Radio-Navigation Receivers and the Tuning Hierarchy

In the GPS era, VOR, DME, and ADF are like a row of veterans put out to pasture — invisible day to day, yet the FM's radio position leans on DME, and if GPS is jammed they are recalled to duty; after both FMGECs die, the RMP backup is the only way left to tune a station. This article covers the receiver layer (who, how many, on what principle) and the tuning layer (who has the authority, how it is handed over).

1. Three layers of tuning — automatic is normal, manual interrupts, backup is a coup

The text for this configuration reads. Per FCOM DSC-34-10-30-10:

In normal operation, the FMGEC tunes navaids automatically, with each FMGEC controlling its own receivers.

If one FMGEC fails, the remaining FMGEC controls both sides receivers, after activation of the FM selector switch.

What is tuned automatically, and by what selection priority, is the FM's business (auto-flight chapter); ATA-34 governs "how the wires are connected." The AMM gives the wiring detail: each receiver has two management ports — the primary port reaches its own FMGEC through its own RMP (relay mode), the secondary port reaches the opposite FMGEC directly — with the choice of port set by a discrete the FM selector sends via the RMP. So "one FMGEC controls both sides" needs no re-wiring, just a flick of the SWITCHING panel's FM selector. Per AMM 34-51-00:

With failure of one FMGEC, the other FMGEC can control the two DME interrogators, one directly, the other through its RMP.

The second, manual layer. Per FCOM DSC-34-10-30-10:

The flight crew can use the MCDU to override the automatic tuning of navaids by FMGEC in order to select a specific navaid for visual display. This does not affect the automatic function of the FMGEC.

Note: manual tuning ≠ switching automatic off — lock a VOR on the RAD NAV page to read raw data and the FM still tunes what it needs for radio position in the background. (This configuration's text has no "manually tuned ILS forces LOC/G-S guidance" clause; that belongs to other configurations.) The third-layer coup is §2.

2. The RMP — a runner in peacetime, a regent at the end

The Radio Management Panel (RMP) is normally a comms-frequency panel and an FMGEC relay; its navigation power hides under a NAV key with a clear guard. Per FCOM DSC-34-10-30-30:

Pressing this key engages the radio navigation backup mode. It takes control of the VOR, ILS, GLS and ADF receivers away from the FMGEC and gives it to the RMP. The green monitor light comes on.

Three sub-clauses, each an exam point. Per FCOM DSC-34-10-30-30:

The flight crew must select this backup tuning mode on both RMP1 and RMP2 if both FMGECs or both MCDUs fail. In the emergency electrical configuration, only RMP1 receives power.

Setting one RMP to NAV backup mode removes navaids tuning from both FMGECs.

Pressing the NAV key on RMP 3 has no effect.

"One key seizes both sides" is the sharpest clause: even RMP 1 alone entering NAV puts all FMGEC auto-tuning out of a job — which is why this is never a "try it and see" key (the point of the guard). Division of labour: the captain's RMP controls VOR 1 + ADF 1, the F/O's controls VOR 2 + ADF 2; ILS can be controlled from either RMP only after both are set to STBY NAV (revisited in the CAT-capability context of the ILS article). Panel vocabulary: the STBY NAV selection keys (VOR/ILS/GLS/MLS/ADF), the dual concentric knobs (outer = integer, inner = decimal, accelerating when spun fast, only the outer sets course), the transfer key swapping ACTIVE/STBY, the amber SEL light meaning "another RMP tuned your station," the LOAD key loading a CPDLC uplinked frequency, and the BFO key (§5). A frozen RMP (ACTIVE/STBY will not swap) has a QRH reset. Per QRH 02.02A:

In the case of freezing of one RMP (impossibility to interchange the ACTIVE and STBY radio navigation or communication frequencies,...), all the RMP's have to be reset, one after the other, to recover a normal operation.

This is one of the few authorised resets in the ATA-34 neighbourhood.

3. VOR — comparing phase, not amplitude

Per FCOM DSC-34-10-30-20:

The aircraft has two VOR receivers.

