Abnormal II: IR and Position Fault Family

The other half of the ADIRU — the inertial reference (IR) — fails more grievously than the ADR: it is the only inertial source of attitude, heading, and position. A single loss means re-sourcing; a dual loss means law degradation plus pulling the trim-tank fuel forward; and IR DISAGREE is the only fault in all of ATA-34 that drops the aircraft to direct law, its handling containing the rare step "turn off the IR + reset the three PRIMs one by one." This article covers IR single/dual/triple faults, IR DISAGREE, IR NOT ALIGNED, ATT-mode re-alignment, and the position-layer FM/IR, FM/GPS POS DISAGREE and GPS FAULT.

1. IR single fault — re-source, knock-on, and "reviving TCAS by hand"

Per FCOM PRO-ABN-NAV:

The alert triggers when the IR 1(2)(3) is failed.

The re-source and shut-off actions (procedure): the ATT HDG SWTG switches the faulty side to IR 3 (CAPT ON 3 or F/O ON 3; NORM if IR 3 is the sick one) — note this switch is not the AIR DATA SWTG of the previous article; the attitude/heading side and the air-data side are two independent rotaries, which is why "same-ADIRU dual fault does ADR before IR" (the two procedures move different switches, and the wrong order swaps the good source away). The transponder moves too (the XPDR altitude datum is the ADR's, but the Mode S magnetic heading etc. are the IR's): IR 1 fault → ATC SYS 2, IR 2 fault → SYS 1. The sick unit's disposition: if it still offers ATT mode, set the MODE selector to ATT (keep attitude, lose navigation); if totally failed, set its pb OFF.

STATUS bill: CAT 3 SINGLE ONLY; INOP that IR. IR 1's knock-on tail is the longest — GPWS TERR (the EGPWS's inertial/track source is ADIRU 1, so the EGPWS taproot holds on the IR side too), ADS-B TRAF (with ATSAW, not fitted here), and the trickiest, TCAS. Per FCOM PRO-ABN-NAV:

In the case of NAV IR 1 FAULT, the TCAS may be inoperative (depending on the TCAS manufacturer). If the IR 1 is available in ATT mode, the TCAS can be recovered by entering the aircraft magnetic heading into the CDU

This configuration carries a Honeywell TCAS, right in the "depending on the manufacturer" affected set — the TCAS needs the aircraft's heading to compute target bearing, so IR 1's death cuts the heading source and TCAS goes on strike; but as long as IR 1 retains ATT mode, feeding a magnetic heading by hand revives it (the ATT re-alignment procedure of §5 is the entry to this lifeline). This is TCAS's third cause of death, beyond "dual XPDR / dual RA loss," and the only one manually reversible.

2. IR dual fault — law degradation and "pulling the centre of gravity forward"

Per FCOM PRO-ABN-NAV:

The alert trigger when the IR 1+2(1+3)(2+3) are failed.

Set the SWTG to 3 by combination, handle both sick units (use ATT if available, else pb OFF); the faulty-side crew can borrow the opposite side via the EFIS DMC rotary; XPDR: IR 1+3 → SYS 2, IR 2+3 → SYS 1; no speed brakes. The most instructive is the trim-tank action (procedure): with the trim-tank pump available, if the CG is aft of 32%, set T TANK mode to FWD (pump the trim-tank fuel forward, fuel burn up ~1%); with the pump inoperative or not fitted, set FWD only when CG aft of 32% and speed > 270 kt and not climbing, all three together.

Mechanism: in cruise the A330 keeps the CG well aft with the trim tank (reducing trim drag to save fuel); losing two IRs, the law reverting to ALTN (PROT LOST), with no inertial reference to help the flight controls finely stabilise pitch — an aft CG plus low augmentation is a nursery for pitch divergence, so the CG is actively pulled forward to 32% to restore natural pitch stability. With the pump dead the fuel cannot be actively pumped, only moved passively by gravity/aerodynamics at high speed (> 270 kt) in level flight or descent — a climb attitude would pour it back aft, hence the "not climbing" condition. The bill: ALTN (PROT LOST) + 330/.82 + FLAP 3 + APPR 1 ONLY (CAT 1 only) + FLS limited to F-APP+RAW; a long INOP queue including dual AP, A/THR, F/CTL PROT, AUTOLAND, TCAS (with IR 1 involved).

