Radio Management Panels (RMP)

The RMP is the communications chapter's frequency selector: the crew set what frequency each VHF/HF transceiver is tuned to. Of the two-panel division of labour from article 1 — the RMP chooses the frequency, the ACP handles transmit/receive — the RMP is the frequency half. Three identical panels manage every communications frequency, plus one sideline: in a double FMGEC failure they become the flight-management system's backup for tuning the navigation aids (VOR/DME/ILS/ADF) — the sideline that connects this chapter to navigation (ATA-34).

This is the first of the two "core control" articles (the other is article 3, the ACP/AMU). By the end you should be able to say how the three RMPs share the work, why RMP 3 on the overhead can still control every radio when 1 and 2 are off, why only RMP 1 survives on emergency power, what the amber SEL light is telling you, how an ATC-uplinked frequency jumps into the standby window, and how to handle one or two failed RMPs.

1. Three identical panels and the leader priority

The FCOM gives the RMP's two jobs and its layout. Per FCOM DSC-23-10-20:

Identical RMPs : Give the flight crew control of all radio communication systems (VHF and HF frequency control). Provide backup to the FMGECs for controlling radio navigation systems.

Two RMPs are on the center pedestal, and a third on the overhead panel. Each RMP can control any VHF or HF transceiver. RMP1 and 2 are connected directly to all VHF and HF transceivers, whereas RMP3 is connected to them via RMP1 and 2. RMPs are connected together so that each RMP is updated with the selections made on other RMPs.

The three are identical and interchangeable, distinguished only by a three-position code (SDI, Source/Destination Identifier) on the rear connector. They dialogue continuously over three buses — change VHF 1 on RMP 1 and RMP 2 and RMP 3 immediately show it.

What matters in a failure is who leads. Per AMM 23-81-00:

The priority procedure can be summed up as follows: RMP1 > RMP3 > RMP2 (The RMP3 does not process any radio navigation information).

Why the odd order that skips number 2? Because RMP 3 (the overhead one) sits physically downstream — routed through RMP 1 and 2 — so it is the natural convergence point of the whole network. The AMM notes that when RMP 1 or 2 is failed or off it becomes "transparent" to RMP 3, connecting RMP 3's buses directly to the transceivers. So when RMP 1 falls, the convergence point (RMP 3) is the smoothest to take the lead; RMP 2 is a local panel and comes last. Remember "1 → 3 → 2" and the takeover order in the degradation article is settled.

2. Power supply — why RMP 1 lives on emergency power

The three supply feeds decide the RMPs' order of survival under electrical degradation. Per AMM 23-81-00:

The RMPs are supplied with 28VDC. The RMP1 is connected to the ESS sub bus-bar 403PP. The RMP2 is connected to the sub bus-bar 202PP. The RMP3 is connected to the sub bus-bar 101PP.

RMP 1 hangs on the essential sub-busbar — the physical root of "only RMP 1 in EMER ELEC": lose main generation and 202PP/101PP go dark, killing RMP 2 and 3, while 403PP stays alive and RMP 1 survives alone (electrical chapter: ATA-24). Combined with the "two failed → one controls all" reconfiguration (§7), that lone RMP 1 can still tune VHF 1 and HF 1. There is also a power-friendly detail. Per AMM 23-81-00:

In the event of power supply cut-off the RMP memorizes the context. Consequently the last configuration selected can be restored when the power supply becomes available again.

A micro-interruption under 200 ms has no effect; after a longer cut, the frequencies do not have to be re-dialled on power return — the panel remembers.

3. The panel face

The face of one RMP carries two high-contrast LCD windows — ACTIVE (the frequency the RMP is outputting) and STBY/CRS (the standby frequency, plus course when tuning a nav aid); a transfer key that swaps ACTIVE and STBY (the last action of tuning); five radio selection keys (VHF 1/2/3, HF 1/2) with green feedback lights; a dual concentric knob (outer knob for whole numbers, inner for decimals); an AM pushbutton (HF defaults to SSB, press for AM); the SEL amber indicator (§5); a guarded NAV pushbutton for the navigation backup mode (§7); a LOAD key with indicator (§6, in the configuration covered here); and an ON/OFF latching switch.