(The two receivers share one antenna, so "VOR antenna icing" is a common cause that takes both down together.) The direction-finding principle. Per AMM 34-55-00:

The principle of the VOR navigation is a comparison between a reference phase signal and a variable phase signal. The ground station generates these two signals. The phase difference between the reference and the variable phase is a function of the position of the aircraft with respect to the ground station.

The ground station is a "phase lighthouse": the reference signal is in phase in all directions, the variable signal's phase changes with the radial — whichever radial you are on, the phase difference is your bearing. The display entry. Per FCOM DSC-34-10-30-20:

The NDs display VOR 1 and VOR 2 information, in accordance with the position of the ADF /VOR sw on the EFIS control panel (Refer to DSC-31-50 EFIS Control Panel).

Single needle = VOR 1, double needle = VOR 2 (white); the ROSE-VOR mode gives the full HSI experience. The tuning-mode annotation beside the station is the "signature" of the three-layer authority structure. Per AMM 34-53-00:

mode of tuning (nothing if automatically tuned, M underlined if manually tuned by the MCDU and R underlined if tuned by the RMP).

A glance at the ND tells you who holds power right now. VOR auto-tuning is trivially simple — automatic if no VOR is selected manually — so do not be surprised when the FM changes the ND's VOR station for radio position.

4. DME — interrogate-and-reply ranging, one mind serving five

Per AMM 34-51-00:

The principle of the DME navigation is based on the measurement of the transmission time. Paired interrogation pulses go from an onboard interrogator to a selected ground station. After 50 microseconds, the station transmits the reply pulses to the aircraft.

The 50 µs fixed delay is the trick: subtract it from the total round-trip time and what remains is pure propagation × speed of light ÷ 2 = the slant range (the hypotenuse, not the ground distance — directly over a station at 6000 ft, DME reads 1 NM, not 0). Interrogation 1041–1150 MHz, reply 962–1213 MHz, offset so your own question does not pollute your own ear. One interrogator serves several customers at once. Per FCOM DSC-34-10-30-20:

One DME receiver is capable to tune simultaneously several frequencies from different DME ground stations.

Some frequencies are tuned for FMS radio position computation in VOR/DME mode and in DME/DME mode. Other frequencies are tuned for VOR/DME display and ILS/DME display.

The AMM gives the ceiling: interrogating five ground stations at once — you see one DME range on the ND while it may be quietly interrogating four others for the FM's DME/DME position. Between the two systems is an anti-interference line. Per AMM 34-51-00:

a suppressor coaxial prevents transmission from one system while the other is in reception mode

Display: the ILS/DME range magenta at the PFD's lower left (LS key, ILS article), the VOR/DME range green in the ND corner; the VOR-D key paints database stations on the map; on failure the DME 1(2) flashes red for 9 s then goes steady.

5. ADF — a sense of direction built from two loop antennas

Per AMM 34-53-00:

This is obtained by the combination of: - the signals from two loop antennas positioned 90 deg. apart - the signal from an omni-directional sense antenna.

A loop antenna's received strength varies as the cosine of the wave's direction of arrival — two orthogonal loops measure the "north-south" and "east-west" components at once, and the arctangent gives the bearing; but the cosine cannot tell 180° apart, so the omni-directional antenna is the phase referee that casts the final vote. The four auto-tuning cases are worth taking whole. Per AMM 34-53-00:

- tuning is automatic if the following conditions are met: . no ADF has been manually selected on the MCDU . a NDB station is specified for the approach phase . a NDB is a TO waypoint in the F-PLAN: this NDB is autotuned . a NDB is a FROM waypoint in the F-PLAN: this NDB is autotuned.

Two audio buttons divide the labour. Per AMM 34-53-00:

With NDB stations modulated in A1 mode (in automatic or manual mode) it is necessary to select the BFO function on the MCDU (or on the RMP in backup mode) in order to hear clearly its Morse identification signal.

An A1 station transmits a bare carrier chopped into dots and dashes — the ear cannot hear an unmodulated carrier, so the BFO (beat-frequency oscillator) mixes in a local tone to "translate" it into a beep. When an NDB also carries ATIS voice, the reverse applies. Per AMM 34-53-00:

it is necessary to push the ON VOICE pushbutton switch on one ACP in order to hear clearly this information without Morse signal.

