Dual COM/NAV Computers

[Atomic_Sheep]

Hi,

I'm trying to wrap my head around redundant systems on aircraft i.e. dual computers FMC's/MCDUs or RMUs.

So a plane has two separate COM/NAV systems.

I just want to discuss COM at this stage.

A pilot flying still needs to monitor ATC so he's listening in on the active frequency. However, the computer next to the co-pilot is the second system and therefore his frequency is different to that of the one that is input on the pilot's side.

I'm guessing this is where the Audio select panel comes in i.e. you select which computer you want to listen in to? So the pilot would set his audio to come from the co-pilots side?

What about NAV then? How do you manage two computers from a NAV perspective? Do you need to input the same flight plan into both computers?

Also, why have two separate transponders?

[PA28161]

They're called nav/com radios (not computers although they do have a lot of PCB's etc in their construction)

In an aircraft with two nav/com units, the active com-1 frequency is the one both you and your passenger/co-pilot will be monitoring. Hopefully, you will have entered the standby frequency (i.e your next controller to call) on com-2. When you want to make the standby frequency the active frequency you advise the controller on the first active frequency that you are changing to 123.xxx, say, and then press the toggle key to make com-2 the active. The audio panel will sort out the electronics switching, but in any case, on most audio panels the active will cut in when a controller speaks even if you are monitoring audio on the standby com-2.

[Piltdown Man]

An audio selector label allows you to listen to whatever had been plumbed into it. Selectors normally exist for COM 1/2/3, NAV 1/2, DME 1/2, ADF 1/2, HF, intercom, PA etc. You listen to as much or as little as you need to. This allows two active frequencies (if you wish) and however many standbys as your systems allow.

When you have multiple FMS boxes you typically find they speak to each other. Some systems call this "Crosstalk". So an input on one box updates the other.

[Alex Whittingham]

If you have 3 comms boxes you can tune in to 3 separate frequencies. In each case the next anticipated frequency will usually be pre-selected on the box so that as you pass from one to the other you can just swap the active frequency. For example the active ATC comms may be set in box 1, with the next frequency waiting, box 2 may be being used to listen to the ATIS and box 3 to talk/listen to company ops. The pilots can choose which box(es) they listen to by selecting them on the Audio select panel. Although you can listen to several at a time (with practice) you can only choose one box to transmit on.

If there are two VHF nav boxes you can manually tune them to two separate VOR/DMEs or ILSs and both will be displayed on the ND. More commonly en-route the tuning control will be given to the Flight Management System(s) so that they can autotune the most relevant nav aids. the ND shows which navaids are 'active', or autotuned.

[500 above]

Our SOP;

Com 1 active atc freq
Com 2 usually 121.5, ATIS
Com 3 datalink.

FMS’s - all 3 are slaved. Input on one crossfills to all. They can however be set up in independent mode by selection.

Two transponders, incase one fails!