4. Normal tuning and the "three presets"

In normal mode the RMP manages VHF/HF frequencies. The sequence is: press the radio key → rotate the knob to change the STBY preset → press transfer to activate. A useful and easily-missed mechanism. Per AMM 23-81-00:

The preset frequencies cannot be exchanged between the RMPs. Consequently a transceiver can have a maximum of three different preset frequencies.

So each of RMP 1, 2 and 3 can hold a different next-frequency for the same VHF 1 — captain, first officer and overhead panel each keeping one up their sleeve. The active frequency is shared network-wide (synchronised over the dialogue buses); only the preset is private to each panel. VHF has two spacings: 25 kHz (the kHz digit is not displayed, e.g. 118.25) or 8.33 kHz (displayed, e.g. 118.255).

5. The SEL amber light — a cross-selection reminder

Normal allocation is "each panel tunes its own": RMP 1 tunes VHF 1, RMP 2 tunes VHF 2, RMP 3 tunes VHF 3/HF. The moment someone crosses over, SEL illuminates. Per FCOM DSC-23-10-20:

The SEL indicator comes on in amber on both RMPs when a transceiver normally associated with one RMP is tuned by another: VHF1 tuned by RMP2 or 3, VHF2 tuned by RMP1 or 3, VHF3, or HF tuned by RMP1 or 2.

The AMM adds that the SEL lights stay on as long as the cross-selection persists, until the radio is selected back on its associated RMP.

[!warning]- SEL on is not a fault — it means "you are on someone else's panel" Crews often read amber SEL as a malfunction. It is not — it only reminds you that at least one radio is currently being tuned on a panel other than its associated RMP. Article 11 covers the technique: after tuning, select the radio back on your onside RMP and SEL goes out. The reminder matters because during a cross-selection the captain may not be able to read the onside VHF frequency (it is being managed by the other side).

6. The LOAD function — one-key load of an ATC frequency

In the configuration covered here the RMP carries a LOAD key — a datalink-era function. When ATC uplinks a "contact/monitor this frequency" instruction by CPDLC, you do not dial it by hand; one key press loads it into the standby window. Per FCOM DSC-23-10-20:

When the ATC sends (CONTACT or MONITOR) CPDLC messages to the flight crew, a white light above the LOAD key comes on to indicate that the sent frequency is available to be loaded.

The chain behind it — the deepest datalink interface in this chapter — is spelt out by the AMM. Per AMM 23-81-00:

When the ATSU receives a frequency from the ground station and validates it, the ATSU sends the data to the Datalink Control and Display Unit (DCDU). The DCDU shows the data. At the same time, the ATSU transmits to the RMPs a signal to start the LOAD function. When the RMP receives the signal, and in relation to the radio selection, the RMP starts the white light of the LOAD indicator.

Press LOAD and the frequency blinks for 300 ms, then displays steadily in STBY/CRS; press transfer to activate it. Per FCOM DSC-23-10-20:

When the LOAD function is activated, the flight crew can repeatedly load the frequency on each RMP as often as necessary; except if DATA is present on standby window.

[!warning]- LOAD is the ATA-23 ↔ ATA-46 interface The white LOAD light means the ATSU (datalink application layer, ATA-46) validated and pushed a frequency; the LOAD key, the STBY window and transfer (hardware) are this chapter. The ATSU uses ARINC 429 label 273 to signal "a frequency is available". This is the deepest point at which this chapter touches the datalink; how the CPDLC message itself is read and answered belongs to ATA-46.

7. The navigation backup mode

The RMP's sideline: on a double FMGEC failure the RMP hand-tunes VOR/DME/ILS/ADF. Per AMM 23-81-00:

In normal mode, the RMP1 (RMP2) receives the FMGEC1 (FMGEC2) management bus. It transmits the frequencies to the radio navigation equipment on a dedicated output bus. Internal relays interconnect these inputs and outputs directly. The RMP is thus transparent to the onside FMGEC. In back up mode, the output bus transmits frequencies generated by the RMP.

