Thrust Mode Degradations

The common theme of this family is that the thrust accounting goes wrong — the unit of measure degrades (EPR → N1), the account freezes (THRUST LOCKED), bleed configuration scrambles the books (THRUST LOSS), two ledgers refuse to reconcile (the two DISAGREEs), or the account becomes recoverable (EPR MODE RECOVERABLE). The engine itself stays healthy throughout this family — what fails is measurement, mode, or cross-checking. That is the essential boundary against the core-failure family of article 18.

One exception: THRUST LIMITED is not an accounting problem at all — it is the IP turbine overspeed protection actively docking thrust, the cockpit face of the protection logic in article 05.

1. ENG 1(2) EPR MODE FAULT — the master alert of the degradation chain

"This alert triggers when FADEC of the affected engine has automatically reverted to the N1 mode."

This is the procedural face of article 07. The procedure forks into the two worlds established there — rated and degraded N1 mode:

On the ground. Rated → the STATUS page carries T.O THRUST LIMITED and dispatch goes through the MMEL thrust penalty; the procedure also commands ALL ENG N1 MODE ON:

"The other engine must be reverted to N1 mode, to ease thrust setting"

— the healthy engine is deliberately downgraded so both engines read the same ruler; symmetric thrust is easier to set when both gauges speak N1 (synthesis). Degraded → the N1 DEGRADED MODE status appears, and:

"Dispatch in degraded N1 mode is not allowed"

Degraded mode grounds the next sector — full stop.

In flight. Rated → ALL ENG N1 MODE ON, and "Autothrust is available in rated N1 mode". Degraded:

"N1 DEGRADED MODE / DO NOT EXCEED N1 LIMIT — As there is no N1 rating limit, an overboost can occur at the full forward thrust lever position. CAUTION: Before selection of N1 mode when the autothrust is not available, THR LEVERS should be set to a detent not higher than CLB, in order to prevent an overspeed."

That CAUTION is article 07's "naked lever" turned into procedure (synthesis): in degraded mode the lever is re-scaled so that full forward = red-line N1, with no rating ceiling in between. If your lever is sitting high when you press the N1 MODE pushbutton, the engine will chase the new scale upward the instant it switches. Retard the lever first; change the ruler second.

Autothrust survives degraded mode only while no more than one engine is degraded (the availability table of article 07); with it gone, MAN THR ADJUST, with the power-management tables in the EFB. The approach-capability arithmetic on STATUS: all engines in N1 mode with A/THR = CAT 1 WITH A/THR / CAT 2 WITH MAN THR; any engine not in N1 mode, or no A/THR = CAT 2 ONLY; CAT 3 INOP.

2. ENG EPR MODE RECOVERABLE — regained, but disconnect first

"This alert triggers when, after an ENG 1(2) EPR MODE FAULT, at least one of the engines is in N1 mode and all the engines can recover the EPR mode."

"With the current EEC Standard, there is no thrust lock at A/THR disengagement when reverting to EPR mode. To avoid a thrust increase, disconnect the A/THR before reverting to EPR mode. — ALL ENG N1 MODE OFF / If A/THR is not available: MAN THR ADJUST."

How this sits with article 07's recovery conditions (fault gone + on ground + pushbutton OFF) — stated plainly, side by side (synthesis): the systems description gives the conditions under which recovery is possible; this alert is the flight warning computer telling you those conditions are now met. The executing action is identical (pushbutton OFF), and "disconnect the A/THR first" is the anti-jump guard under the current EEC standard — reverting to EPR does not engage a thrust lock, so an engaged autothrust could command a thrust increase mid-switch. The two texts contain no explicit phase conflict; the executing moment follows the systems-description ground conditions, as locked down in article 07.

3. ENG 1(2) THRUST LIMITED — the IP turbine protection is docking your thrust

"This alert triggers when the FADEC limits the engine thrust to up to 30 % of the maximum takeoff thrust, to protect the integrity of the Intermediate Pressure Turbine. — After 10 min: THR LEVER 1(2) IDLE."

This is the "limit to 30 %" branch of article 05's IP-turbine overspeed logic, arriving in the cockpit. Both numbers carry meaning (synthesis): 30 % is the thrust equivalent of the speed ceiling that keeps the IP turbine intact; the 10 minutes then retire the engine to idle — once a powerplant has been docked to a third of its takeoff thrust, its residual value past the immediate phase does not justify keeping a protection-limited engine working. Note the upstream context: the turbine overheat detection that forms one of this protection's triggers lives with the exceedance alerts in article 28.

4. ENG THRUST LOCKED — a frozen throttle that nags every five seconds

"This alert triggers when the thrust is frozen on one or more engines after a failure or an involuntary autothrust disconnection. — This alert is automatically repeated every 5 s until thrust levers are moved. — THR LEVERS MOVE."

The mechanism, via article 08 (synthesis): at the instant of an involuntary autothrust disconnection, the levers are still parked in the CL detent while actual thrust is whatever the autothrust last commanded — lever position and thrust have come apart. The FADEC freezes thrust at the disconnection value (preventing a sudden jump to the lever-position value), then calls "move the levers" every five seconds — because moving the levers is the only unlock: the movement re-declares the lever is authoritative again.

