Abnormal III: Surveillance and Radio-Nav Fault Family

This article closes the ATA-34 abnormal spectrum: the fault families of the radio altimeter, transponder/ADS-B, EGPWS, weather-radar PWS, and landing system, plus one reset discipline running through the whole chapter. The headline act is dual RA failure — the radio-altimeter article drew its downstream user network; here it is realised: how the loss of one sensor takes GPWS, TCAS, both windshear detections, auto callouts, and flare-law switching offline in the same second, and forces out "USE MAN PITCH TRIM."

1. RA single failure — a one-line bill

Per FCOM PRO-ABN-NAV:

This alert triggers when the RA 1(2) is failed.

No handling action (pure awareness) — the remaining unit automatically takes over both PFDs, and the callouts, GPWS, and TCAS continue on it. The only bill is capability: CAT 2 ONLY (CAT 3 lost — CAT 3 needs two RAs). After landing there is a maintenance-only prompt. Per FCOM PRO-ABN-NAV:

This alert triggers after landing when a discrepancy between the two radio altimeters is detected. Maintenance action is due.

(Both are still working but their readings are diverging — fix it before it fails, a preventive maintenance of CAT 3 capability.)

2. Dual RA failure — one sensor, half the warning network

The ECAM trigger is one line (This alert triggers when the RA 1+2 is failed.), but the STATUS INOP list is one of the longest knock-ons in all of ATA-34: A/CALLOUT (auto callouts), GPWS, PRED W/S DET, TCAS, GLS AUTOLAND, REAC W/S DET — plus CAT 2. The user network collapses here in full. The FCTM turns the consequence into a lesson. 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. Although the ECAM procedure for NAV RA 1 AND 2 FAULT is straightforward, the effects of the failure on the aircraft operation require consideration.

Without RA, how do the flight controls know their height above the ground? — they switch to the landing-gear wheel-load signal (LGCIU). Per FCTM PR-AEP-NAV:

Instead of using RA information, the flight control system uses inputs from the LGCIU to determine mode switching.

Three phases of alternate triggering: take-off — normal law when the main gear is no longer compressed and pitch > 8°; landing — flare law when the gear is down and the AP disconnected, with a manual-trim prompt. Per FCTM PR-AEP-NAV:

On approach, flare law becomes active when the L/G is selected down and provided AP is disconnected. At this point, USE MAN PITCH TRIM is displayed on the PFD.

(Normal flare law is handed over smoothly by RA height with automatic trim; with no RA, flare law is hard-triggered by "gear down + AP off," and the automatic trim fails, so you crank the trim wheel by hand — one of the A330's rare USE MAN PITCH TRIM scenarios.) Landing capability and capture. Per FCTM PR-AEP-NAV:

It is not possible to capture the ILS/FLS /SLS /GLS using the APPR pb and the approach must be flown to CAT 1 limits only. However, it is possible to capture the localizer using the LOC pb.

(Why can LOC but not G/S? The G/S auto-flight monitoring chain needs RA to backstop the flare; the LOC is lateral guidance, independent of height. So "lateral on the beacon, vertical on hand skill.") The last paragraph translates the knock-on into airmanship. Per FCTM PR-AEP-NAV:

There will be no auto callouts on approach, and no "RETARD" call in the flare. The GPWS/EGPWS will be inoperative, therefore, terrain awareness becomes very important. Similarly, the "SPEED, SPEED, SPEED" low energy warning is also inoperative, requiring increased awareness.

The chain closes: a pair of 4 GHz small antennas support the entire safety net of "the last 2500 feet"; lose both and you simultaneously lose the ear of terrain warning, the eye of collision avoidance, the mouth of callouts, and the automatic hand of the flare — landing on visual height judgement, manual trim, and CAT 1 limits.

3. Transponder and ADS-B fault family — mostly switch sides

ADS-B RPTG single failure is treated by "switch transponder" (procedure): ATC/XPDR to the other SYS — because ADS-B Out position comes from the onside GPS, switching means switching to an intact GPS broadcast chain. Transponder single failure switches the same way, but RVSM airspace adds a pairing. Per FCOM PRO-ABN-NAV:

In case of flight in RVSM airspace, select AP1 if SYS1 is used and AP2 if SYS2 is used.

(Make "the you the controller sees" and "the you the AP flies" share a source, so the RVSM separation lives up to the arithmetic.) A dual transponder failure's INOP list has TCAS as its first line — the TCAS's mouth and ears are the transponder's, so dual death = collision avoidance dead too. Selecting the transponder to STBY manually also raises a prompt (NAV ATC/XPDR STBY, awareness) that you have "disappeared" from radar.

4. EGPWS fault family — two levels of failure, two keys

NAV GPWS FAULT (the basic modes ailing) handling (procedure): set the GPWS SYS key OFF, and this ECAM line "remains on screen after the action" to remind you the protection is lost. All five basic modes lost = no ground-proximity warning at all. NAV GPWS TERR DET FAULT (only the prophet ailing) is the lighter level. Per FCOM PRO-ABN-NAV:

This alert triggers when the enhanced Terrain Clearance Floor and Terrain Awareness Display modes of the EGPWS are inoperative.

The handling sets the TERR key OFF, and the key difference is. Per FCOM PRO-ABN-NAV:

The basic GPWS Modes 1 to 5 are still operative, if the SYS pb FAULT or OFF lights are not on.

The prophet is dismissed, the veteran remains — lost is the forward-looking terrain database, while the reactive five basic modes still guard "underfoot." Teaching anchor: two ECAM names differ only by "TERR DET," yet the capability is worlds apart — grade it at a glance.