3. IR DISAGREE — the only navigation fault that drops to direct law

Per FCOM PRO-ABN-NAV:

This alert triggers when one IR is failed or rejected by the PRIM, and the information received from the two remaining IRs is different.

(Structurally the same source as ADR DISAGREE — two argue after the third is gone, with no arbiter; but an attitude disagreement is far fiercer than a speed one.) The consequence is immediate. Per FCOM PRO-ABN-NAV:

Direct law becomes active. All pitch and roll protections are lost.

Why direct law (rather than ADR DISAGREE's ALTN)? Attitude is the bedrock of flight-control augmentation: two IRs giving contradictory attitudes leave the flight controls unable to trust either for augmentation, so stick commands pass straight to the surfaces (direct law). Finding the culprit takes the aircraft's most independent eye. Per FCOM PRO-ABN-NAV:

Use the standby horizon to determine the faulty IR.

(The ISIS has its own attitude gyros, independent of the ADIRU — the neutral judge in this "two witnesses accusing each other.") After confirmation comes the rare restart sequence (procedure): with the disagreement confirmed, set the faulty IR OFF, then reset PRIM 3, PRIM 2, PRIM 1 in turn (OFF then ON, one at a time). The purpose is stated plainly. Per FCOM PRO-ABN-NAV:

After corrective action (the faulty IR is switched off and PRIMs are reset), pitch alternate law is recovered with reduced protections.

The chain: direct law is the temporary state of "two witnesses arguing, the judge (PRIM) unsure whom to believe"; you use the ISIS as neutral arbiter to identify and turn off the bad IR, then let the three PRIMs "reboot" one by one — on reboot they see "two agreeing IRs" (the sick one offline) and dare to recover to alternate law (one protection more than direct law, but still short of normal because an IR is lost). One at a time, not together, so a PRIM is always online to control the aircraft. The branch (procedure): if the disagreement is not confirmed (the ISIS cannot tell either, both IRs kept on), maintain F/CTL DIRECT LAW — do not turn off either, and land in direct law. Better fewer protections than the wrong attitude source off.

4. IR NOT ALIGNED and the "gentle faults" of the position layer

IR NOT ALIGNED has four faces (ECAM lines): POSITION MISMATCH / POSITION MISSING (→ INSERT PRESENT POSITION), EXCESS MOTION, IR…IN ALIGN. Against the alignment mechanism: MISMATCH/MISSING = a problem in position entry (mis-entered or not entered — the step before "twice-wrong → IR FAULT"), fixed by re-entering; EXCESS MOTION = the aircraft pushed/blown above 0.5 ft/s during alignment, reverting to redo; IN ALIGN = not finished turning (longer at the equator/high latitude). This caution is available only in phase 2 (one engine running) — because alignment should be complete before pushback. The position layer's two gentle faults are advisory only. Per FCOM PRO-ABN-NAV:

This alert triggers when a significant drift is detected between any of the FMS positions and any of the IRS positions.

Handling is only "check position + thereafter periodic NAV ACCURACY" (using the POSITION and IRS MONITOR pages). Alongside FM/GPS POS DISAGREE: the former compares "FMS vs inertial," the latter "FMS vs satellite" — neither degrades nor re-sources, because the FMS position is a GPIRS hybrid, and a single over-tolerance comparison only says "a source is drifting," not "position is unusable." The real handling is to keep watching NAV ACCURACY and deselect a source if needed. This is why they are awareness-level: the robustness of the hybrid position gives the crew room to handle it while watching.

