Overlimit & Overspeed

The red lines of article 00 marked where the engine must not go; this article covers the three alerts that fire after it has gone there. All three share a handling philosophy gentler than you might expect: first, a chance to come back inside the limit (lever below the limit → "normal operation may be resumed"), and only when there is nothing left to retard — the lever already at idle and the parameter still exceeding — does the shutdown line apply. And the wait at idle differs: 3 seconds for overspeed, 5 for overtemperature. The article closes with the grand ledger of the four-layer overspeed protection system built up across seven earlier articles.

1. EGT OVERLIMIT — three gates, each guarding its own regime

"This alert triggers when: ‐ EGT is above 920 °C, or ‐ EGT is above 900 °C for more than 20 s, or ‐ EGT is above 850 °C when the throttle resolver angle is lower than MCT."

The first two gates are the red-line criteria locked down in articles 00/15 — the 920 °C transient ceiling, and the 20-second tolerance band between 900 and 920. The third gate is new knowledge: 850 °C is the MCT continuous limit — so an EGT above 850 with the lever below MCT means thrust and temperature don't match. An engine running hot on a low lever is itself a disease (synthesis) — and precisely the mirror image of the advance-and-EGT-rises stall trap in article 27.

The handling ladder:

"Maximum pointer indications: THR LEVER (AFFECTED ENGINE) BELOW LIMIT — Normal operation may be resumed. Report in maintenance logbook. — For aircraft equipped with GE or RR engines: If THR LEVER at idle for more than 5 s: ENG MASTER OFF — If conditions do not permit engine shut down, land as soon as possible using the minimum thrust required to sustain safe flight."

2. N1/N2/N3 OVERLIMIT — the same script, with a 3-second wait

"This alert triggers when the corresponding N1/N2/N3 value is above the red threshold. — Maximum pointer indications: N1 above 99 % / N2 above 103.3 % / N3 above 100 % — THR LEVER BELOW LIMIT — Normal operation may be resumed. Report in the maintenance logbook. — If THR LEVER at idle for more than 3 s: ENG MASTER OFF — If conditions do not permit engine shutdown, land as soon as possible using the minimum thrust required to sustain a safe flight."

The 3-versus-5 asymmetry is worth memorising (synthesis): both waits are waiting for the same thing — with the fuel cut, will the parameter come back by itself? Rotor speed answers the throttle faster than temperature does (the hot section carries thermal inertia), so overspeed gets 3 seconds, overtemperature gets 5. If the parameter still hasn't returned, the problem is not the throttle — a jammed metering valve, a runaway from a broken shaft, internal damage — and the shutdown cuts the losses.

Read the resumption clause precisely too: "normal operation may be resumed" applies to the case where retarding brought the parameter back inside the limit — but the red exceedance markers stay pinned on the display (article 15), and "report in the maintenance logbook" is not a courtesy: the peak value and its duration decide the scope of the borescope inspection waiting on the ground (article 01's inspection map).

3. TURBINE OVHT — a cooling-environment alert, and the relay to the IP-turbine protection

"This alert triggers when a turbine overheat is detected. — THR LEVER (AFFECTED ENGINE) IDLE — IF WARNING PERSISTS: ENG MASTER OFF."

Article 14 established what this sensor actually measures: not the disc itself but the temperature of the cooling air around the disc — the downstream symptom of a failed seal-air supply (article 03). That is why this alert has no "come back inside and carry on" option: a broken cooling environment does not heal because you retarded the lever. Two settings only — cool it (idle) or cut the losses (shutdown).

Its relay to the IP-turbine overspeed protection was laid in articles 14/21: overheat detection is one of that protection's triggering contexts — so if THRUST LIMITED shares the screen (article 21), the protection has already acted and docked thrust to 30 %.

4. The four-layer overspeed grand ledger — the chapter's closing table

How to read the table (synthesis): the red lines are the referee's whistle — after the whistle, the handling is yours (this article's three alerts). The 110/117 thresholds and the two turbine protections are the security guard's hands — when the whistle isn't enough, they execute the cut themselves. The fail-safe shaft is the last mechanical guardrail. And every layer's own failure has its own alert (article 20) — the protection system is itself protected by monitoring.

5. Scenario walk-throughs

Takeoff EGT peaks at 915 °C and falls back. A lawful transient inside the 20-second tolerance band — and still an exceedance to report faithfully after landing.

N1 peaks at 101 % during a windshear recovery. Retard inside the limit, resume operation, and write the peak and its duration in the logbook — that line decides the borescope.

TURBINE OVHT and THRUST LIMITED on screen together. Read the plot: overheat triggered the IP-turbine protection, thrust is already docked to 30 %, the lever goes to idle after 10 minutes (article 21), and this engine flies the remainder limited.

Replaying article 09's scenario — an N1 at 108 % with no shutdown is not a failed protection: that is the designed buffer between the referee (red line, 99 %) and the guard (110 %).

Self-test

[!note]- Q1. Cruise, EGT 862 °C, lever in CLB — does an alert trigger? Yes — the third gate: above 850 °C with the throttle resolver angle below MCT. Running hot on a low lever is a mismatch, and the mismatch itself is the disease.

[!note]- Q2. Why 3 seconds for overspeed but 5 for overtemperature? Both waits ask "will it come back by itself with the fuel cut?" Speed answers the throttle quickly (3 s); temperature carries thermal inertia (5 s). Past the wait, the problem isn't the throttle — shut down.

[!note]- Q3. N2 spiked to 104.5 %, you retarded, it's back at 99 % — can the engine be used? And after landing? The procedure allows normal operation to be resumed — but the red exceedance marks stay on the system display (article 15), and the logbook entry is mandatory: peak and duration set the inspection scope.

[!note]- Q4. Why does TURBINE OVHT lack a "come back inside and carry on" option? Because it detects cooling-air temperature around the discs — an environment failure that retarding cannot repair (article 03's seal air). Two settings only: idle, or shutdown. The IP-turbine protection may simultaneously be docking thrust (THRUST LIMITED).

[!note]- Q5. The four protection layers, soft to hard? EEC red-line limiters (control-level fuel reduction) → OPU at 110/117 % (independent shutdown) → LP TOS (broken shaft = immediate fuel cut) → IPTOS (30 % limit or shutdown). Mechanical backstop: the fail-safe shaft. Each layer's own failure alerts live in article 20.

Key takeaways

References

• FCOM PRO (engine abnormal procedures: EGT OVERLIMIT three triggers and ladder; N1/N2/N3 OVERLIMIT complete section; TURBINE OVHT) — quoted verbatim; engine-type-specific branches of other manufacturers excluded per type discipline.
• Mechanism articles referenced: 00 (red lines), 04/09 (OPU layers), 05 (TOS/IPTOS), 14 (exceedance recording; overheat = cooling-air sensing), 15 (red markers).
• Integrative synthesis (marked in text): the low-lever-high-heat reading of the third gate; the thermal-inertia explanation of 3 vs 5 seconds; the logbook-to-borescope link; the no-self-healing reading of TURBINE OVHT; the referee/guard/guardrail 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.