Thrust Management — EPR / N1 Modes, Thrust Levers and Rating Limits

FADEC reads the thrust-lever angle and drives the FMU. This article follows that one chain in full: which parameter the FADEC controls thrust on (EPR or N1), what the four thrust-lever detents mean, how the rating limit is chosen, and how manual thrust differs from autothrust. It is the control the crew uses on every take-off, climb and approach.

(The DSC-70-35 sections carry a combined FCOM heading covering several Rolls-Royce variants; only the Trent 700 content is taken here.)

1. The FADEC controls thrust

A FADEC dedicated to each engine controls thrust. The pilot uses the thrust levers to set the thrust in manual mode, and the FMGS sets the thrust in automatic mode. The FADEC prevents the thrust from exceeding the limit for the thrust lever position.

Per DSC-70-35-10: in manual mode the pilot sets thrust with the levers, in automatic mode the FMGS sets it, and in both the FADEC holds the ceiling for the current lever position.

2. Thrust levers — manual only, four positions, three segments

The thrust levers can only be moved manually. The range of movement is divided into 3 operating segments. The sector has 4 positions defined by detents or stops. Thrust lever position is transmitted to the FADEC which computes and displays the thrust rating limit and the EPR TLA.

Per DSC-70-35-30, four detent/stop positions divide the travel into three segments:

The autothrust active ranges (per the TLA-range diagram): both engines IDLE → CL; with one engine inoperative, extended to IDLE → MCT.

[!warning]- The thrust levers move only manually — they do not move under A/THR Unlike autothrottle designs that back-drive the levers, these levers move only by hand (DSC-70-35-30). Under autothrust the lever stays in a detent and the FADEC trims thrust within the range that detent allows. So lever position ≠ actual thrust — read the ECAM thrust indication.

3. EPR mode — the normal mode

EPR mode is the normal mode to control the thrust. The required EPR is set by controlling the fuel flow. The FADEC computes the command EPR as a function of: ‐ Thrust Lever Angle (TLA) ‐ Altitude ‐ Mach number ‐ Air data (static pressure/temperature, total air pressure/temperature) ‐ Service bleed. Note: During reverse operation, the thrust is controlled as a function of N1.

Per DSC-70-35-20, EPR (engine pressure ratio) is the normal thrust-control parameter on the Trent 700; the FADEC computes the command EPR from TLA, altitude, Mach, air data and service bleed, and meters fuel to achieve it. N1 is a monitored parameter here — except in reverse, where thrust is controlled on N1 (see Thrust Reverser).

4. N1 mode — automatic reversion

In the event of no EPR available the affected FADEC will automatically revert to N1 mode. At the reversion to N1 mode, an equivalent thrust to that achieved in EPR mode is provided, until a thrust lever position change. In case of dispatch in N1 mode, flex take-off is not available. Depending on the failure case leading to EPR mode loss, the FADEC will revert to either rated or degraded N1 mode.

With the FADEC in either rated or degraded N1 mode, switching OFF the ENG N1 MODE pushbutton on the overhead panel will permit to return to the EPR mode, if the failure has disappeared and the aircraft is on ground.

Per DSC-70-35-20, with no EPR available the FADEC reverts to N1 mode automatically, holding equivalent thrust until the lever is moved. Three points for the crew:

[!note]- The three pilot points about N1 mode ① Automatic — no EPR, the FADEC switches itself, no prior crew action. ② Equivalent thrust held — no thrust jump at reversion, until the lever moves. ③ Cost — dispatched in N1 mode, flex take-off is not available (full-thrust take-off only); returning to EPR needs on-ground + failure cleared + ENG N1 MODE pb OFF. Reversion is to rated or degraded N1 mode depending on the failure.

5. Rating limits — the detent logic

The FADEC computes the thrust rating limit for each thrust lever position... If the thrust lever is set in a detent, the FADEC selects the rating limit corresponding to this detent. If the thrust lever is set between two detents, the FADEC will select the rating limit corresponding to the higher detent.

Per DSC-70-35-40, a lever in a detent gives that detent's rating limit; a lever between two detents gives the higher detent's limit.

[!warning]- Between two detents, the FADEC takes the higher detent's limit Not the lower, and not an interpolation — the higher detent (DSC-70-35-40). A lever not fully seated (between detents) is allowed up to the upper detent's rating — relevant to thrust-setting precision and overtemperature understanding.

The rating-limit geometry differs between "in flight or on ground with engine not running" and "on ground with engine running."

6. Manual vs autothrust

The engines are in the manual mode, provided the A/THR function is: ‐ Not armed, or ‐ Armed and inactive (thrust lever not in the A/THR operating range and no alpha floor). In manual mode, each FADEC computes the associated engine thrust based on the position of the corresponding thrust lever. The flight crew can move the thrust lever from the idle (0) detent to the TOGA detent. The position of the thrust lever corresponds to a specific percent of maximum thrust.

Per DSC-70-35-50, the engines are in manual mode when A/THR is not armed, or armed but inactive (lever outside the A/THR range, no alpha floor); then thrust follows lever position, each position being a specific percentage of maximum thrust. Under active A/THR (lever in the range, A/THR engaged) the FADEC trims thrust to the FMGS command within the lever's range.

7. Counterintuitive points

[!warning]- Three core points ① The levers move only manually — under A/THR they don't move, so lever position ≠ thrust. ② EPR loss reverts to N1 automatically, but flex take-off is unavailable in N1 mode. ③ Between two detents, the higher detent's rating limit applies.

Self-test

[!note]- Q1. What parameter does the Trent 700 normally control thrust on? EPR (engine pressure ratio) — the normal mode; the FADEC computes command EPR from TLA, altitude, Mach, air data and service bleed. N1 is the failure-reversion mode.

[!note]- Q2. When does it go to N1 mode, and at what cost? No EPR available → automatic reversion to N1 (rated or degraded), equivalent thrust held at reversion. Cost: flex take-off unavailable if dispatched in N1 mode; return to EPR needs on-ground + failure cleared + ENG N1 MODE pb OFF.

[!note]- Q3. What are the four thrust-lever detents? TOGA / FLX-MCT / CL / IDLE(0) — four detents/stops, three segments.

[!note]- Q4. Lever between two detents — which rating limit? The higher detent's rating limit.

[!note]- Q5. Manual vs A/THR, and the both-engine A/THR range? Manual when A/THR not armed, or armed-and-inactive (lever outside range, no alpha floor); thrust follows lever. Both-engine A/THR range IDLE → CL; single-engine extended to IDLE → MCT.

Key takeaways

References

• FCOM DSC-70-35-10 — FADEC controls thrust; manual (levers) / automatic (FMGS); ceiling held.
• FCOM DSC-70-35-20 — EPR normal mode; automatic N1 reversion (rated/degraded, flex unavailable, return conditions); reverse on N1.
• FCOM DSC-70-35-30 — levers manual only, 3 segments, 4 positions, position → rating limit + EPR TLA.
• FCOM DSC-70-35-40 — rating limit per detent; between two, the higher detent.
• FCOM DSC-70-35-50 — manual-mode definition; lever position → percent of maximum thrust.
• TLA-range and rating-limit diagrams (FCOM) — four detents, A/THR ranges.

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.