This is the only alert in the section that automatically repeats. Every second spent in the frozen state is a second in which thrust answers to nobody — the nagging is proportionate.

5. ENG THRUST LOSS — the name says thrust, the culprit is bleed

"This alert triggers when the actual bleed configuration, as seen by the FADEC, is not in accordance with the bleed configuration requested by the crew. — If abnormal engine anti-ice: ABNORM ENG A.ICE / THR LEVERS IDLE; If abnormal wing anti-ice: ABNORM WING A.ICE / THR LEVERS IDLE; If abnormal bleed or pack: ABNORM BLEED OR PACK / THR LEVERS IDLE."

The most misleadingly named alert in the family (synthesis): the engine is fine, the thrust system is fine — the bleed configuration is saying one thing and doing another. Articles 05/09 showed the FADEC correcting its thrust accounting for bleed extraction; a configuration mismatch means the books are wrong, and actual thrust is lower than (or different from) what you believe you have — hence "thrust loss". All three branches retard to idle for the same reason: first bring thrust back to a regime that needs no precise accounting, then chase the bleed side (the air-system procedures take over from there).

6. The two DISAGREEs — the auditor's alerts

"ENG T.O THRUST DISAGREE — This alert triggers when a rating discrepancy between two engines is detected. — Crew awareness."

"ENG TYPE DISAGREE — This alert triggers when: ‐ The engine ratings seen by the FWC are not identical for all engines, or ‐ The engine ratings are different from the rating memorized by the FWC. — Crew awareness."

Both are findings by the bookkeeping auditor — the flight warning computer (synthesis). T.O THRUST DISAGREE asks "are both engines using the same rating for this takeoff?" (one at FLEX while the other is at TOGA — an MCDU-entry or data inconsistency). TYPE DISAGREE asks "do the engines' rating identities match each other, and match what I have memorised?" (the engine-rating-selection domain of article 04's Data Entry Plug). Both are awareness: the auditor reports the mismatch; the human arbitrates, nothing acts automatically.

7. The family in one table

8. Scenario walk-throughs

Cruise, EPR MODE FAULT, rated. Both engines to N1 mode, autothrust re-engaged, and brief the approach around CAT 1 WITH A/THR.

Same alert, degraded. Autothrust is gone — the lever becomes your ruler (article 07), the approach is CAT 2 ONLY, and one more item for the ground call: this aircraft does not dispatch in degraded mode — the maintenance conversation starts at the gate, not at scheduling time.

Autothrust kicked off by a failure, ECAM chiming every five seconds. Don't resent it — it is waiting for you to move the levers and take thrust back under command.

T.O THRUST DISAGREE before takeoff. Back to the MCDU: compare both PERF entries against the rating labels on the EWD (articles 08/15 own that cross-check chain).

Self-test

[!note]- Q1. In-flight EPR MODE FAULT (rated) — why does the healthy engine also get switched to N1 mode? "To ease thrust setting" — both engines onto the same ruler, so thrust can be set symmetrically. One side's fault converts both sides' gauges.

[!note]- Q2. What is the CAUTION before selecting N1 mode in degraded conditions, and why? Retard the levers to a detent not higher than CLB first. Degraded mode has no rating limit — switch scales with the lever high and the engine overboosts/overspeeds chasing the new scale. The naked-ruler effect.

[!note]- Q3. THRUST LIMITED — what are the 30 % and the 10 minutes? 30 % of maximum takeoff thrust = the cap the FADEC imposes to keep the IP turbine intact; after 10 minutes, lever to IDLE — an engine docked to a third of its thrust has done its immediate-phase work and gets retired.

[!note]- Q4. THRUST LOCKED keeps chiming. How do you silence it? Move the thrust levers. The alert repeats every 5 seconds until they move; the movement re-declares the lever as authoritative and releases the freeze.

[!note]- Q5. ENG THRUST LOSS triggers. Is the engine broken? No — the trigger is a bleed configuration mismatch (the FADEC's thrust accounting scrambled by bleed). All three branches retard to idle first, then the abnormal-bleed/anti-ice indications route you to the air-system side.

Key takeaways

References

• FCOM PRO (engine abnormal procedures: EPR MODE FAULT complete section incl. CAT capability and dispatch notes; EPR MODE RECOVERABLE; THRUST LIMITED; THRUST LOCKED; THRUST LOSS; T.O THRUST DISAGREE; TYPE DISAGREE) — all triggering conditions and procedures quoted verbatim.
• FCOM DSC-70 (thrust-mode reversion and recovery conditions) — the ground-recovery conditions referenced via article 07.
• Integrative synthesis (marked in text): the same-ruler and retard-first readings; the reconciliation of RECOVERABLE with the ground conditions; the 30 %/10-minute interpretation; the freeze mechanism and lever-as-authority reading; the scrambled-books framing; the auditor framing.

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.