three independent state machines for three independent user interfaces and also organize data display updates on all three MCDUs. This complicates the program quite a bit to say the least.

ACME, there is no third FMC! There are just two FMCs. But yes there are three MCDUs, and the aircraft has limited support for a fourth, spare, but sort of active MCDU in the main equipment center. Not often used, but possible.

The three MCDUs are technically identical but they are not wired to the aircraft in exactly the same way. Therefore not all subsystems can be reached from each MCDU.

The most important function of the MCDUs is to be a terminal device to the subsystems; these terminals (display and keyboard) are very dumb devices and the subsystems must do 99.9% of the work, even including scratch pad editor support, character for character. But when other equipment, such as the MCP or the EFIS control panel, fails, the MCDUs can take over as they have backup menus for these panels. Of course this costs a lot of extra wiring. When both FMCs fail, the MCDUs even have a mini LEGS page that is kept up to date and can provide limited support for automatic route navigation. Enough to get you home.



Hi Jeroen,

thanks for these elaborate explanations.
You must be a professional pilot to know so much about these devices and their configurations.
I spent a lot of time with the PMDG 747 simulation and thought this was already incredibly detailed, but PSX is on a different planet; simply perfect.
Great to have such pros in this forum.


Jeroen Hoppenbrouwers

Not a pilot, neither professional nor amateur.  Pilots don't know much about this   :twisted:

However I am a professional avionics developer and I daily fight with MCDU programming and other associated box fumbling from both the inside and the outside.

This is my planet (note: shameless plug):

Working on bringing it to PSX... "because we can".


Hardy Heinlin


some practical hints re subsystems:

Subsystem FMC L can be controlled by CDU L & R simultaneously.
Subsystem FMC R can be controlled by CDU L & R simultaneously.
Subsystem ACARS can be controlled by a single CDU (L or R or C).

Now to demo this stuff a little bit more, you may want to load this situation:

00 Parked 001 - Sydney.situ

On CDU L (captain's CDU), press (6L), then MAINT> (6R), then BITE> (1R). Check the page title, it should read:

Look at the captain's NAV source selector (see page 283); it's located left of the captain's clock. Move this selector from the FMC L position to the FMC R position. Notice the title on CDU L now reads:
CDU L now controls FMC R.

On CDU R (F/O's CDU), press (1L) and go that CDU's BITE page. Check the page title, it should read:

Look at the F/O's NAV source selector; it's located right of the F/O's clock. Move this selector from the FMC R position to the FMC L position. Notice the title on CDU R now reads:
CDU R now controls FMC L.

Look at the circuit breakers on the overhead panel. Pull the circuit breaker C23 ("FMC R"). FMC R is now inoperative and no longer a subsystem of CDU L. CDU L automatically returns to its MENU page and the prompt is gone. You may now assign FMC L to CDU L: Turn the captain's NAV source selector to "FMC L". The prompt reappears. But don't select it; for this demo select
On CDU C press the
Once ACARS is connected to CDU C, it remains connected to CDU C because at this time there are no further add-ons available on the center CDU's MENU that you may select (just the selection of another subsystem deselects the active subsystem).

(Hoppie, I just realized that the CDUs will keep their ACARS when I pull and push the ACARS CBs G19 and G20 on P7. I can't remember if this has been tested on the real deck. If I unpower and repower an FMC, the respective CDU will lose the FMC status on the MENU, as we know. I guess this principle should be applied to ACARS as well?)



Jeroen Hoppenbrouwers

Affirm, subsystems die on the MCDU after 3000 ms of no signal.


Hardy Heinlin

I mean, will the CDU autoreconnect to the last connected (now lost) subsystem when that subsystem becomes alive? (Provided no other subsystem is connected at this time.)

Status A: blank
Status B: Status C:

I mean status C.


Edit: Please ignore. My brain got corrupted over Christmas. The FMC status is not lost after FMC long term power interruption. When the label reappears as well if it was selected before.

Jeroen Hoppenbrouwers

FMCs operate in pairs; although each CDU connects to two FMCs, there are special dual subsystem rules in place. It may be possible (likely) that there is a quiet failover.

For ACARS, I am positively sure that there is NO automatic pickup if the subsystem re-appears after > 3000 ms of absence. The CDU will say TIMEOUT RESELECT and bail out to MENU.