Radio Altimeter and Automatic Callouts

The radio altimeter (RA) is the sole supplier of "the last 2500 feet": the bouncing green radio height on the PFD, the quickening string of callouts, the timing of the AP flare, every basic GPWS mode, the TCAS low-altitude inhibitions, the PWS activation window, and even the flight-control law changes between take-off and landing all feed on its data. That is why "dual RA failure" is one of the most alarmingly far-reaching single failures in the abnormal chapter. This article first covers how it measures (FM/CW leading-edge tracking, an elegant design), then the full display and callout rules.

1. Definition — wheel height above the ground, distance to the nearest obstacle

Per AMM 34-42-00:

The function of the Radio Altimeter (RA) is to provide, below 2500 feet, the crew and the system users with the radio height data, which is the main landing gear wheels height above the ground.

Two words to pick at: (1) it is only in business below 2500 ft — no radio height on the PFD in cruise is not a failure; (2) the datum is the main gear, the antenna is on the belly, and an internal offset zeros the mounting so the reading at touchdown ≈ 0, not the two or three metres of antenna height. The third point is its character. Per AMM 34-42-00:

One of the main characteristics of the system is that it locks onto the leading edge of the reflected wave. This permits to measure the distance between the aircraft and the nearest obstacle. The radio altimeter can therefore operate over non-flat ground surface.

The beam is not a line but an illuminated patch; over a slope, the points in the patch differ in range and the echo is a train — take the first to return (the leading edge) = the nearest point. Conservatively safe: the peak reads before the valley. Band. Per AMM 34-42-00:

The system operates between 4.2 GHz and 4.4 GHz and is defined in accordance with ARINC Specification 707.

2. FM/CW — no pulses, a "tone slide"

Per AMM 34-42-00:

The operating mode of the RA transceiver is based on the leading-edge tracking FM/CW (Frequency Modulation/Continuous Wave) principle:

It transmits continuously a signal whose frequency ramps linearly with time (a tone slide). When the echo returns after a delay t, the aircraft has already "slid" to a higher frequency — so the transmit-minus-receive frequency difference (the beat frequency fb) is proportional to the delay t, and thus to height. The clever step is in the servo. Per AMM 34-42-00:

- delta F is a constant, and the frequency-modulation period T is adjusted to keep the frequency difference fb equal to a value of 25 KHz. The height information is then derived from the measurement of T

Rather than measure that small, jittery beat, it locks the beat at 25 kHz and lets the period T vary: the higher you are, the longer T needs to be — and measuring a period is steadier and more precise than measuring a frequency. The multi-echo choice. Per AMM 34-42-00:

A beat frequency corresponds to each reflection point and, for accuracy and aircraft safety, the smallest beat frequency is isolated to measure the nearest distance to the ground.

Smallest beat = shortest delay = nearest point — the frequency-domain statement of "leading-edge tracking." Each system's kit. Per AMM 34-42-00:

Each system consists of: - one transceiver 1SA1(1SA2) - one transmission antenna 6SA1(6SA2) - one reception antenna 5SA1(5SA2) - one fan 3SA1(3SA2).

Separate transmit and receive antennas are standard for FM/CW: transmission is continuous, and on one antenna the transmit leakage would swamp the weak echo, so they live apart.

3. Display — a reading that changes font size

Per FCOM DSC-34-10-40-10:

The aircraft has two radio altimeter (RA). CAPT PFD displays the RA1 height, and the F/O PFD displays the RA2 height. If one RA fails, both PFDs display the height from the remaining RA.

A single failure is taken over fully automatically — no crew switching (unlike the ADIRS with its rotary; with only two RAs there is no choice). The PFD radio-height "wardrobe rules": font and colour — ≥400 ft, 3 mm green; 400 ft to DH+100, 4 mm green (enlarged to flag the low-altitude regime); below DH+100, 4 mm amber (nearing decision); with no DH set, it turns amber below 400 ft. Resolution — 10 ft steps above 50 ft, 5 ft steps from 50–5 ft, 1 ft steps below 5 ft (fine down to the ankle near the ground). Two "graphical grounds":

When the aircraft is below 570 ft height above the terrain: a red ribbon comes into view on the bottom and at the right of the altitude scale and moves up as the aircraft is in the descent phase (Item 2).

That red ribbon growing from the bottom of the barometric tape is the ground itself — at touchdown its top reaches the middle of the altitude window. The other is on the attitude ball. Per AMM 34-42-00:

Below 150 ft, the height is shown by the distance between the horizon line and the limit of the sector 2.

The distance between these two lines is proportional to the ground height (sensitivity 5 ft/mm).

In the flare, eyes outside, the "brown ground climbing toward the horizon" in peripheral vision is this 5 ft/mm animation. The dual-RA-failure flag has an exam-worthy condition set. Per AMM 34-42-00:

a red RA flag (item 4) is shown on the lower sector of the PFD when both radio altimeters are invalid, the attitude is not excessive and the aircraft is below the selected transition altitude. This flag flashes for nine seconds as soon as the transition altitude is reached, then remains steady.

Both RAs failing in cruise shows no flag — at that altitude it is not in business anyway; the flag appears only when you descend to the transition altitude (about to need it).

4. Automatic callouts — the FWC's voice, the RA's numbers

Per FCOM DSC-34-10-40-10:

The FWC generates a synthetic voice for radio height announcement below 2 500 ft. These announcements come through the cockpit loudspeakers, even if the speakers are turned off.