5. ATT-mode re-alignment — give the inertial system a heading and it revives

The recovery procedure when alignment is lost in flight (red ATT flag / red HDG flag) (QRH 23.05A, procedure): (1) the sick unit's MODE selector → ATT; (2) keep speed, heading, and altitude constant for 30 s; (3) the FMS DATA page, press IRS MONITOR; (4) enter the aircraft heading with SET HDG; (5) thereafter cross-check with the standby compass regularly and update as required.

The chain: ATT mode gives up navigation (no position, no true heading), rebuilding attitude only against the known direction of gravity — hence "steady for 30 s" to find the vertical undisturbed (the coarse-levelling principle, but with no gyro-compass search for north); the inertial system cannot recover heading itself, so a value is read manually from the standby compass and fed in, and because the magnetic compass drifts with manoeuvring, it must be re-checked against the standby compass periodically. This procedure has three identities: (1) the recovery for an in-flight IR alignment loss; (2) the entry to "reviving the IR 1-side TCAS by hand" of §1 (feeding a heading = giving the TCAS its target-bearing reference back); (3) the crossing point of this article and the TCAS article.

6. The GPS fault family — the composure of a hybrid position

Per FCOM PRO-ABN-NAV:

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

A single GPS FAULT is awareness + INOP only — the GPIRS still has the other GPS plus three IRs (the 18-state Kalman transitions smoothly to IRS/DME/DME etc., almost imperceptibly). Only a dual loss begins a capability bill (STATUS, procedure): dual GPS failure → FLS limited to F-APP+RAW (FM managed approach + raw data).

(FLS is not fitted, so this line is empty for this configuration — but the mechanism matters: a GPS-based approach's position source is gone, reverting to FM managed approach + raw-data monitoring.) Against the interference article: there, external interference makes GPS data wrong/lost; here, the receiver itself fails — similar symptoms (GPS PRIMARY LOST, accuracy degradation), but interference knocks on a whole crowd (false TAWS, clock, ADS-B), while a plain GPS FAULT loses only the navigation-accuracy line.

Key numbers

Self-test

[!note]- Q1. IR 2 single fault: which switch, to whom? Which XPDR SYS? How does it differ from an ADR 2 fault's switch? ATT HDG SWTG to F/O ON 3; XPDR to SYS 1. It differs from ADR 2's AIR DATA SWTG — attitude and air-data sides are independent rotaries.

[!note]- Q2. Why does IR 1 failure strike TCAS here but not on all A330s, and how to revive it? The Honeywell TCAS needs aircraft heading for target bearing; IR 1's death cuts it. If IR 1 retains ATT mode, enter a magnetic heading in the CDU (per ATT re-alignment) to revive it.

[!note]- Q3. IR dual fault, CG at 35%, trim-tank pump normal — action? Pump dead? Why not while climbing? Pump normal: CG aft of 32% → T TANK FWD. Pump dead: only if also > 270 kt and not climbing (fuel moves passively; a climb attitude pours it back aft).

[!note]- Q4. Why is IR DISAGREE direct law while ADR DISAGREE is only ALTN? Which instrument identifies the bad unit? Attitude is the bedrock of augmentation; two contradictory attitudes leave nothing to trust, so direct law. The ISIS (standby horizon) identifies the bad IR.

[!note]- Q5. After off-bad-IR + PRIM reset one by one, which law recovers, why not normal, and why one at a time? Reduced-protection alternate law (one IR lost, so not normal). One at a time so a PRIM is always online to control the aircraft.

[!note]- Q6. The ISIS cannot tell which IR is right either — which law do you land in, and why is that correct? Direct law (do not turn off either). Better fewer protections than turning off the wrong attitude source.

Key takeaways

References

• FCOM PRO-ABN-NAV — IR single/dual/DISAGREE/NOT ALIGNED, IR 1 → TCAS revival, trim tank, FM/GPS·FM/IR POS DISAGREE, GPS single/dual.
• QRH 23.05A — ATT-mode re-alignment.

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.