Entering the backup mode means lifting the guard and pressing NAV (the guard prevents inadvertent selection — normal flight should never enter it). There is a visible consequence. Per AMM 23-81-00:

NOTE: As the Nav mode is selected on any RMP (1 or 2), the radio Nav pages on the MCDU's are inhibited.

Tuning authority passes from the FMGS (managed on the MCDU) to the RMP, so the two cannot both manage it and the MCDU RAD NAV pages lock out. Two details: the ILS frequency and course are the only navigation data exchanged between RMP 1 and 2 over the dialogue buses (keeping ILS 1 = ILS 2); and the DME has no separate selection — it follows the paired VOR/ILS frequency. The tuning operation and its place in the navigation chain are developed in ATA-34-06; this article covers only the RMP-side mechanism.

8. Failure and degradation

The RMP's degradation path is simple and robust. Per FCOM DSC-23-10-20:

If two RMPs fail, the remaining one controls all the VHF and HF transceivers.

When a single RMP fails its displays go black; to let a surviving RMP take over the radios it was blocking, set the failed RMP's ON/OFF switch to OFF — it then goes "transparent" and internal relays connect the upstream RMP's buses straight to the transceivers. The DSC-23-50 handling (articles 11, 12) is exactly this. Per FCOM DSC-23-50:

When an RMP fails : The affected RMP no longer controls the selected receiver. The frequency displays disappear, and the green VHF or HF lights go out. Affected RMP ... SWITCH OFF. One RMP can control all receivers.

And the easily-missed overhead design. Per FCOM DSC-23-10-20:

Note: RMP3 is able to control VHF and HF transceivers through RMP1 and RMP2 even when they are OFF.

Switching RMP 1/2 to OFF does not cut RMP 3's path — it clears the way for it (transparency). That is the operating face of the "1 → 3 → 2" leader priority of §1.

Self-test

[!note]- Q1. How are RMP 1/2 connected versus RMP 3? Why does switching RMP 1 and 2 to OFF let RMP 3 control every radio? RMP 1 and 2 connect directly to all transceivers; RMP 3 connects through them. Setting 1/2 to OFF makes them transparent, connecting RMP 3's buses straight to the transceivers.

[!note]- Q2. Why is the leader priority "RMP1 > RMP3 > RMP2", skipping number 2? RMP 3 is the routed-through convergence point of the network, so it is the smoothest to take the lead when RMP 1 fails; RMP 2 is only a local panel and comes last.

[!note]- Q3. Why is only RMP 1 available on emergency power? RMP 1 is fed from the essential sub-busbar (403PP); RMP 2 (202PP) and RMP 3 (101PP) are not.

[!note]- Q4. What does an amber SEL light mean? Is it a fault? How do you clear it? A radio is being tuned on a panel other than its associated RMP. Not a fault — select the radio back on the onside RMP and it goes out.

[!note]- Q5. What has happened when the white LOAD light is on, and where does the frequency go when you press LOAD? Why is this the 23/46 interface? The ATSU validated and pushed an ATC frequency. Pressing LOAD places it in the STBY/CRS window. The key/window/transfer are ATA-23 hardware; the ATSU/CPDLC content is ATA-46.

[!note]- Q6. Why is the NAV key guarded, and what changes on the MCDU when it is pressed? Why are ILS 1 and 2 always identical? The guard prevents inadvertent entry to the navigation backup mode; selecting it inhibits the MCDU RAD NAV pages. The ILS frequency/course is the only nav data exchanged between RMP 1 and 2, so ILS 1 = ILS 2.

Key takeaways

References

• FCOM DSC-23-10-20 — RMP jobs, layout, direct/via connection, EMER ELEC, SEL, LOAD, RMP 3 transparency.
• FCOM DSC-23-50 — RMP failure handling (switch OFF, one controls all).
• AMM 23-81-00 — SDI, leader priority, supply busbars, context memory, three presets, LOAD chain (ATSU→DCDU→RMP), navigation backup and MCDU inhibit.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.