Overspeed / Severe Damage — Three-Layer Overspeed Protection, Shaft Limits, SHAFT FAILURE

This article gathers the overspeed-protection mechanisms (from 04/11/20), the shaft-speed limits (LIM-ENG), and severe-damage / ENG SHUT DOWN.

1. Three-layer overspeed protection

[!warning]- The logic of the three layers (synthesis across verbatim) In a three-spool Trent, a shaft break unloads its turbine, which instantly overspeeds (catastrophic). Three lines of defence: FMU cuts fuel + the overspeed protection unit monitors N1/N2/turbine + SHAFT FAILURE auto-shutdown. ①②③ are each verbatim from earlier articles, gathered here.

2. Shaft-speed and EGT limits (LIM-ENG)

Maximum N1 ... 99 %. Note: The N1 limit depends on the ambient conditions and on the configuration of the engine air bleed. These parameters may limit N1 to a value that is less than the above-mentioned N1 value.

(EGT Limits) 20 s 920 °C ... Takeoff and Go-around ... 900 °C Maximum Continuous Thrust (MCT) ... 850 °C ... Starting On ground 700 °C / In flight 850 °C

Per LIM-ENG: Maximum N1 = 99 % (the N1 limit depends on ambient conditions and bleed configuration, which may limit N1 lower). EGT limits: take-off/GA 20 s 920 °C / MCT 900 °C / 850 °C (low thrust) / starting ground 700 °C, in flight 850 °C (confirming 10/18/23).

[!note]- The three shaft red lines (LIM-ENG SHAFT SPEEDS) LIM-ENG SHAFT SPEEDS gives all three verbatim: Maximum N1 = 99 % / Maximum N2 = 103.3 % / Maximum N3 = 100 % — all three shafts complete (see 10). N1 carries the note that ambient + bleed configuration may limit it lower.

3. Overspeed / severe-damage handling

 overspeed / severe damage
   ├─ FMU automatically cuts fuel (close HP shut-off valve, 04)
   ├─ overspeed protection unit intervenes (N1/N2/turbine, 11)
   ├─ SHAFT FAILURE → FADEC auto-shutdown (20)
   └─ crew: confirm the failed engine → secure (THR IDLE / MASTER OFF) → LAND ASAP (full in PRO-ABN-ENG)

4. Counterintuitive points

[!warning]- Overspeed protection is fundamentally "cut fuel," executed automatically by the FMU/FADEC Protection runs on the FMU closing the HP shut-off valve to cut fuel (04) + the overspeed protection unit (11) + SHAFT FAILURE auto-shutdown (20) — automatic fuel cut / shutdown, not relying on prior crew action.

[!warning]- N1 max is 99 %, but it may be limited lower N1 limit 99 %, but ambient conditions + bleed configuration may limit N1 lower (LIM-ENG) — 99 % is a ceiling, not a fixed value.

Self-test

[!note]- Q1. The three layers of overspeed protection? ① FMU closes the HP shut-off valve to cut fuel (04) ② overspeed protection unit (N1/N2/turbine, 11) ③ SHAFT FAILURE → FADEC auto-shutdown (20).

[!note]- Q2. The maximum N1 limit, and why it may be lower? 99 %; ambient conditions + bleed configuration may limit it lower.

[!note]- Q3. Who shuts down on a Trent shaft failure? The FADEC automatically (SHAFT FAILURE for RR engines).

[!note]- Q4. The three shaft red lines? N1 = 99 % / N2 = 103.3 % / N3 = 100 % (LIM-ENG SHAFT SPEEDS).

Key takeaways

References

• FCOM DSC-70-40-40 (04) / DSC-70-20 (11) / PRO-ABN-ENG (20) — three-layer overspeed protection: FMU cut fuel / overspeed protection unit / SHAFT FAILURE auto-shutdown.
• FCOM LIM-ENG — Maximum N1 99 % (ambient/bleed may limit lower); shaft speeds N1 99 % / N2 103.3 % / N3 100 %; EGT limits 920 · 900 · 850 · 700 °C.

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.