"Sound even with the speakers off" is a callout privilege (the same treatment as a master warning). The predetermined thresholds run 2500, 2000, 1000, 500, 400, 300, 200, 100, 80, 70, 60, 50, 40, 30, 20, 10, 5, plus HUNDRED ABOVE and MINIMUM. The reference choice. Per FCOM DSC-34-10-40-10:

The reference altitude for callouts is the radio height (RADIO or DH), or the baro altitude (BARO or MDA/MDH), depending on the type of approach. Pin programming enables Operators to select the required callouts.

Precision approach uses DH (radio height), non-precision uses MDA (baro): HUNDRED ABOVE / MINIMUM follow whether you entered DH or MDA on the MCDU PERF page. Two "no lingering" rules. Per FCOM DSC-34-10-40-10:

If the aircraft remains at a height that is in the detection zone for a height callout, the corresponding message is repeated at regular intervals.

If the time between two consecutive predetermined callouts exceeds a certain threshold, the present height is repeated at regular intervals. The threshold is: ‐ 11 s above 50 ft, or ‐ 4 s below 50 ft The repeating interval is 4 s.

Drift long without landing and it reads the number to you over and over — silence is designed as the anomaly. The last call. Per FCOM DSC-34-10-40-10:

The loudspeaker announces RETARD: ‐ at 20 ft, or ‐ at 10 ft if the autothrust is active and one autopilot is in LAND mode.

Hand-flown, 20 ft to prompt the closing of the thrust levers; on autoland, 10 ft — because the A/THR closes itself after the flare, so this is more a confirmation of monitoring.

5. The callout source chain — RA 1 lead singer, RA 2 understudy, both dead = silence

There is only ever one voice, but the data source has priority (AMM display section):

- in normal operation (or RA2 invalid), the RA1 provides the altitude information to FWC1 and FWC2 which generated the call-outs. - in case of RA1 failure (or RA1 data invalid or NCD) with RA2 valid, the RA2 provides automatically the altitude information to FWCs. - if RA1 and RA2 invalid, no call-outs generated.

The DH input. Per AMM 34-42-00:

For information: the pilot sets the DH on the MCDU.

Set a DH and the FWC has HUNDRED ABOVE (DH+100) and MINIMUM (DH) to speak; lose the DH information and those displays disappear. The QRH equipment list thus gains a footnote: autoland requires "auto callout: 1 available" — an autoland with no RETARD and no numbers has a broken link in its monitoring chain (the MEL: auto-callout failure → maximum "CAT 2 without autoland").

6. Downstream users — a pair of small antennas supporting half the warning system

The FCTM summarises the RA's "social network" best. Per FCTM PR-AEP-NAV:

The Radio Altimeters (RA) provide inputs to a number of systems, including the GPWS and FWC for auto callouts. They also supply information to the AP and A/THR modes, plus inputs to the flight control laws switching at various phases.

The roll-call (each detailed in its own article): all five basic GPWS modes are built on radio height; the whole TCAS inhibition table is layered by AGL; the PWS 2300 ft activation window; the AP LAND/FLARE/THR IDLE timing; the flight-control-law ground/flight/flare switching — plus most of the "below XXXX ft" inhibition logic on ECAM asks the RA. This user network is the map for the dual-RA-failure handling later: losing the RA is not "one fewer reading" but GPWS / TCAS / both windshear detections / auto callouts / CAT capability going offline in the same second, while the flight controls cannot switch to flare law either (they revert to the LGCIU wheel-load signal, with USE MAN PITCH TRIM). A signpost here; the full account is later.

Key numbers

Self-test

[!note]- Q1. Flying over a V-shaped valley, does the RA report the valley floor or the ridge? Why? The ridge — leading-edge tracking isolates the smallest beat (nearest point), so the peak reads before the valley. Conservatively safe.

[!note]- Q2. What is clever about the "beat held at 25 kHz" servo, and from what quantity is height actually measured? It converts a hard-to-measure small frequency into an easy-to-measure period; height is derived from the modulation period T.

[!note]- Q3. RA 1 fails — how does the PFD change, do the callouts stop, who takes over? Both PFDs show the remaining RA (automatic). Callouts continue: RA 2 automatically feeds the FWCs.

[!note]- Q4. Drifting long on approach, after "10 ft" it will not touch down — what does the loudspeaker do, on what basis? It repeats the present height every 4 s (gap > 4 s below 50 ft). Silence is the anomaly by design.

[!note]- Q5. The 20/10 ft RETARD boundary, and why does autoland call it later? 20 ft hand-flown; 10 ft if A/THR active and an AP in LAND. Autoland calls later because the A/THR closes itself after the flare — the call is confirmation of monitoring.

[!note]- Q6. Both RAs fail in cruise — is there a flag? When and how does it appear? No flag in cruise. The red RA flag appears only below the transition altitude (both invalid, attitude not excessive), flashing 9 s at the transition altitude then steady.

Key takeaways

References

• FCOM DSC-34-10-40-10 — dual RA, display takeover, callout system, RETARD.
• AMM 34-42-00 — wheel-height definition, leading-edge lock, 4.2–4.4 GHz FM/CW, 25 kHz, smallest beat, system parts, display rules, 570 ribbon, 150 ft gap, dual-fail flag, source chain, DH input.
• FCTM PR-AEP-NAV — RA downstream users.

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.