5. Scattered PWS, heading, and landing-system faults

NAV PRED W/S DET FAULT (predictive windshear detection lost) has the cleverest handling (procedure): consider the WXR 1/2 SWTG to the other radar — PWS lives inside the weather radar, so this radar's PWS dead, switching to the other transceiver's PWS may revive it (if the other is good). NAV HDG DISCREPANCY (the two sides' headings argue) handling (procedure): compare the three IR headings on the MCDU, or cross-check the standby compass, then ATT HDG SWTG the IR 3 to the faulty side. The trigger threshold hides a subtlety. Per FCOM PRO-ABN-NAV:

This alert triggers when the difference between the heading on the CAPT and F/O displays is greater than 5 ° in TRUE, or greater than 7 ° in MAG.

(Why is TRUE stricter? The TRUE reference is used mostly in the polar region, where heading accuracy matters more for polar navigation, so the threshold tightens; under MAG the displays also carry the inherent scatter of the magnetic-variation table, so the threshold widens by 2° against nuisance.) In the polar region it often accompanies NAV EXTREME LATITUDE. Landing system: NAV LS 1(2)(1+2) FAULT (awareness, capability to CAT 2, ground LS 1 death knocks on GPWS mode 5) and NAV LS TUNING DISAGREE (the FMGES compares the two MMRs' type/frequency/channel/course, triggering inhibits/disarms APPR, CAT 2 inop) — the mechanism was detailed in the ILS article; here they are only placed in the fault family.

6. Reset discipline — ATA-34 is mostly a "no-go zone," the RMP the only exception

The overarching rule of resets decides life and death in one line. Per QRH 02.01A:

WARNING In all other cases: RESET NOT AUTHORIZED.

Authorisation has only two sources: an ECAM/OEB/QRH procedure explicitly requesting a reset, or the fault being listed and authorised in the reset table. Most ATA-34 systems (ADIRU, GPWS, TCAS, transponder, weather radar, MMR) are not in the reset table — meaning their ECAM faults may not be reset by hand ("turn it off and on to try my luck" is an unauthorised action that may turn a controllable degradation into an uncontrollable loss). And a same-system knock-on rule. Per QRH 02.01A:

In the case of several failures related to the same system, RESET IS NOT AUTHORIZED if a minimum of one reset is not authorized.

The only ATA-34-neighbourhood item in the reset table is the frozen RMP. 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.

The reset in three steps (procedure): the RMP panel ON/OFF selector to OFF → wait at least 5 s → ON, each in turn. Note it is "all RMPs one by one," not just the frozen one — because the three RMPs share the tuning bus (the same "reboot one at a time to keep one online" philosophy as the PRIM reset in IR DISAGREE). The counter-example: the IR DISAGREE PRIM reset is a flight-control reset, explicitly requested by the ECAM (the first authorisation source), not ATA-34 acting on its own — both are "reset one by one," but the authorisation provenance differs; do not confuse them. The teaching line: when ATA-34 equipment fails, the principle is re-source / degrade / switch sides, not reset; the only thing you may actively reset is a frozen RMP.

Key numbers

Self-test

[!note]- Q1. RA 1 single failure: any action? Capability drops to which category? Why do the callouts continue? No action (awareness). CAT 2 ONLY. The remaining RA automatically takes over both PFDs, so callouts, GPWS, TCAS continue.

[!note]- Q2. After dual RA failure, how do the flight controls judge air/ground? When does flare law engage, and why USE MAN PITCH TRIM? On the LGCIU wheel-load signal. Flare law hard-triggers at gear-down + AP-off; automatic trim fails, so manual pitch trim is needed.

[!note]- Q3. The dual-RA INOP list (at least six), and three airmanship points before approach? A/CALLOUT, GPWS, PRED W/S DET, TCAS, REAC W/S DET, GLS AUTOLAND (+ CAT 2). Emphasise: terrain awareness (no GPWS), CAT 1 limits, manual trim in the flare with no RETARD.

[!note]- Q4. With dual RA failure can you still capture LOC? G/S? To which category, and why? LOC yes (lateral, height-independent), G/S no (its monitoring needs RA for the flare). CAT 1 only.

[!note]- Q5. RVSM, XPDR 1 fails, switch to SYS 2 — what else, in one line? Select AP 2, so the reported altitude and the flown altitude share the same-side source.

[!note]- Q6. GPWS FAULT vs GPWS TERR DET FAULT — which key each, and the basic-mode difference? GPWS FAULT: SYS key OFF (all basic modes lost). TERR DET FAULT: TERR key OFF (basic modes 1-5 remain if SYS light off).

[!note]- Q7. A TCAS ECAM fault — turn it off and on to try? On what discipline? And a frozen RMP? No — RESET NOT AUTHORIZED (TCAS is not in the reset table). A frozen RMP is the exception: OFF ≥ 5 s → ON, all RMPs one by one.

Key takeaways

References

• FCOM PRO-ABN-NAV — RA single/dual/degraded; ADS-B/XPDR/GPWS/GPWS TERR/HDG DISCREPANCY/PRED W/S DET.
• FCTM PR-AEP-NAV — dual RA failure: LGCIU law, MAN PITCH TRIM, CAT 1, LOC capturable, knock-on consequences.
• QRH 02.01A/02.02A — reset overview, RESET NOT AUTHORIZED, same-system knock-on, frozen-RMP reset.

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.