EPR & N1 Thrust Modes

Thrust cannot be measured directly — no thrust gauge is bolted to the engine. The Trent 700 chooses a measurable ratio that represents thrust:

"The engine thrust setting is made through control of the Engine Pressure Ratio (EPR). EPR = low pressure turbine exhaust pressure / Engine air inlet air pressure."

"Thus, EPR is directly related to engine thrust and as a result is used as the parameter for its control."

The intuition (synthesis): thrust is, at bottom, "how much the engine accelerates its airflow" — and the ratio of outlet pressure to inlet pressure captures exactly the head and tail of that process. A ratio has a beautiful built-in property: it self-corrects. On a hot day or at a high-elevation airport the inlet pressure falls, P50 falls in proportion, and the same EPR still means the same thing. But a ratio also has an Achilles heel — if the measurement at either end fails, the entire parameter is worthless. Hence the second half of this article: the N1 modes, the fallback from "the pressure ratio across the machine" to the humbler "how fast is the fan turning".

1. The measurement geography

        P20 (denominator)                         P50 (numerator)
   in the intake cowl, top right 15°       five probes in the exhaust-case
   P20/T20 probe                           vanes Nos. 3 · 5 · 7 · 9 · 11
        │ tubing (non-flowing)             (numbered from the top, viewed
        ▼                                   from the rear) — 5 holes per
   P20 accumulator (60 in³ smoothing)       leading edge → manifold
        └──────────────► EEC pressure transducers ◄──────────┘
                         (installed in channel B)
                          EPR = P50 / P20
                  + the DEP's EPR trim (irons out CNA and
                    rear-bearing individual tolerances)
                          ▼ digitised · ARINC
                    DMC → E/WD display

"The P50 sensor and manifold has five probes which measure the total pressure inlets of the exhaust gas stream. The probes are installed into the exhaust gas vanes at the N° 3, 5, 7, 9 and 11 positions (the vanes are numbered from the top viewed from the rear of the engine). Each of the five vanes has five holes in its leading edge."

Five probes, five holes each — twenty-five sampling points averaged around the annulus (synthesis). The exhaust stream is not uniform; a single-point pressure tap would flutter with the flow field, and only a circumferential average earns the title of "the thrust representative". The P20 end sits in the intake cowl at the top-right 15° position (article 04 covers the probe's own anti-icing, contamination slinging and accumulator smoothing). And one quiet single-point dependency is worth recording:

"The pressure transducer is installed in channel B."

The pressure-to-signal conversion for EPR is a single-element installation in channel B — a live instance of the single-sensing-element class from article 06: channel A receives its EPR data across the cross-channel link, and if that link fails, the frozen-last-value behaviour can apply to this very parameter.

One more piece of cockpit hardware belongs to this story: the ENG N1 MODE 1/2 pushbuttons on the overhead panel. Each button runs two discrete lines (HI/LO) separately into EEC channel A and channel B — one button, redundantly heard by both channels, for manually selecting the reversionary mode described below.

2. EPR mode — the normal state

"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."

The fifth input — service bleed — is the one most often read past: drawing bleed for packs or anti-ice taxes the compressor, so the same EPR corresponds to a different engine state, and the FADEC folds the bleed configuration into the commanded value. The note about reverse has its reason in the measurement geography (synthesis): with the reverser doors open, the redirected bypass flow scrambles the exhaust flow field, and P50 — the numerator's home — becomes untrustworthy. N1 is immune to the flow field, so reverse thrust is natively N1 territory (article 05 states the same rule from the control-law side).

3. Reversion to rated N1 — two triggers

"RATED N1 MODE — An automatic reversion to rated N1 mode occurs, when: ‐ Engine P2 (Engine Inlet Total Pressure) and/or P5 (Engine Low Pressure Turbine Outlet Pressure) are not available, or ‐ Engine P2 is lower than ADIRS Pt. The FADEC will compute an EPR COMMAND, depending on the TLA, then convert it into a N1 COMMAND as a function of Mach. The rated N1 mode can also be manually-selected through the ENG N1 MODE pb-sw on the overhead panel."

The two triggers correspond to two ways a ruler can break: one end of the ratio goes missing (P2 and/or P5 unavailable — no ratio can be formed), or the denominator drifts in the dangerous direction (engine P2 below the ADIRS total pressure → EPR computes high → phantom thrust; this is the FCOM face of the asymmetric P20 rule derived in article 05). The essence of rated N1: the EPR brain still works — only the output currency has changed. The FADEC still computes an EPR command from lever angle, then converts it to an N1 command with a Mach correction; ratings, FLEX and DERATE concepts all survive; you simply supervise N1 on the gauge. And the transition itself carries a considerate detail:

"At the reversion to N1 mode, an equivalent thrust to that achieved in EPR mode is provided, until a thrust lever position change."

Thrust does not step at the moment of reversion — the aircraft never lurches. Only after you next move the lever does the new mode's scale take over.

4. Reversion to degraded N1 — three "both-sides-gone" conditions

"DEGRADED N1 MODE — An automatic reversion to degraded N1 mode occurs, when: ‐ Engine P2 (Engine Inlet Total Pressure) and ADIRS 1 + 2 Pt are not available, or ‐ Engine T2 (Engine Inlet Total Temperature) and ADIRS 1 + 2 Tt are not available, or ‐ Engine P0 (Engine Inlet Static Pressure) and ADIRS 1 + 2 Ps are not available."