(The FCOM's RMP section adds a twist: most ADFs need BFO on to hear the ident, but some ADFs need BFO off — a generation difference; if the ident is unclear, try both.) Display: single needle ADF 1, double needle ADF 2 (green).

6. The marker — three station-calls hidden inside VOR 1

Per FCOM DSC-34-10-30-20:

One marker beacon system is included in VOR receiver 1.

Per AMM 34-55-00:

There are three marker transmitters positioned on the ground at known distances from the runway threshold: - the outer marker at approx. 4 NM - the middle marker at 0.6 NM - the inner marker at the runway threshold.

A 75 MHz vertical beam triggers on overflight, and the PFD calls the station at the intersection of the G/S and LOC scales. Per AMM 34-55-00:

- OM for the outer marker (blue) - MM for the middle marker (amber) - IM for the inner marker (white).

The "single set, hidden in VOR 1" architecture leads straight to the MEL knock-on clause: VOR 1 dispatched inoperative means MARKER is automatically considered inoperative; conversely, a MARKER failure alone requires only that "the approach procedure does not need a marker fix."

7. The emergency inventory, and a configuration note

After a total loss of main generation, how much navigation remains? The FCTM's emergency-electrical list gives a "NAVIGATE" line. Per FCTM PR-AEP-ELEC:

NAVIGATE ND 1, FMGC 1, radar, RMP 1, VOR 1, DME 1, (ILS 1, GPS 1 if MMR installed)

The MMR is fitted, so ILS 1 / GPS 1 survive on emergency power (MMR 1 hangs on the static-inverter bus). With the "keep two IRs, heat one probe set" of the overview article, the full picture of emergency navigation is: attitude in duplicate, speed singly, position from GPS 1 + VOR 1/DME 1 raw data, approach on ILS 1, tuning via RMP 1.

Configuration note (DDRMI): the FCOM radio-navigation section references the DDRMI (the electromechanical radio-magnetic indicator) in several places; this configuration does not carry it, its function being fully covered by the ND needle family.

Key numbers

Self-test

[!note]- Q1. Where does each of the three tuning layers act, and which interrupts which? Automatic = FMGEC (no crew action). Manual = MCDU RAD NAV page (overrides display without stopping FM auto-tuning). Backup = RMP NAV key. The RMP coup removes tuning from both FMGECs; the MCDU manual override does not.

[!note]- Q2. RMP 2 is accidentally pushed into NAV in cruise. Consequence, and recovery? All FMGEC auto-tuning stops (one key seizes both sides). Recover by lifting the guard and de-selecting NAV on that RMP.

[!note]- Q3. DME reads 1.0 NM and stops decreasing. Where is the aircraft, and why? Directly over the station (slant range). At overhead, the hypotenuse equals the altitude — 6000 ft ≈ 1 NM — so DME never reads zero.

[!note]- Q4. How many stations does the FM's DME/DME position interrogate at once, and does that conflict with seeing one DME range on the ND? Up to five. No conflict: the ND shows one for display while the interrogator quietly polls the others for the FM's DME/DME position.

[!note]- Q5. You cannot hear an A1-modulated NDB's ident — which button? An NDB carrying ATIS voice — which button? A1 ident: BFO on (mixes a tone to make the bare carrier audible). ATIS voice: ON VOICE on an ACP (to hear voice without the Morse).

[!note]- Q6. VOR 1 is dispatched inoperative under the MEL — can you still use the marker? Why? No. The single marker set lives inside VOR 1, so it is automatically considered inoperative with VOR 1.

Key takeaways

References

• FCOM DSC-34-10-30-10/-20/-30 — three-layer tuning, receiver overview, RMP NAV key and sub-clauses, knobs, SEL, LOAD, BFO.
• QRH 02.02A — frozen-RMP reset.
• FCTM PR-AEP-ELEC — emergency-electrical NAVIGATE line.
• AMM 34-51-00 / 34-53-00 / 34-55-00 — DME (principle, five-station scan, suppressor), ADF (principle, auto-tuning, BFO/ON VOICE, tuning annotation), VOR/marker (phase principle, three markers and colours).

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.