Note the shared sentence pattern in all three conditions: the engine's own parameter and both ADIRS copies of the same parameter, all gone. This is the terminal point of the complementary-sensor and air-data-voting design of articles 04/05: a vote needs at least one voter, and degraded mode is reached only when all three ballots of one parameter class are void. The world inside:

"The N1 is defined as a function of TLA and altitude and is limited by the FADEC to either the smaller of N1 max or N1 redline (if T2 is available), or N1 redline (if T2 is not available). The N1 DEGRADED MODE is an unrated N1 mode. The N1 rating limit, N1 TLA, and N1 max indications on ECAM E/WD are lost."

"Unrated", plus three indications vanishing from the E/WD — this is the cockpit sensation of the "bare ruler" from article 05: nobody is computing today's maximum permitted thrust for you any more. Full forward lever reaches the red line itself (or N1 max, if T2 survives to trim it back slightly). And the emergency-electrical destiny is restated at the FCOM level:

"In electrical emergency configuration, the EPR mode is lost on both engines, and each FADEC reverts to an degraded N1 mode."

5. What survives in N1 mode: A/THR, ALPHA FLOOR, FLEX

"Autothrust control disengages. However it can be re-engaged if no more than one engine is in degraded N1 mode. ALPHA FLOOR protection remains available if one engine is in degraded N1 mode." — "In case of dispatch in N1 mode, flex take-off is not available."

Three survival rules assemble into a capability ledger:

(The table integrates the three verbatim rules; the both-degraded row follows from the "no more than one" condition — synthesis, marked as such.)

6. The road home: EPR recovery — on the ground only

"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."

There is no way back in the air — even if the failure clears. Mode switching in flight would mean rewriting the thrust scale mid-task, and the designers preferred that you finish the sector on N1 (synthesis). This single ground-only condition is also the mechanical foundation of the ENG EPR MODE RECOVERABLE memo in article 21: the FADEC telling you "the failure is gone — recovery is available after landing."

7. What the cockpit shows when EPR fails — and one easily missed sentence

"If a failure occurs on any indication displayed, the indication is replaced by two amber crosses, the analog indication pointer and the marks on the circle disappear, and the circle becomes amber. ‐ The MASTER CAUTION light comes on accompanied by the single chime. ‐ The following message appears on the E/WD: ENG1(2) EPR MODE FAULT. ‐ Both engines are in the N1 Mode. The N1 LIMIT is displayed."

The last line is the one to underline: a single engine's EPR failure reverts both engines to N1 mode. Two engines on the same measuring stick is what preserves manageable thrust symmetry — the procedural consequences are developed in article 21, and the normal-display family of EPR symbols (ACTUAL / MAX / TREND / THROTTLE / LIMIT) belongs to article 15.

8. Where these modes meet operations

Self-test

[!note]- Q1. Where exactly are the numerator and denominator of EPR measured? Numerator P50: five probes in exhaust-case vanes 3/5/7/9/11 (numbered from the top, viewed from the rear), five leading-edge holes each — a 25-point circumferential average. Denominator P20: the P20/T20 probe in the intake cowl, top right 15°. The pressure transducers live in channel B.

[!note]- Q2. Engine P2 reads lower than the ADIRS total pressure. What happens, and why that direction? Automatic reversion to rated N1. A low P2 inflates the computed EPR — phantom thrust, the dangerous direction. A high P2 merely understates EPR (conservative) and does not trigger reversion — the same asymmetric philosophy as the air-data vote.

[!note]- Q3. Why is degraded N1 described as "both sides gone"? Each of the three trigger conditions requires the engine's own parameter and both ADIRS copies of the same class (Pt, Tt or Ps) to be unavailable together — the air-data vote has literally run out of voters.

[!note]- Q4. One engine is in degraded N1. Do you still have A/THR and ALPHA FLOOR? A/THR disengages but can be re-engaged (the condition is no more than one engine degraded); ALPHA FLOOR remains available with one engine degraded. With both engines degraded — the emergency-electrical case — both are gone.

[!note]- Q5. In cruise the EPR failure that caused reversion clears. Can you switch back? No. EPR recovery requires failure cleared + aircraft on the ground + N1 MODE pushbutton OFF — all three. You finish the sector on N1; after landing the RECOVERABLE memo confirms the way home is open.

Key takeaways

References

• FCOM DSC-70 (thrust control, general) — EPR definition and role.
• FCOM DSC-70 (thrust control modes) — EPR mode inputs, rated and degraded reversion conditions, equivalent-thrust transition, A/THR / ALPHA FLOOR / FLEX survival rules, emergency-electrical statement, EPR recovery logic.
• AMM 77-13 (EPR indicating, D/O) — thrust relation, P50 probe geometry, channel-B transducer, display failure effects and the both-engines-to-N1 rule.
• ASM (N1 MODE pushbutton schematic) — dual discrete lines per button into both EEC channels (direct reading).
• Integrative synthesis (marked in text): the ratio's self-correction and Achilles heel; the 25-point average reading; the reverse-flow-field explanation; the two ways a ruler breaks; the both-degraded table row; the finish-the-sector reading of ground-only recovery.

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.