Engine Indication & Power Augmentation
EPR/N1 thrust setting, N1/N2 and EGT indication, plus water injection and afterburning.
Indication & power augmentation, summary notes
- Engine instruments let the crew set power and stay within limits: thrust is set by EPR (engine pressure ratio, turbine-exit ÷ inlet pressure) or by N1 (fan speed) depending on the engine; both are used as the thrust-setting parameter.
- Rotational speeds N1/N2(/N3) are shown as a percentage of a datum RPM, not actual rev/min. EGT/ITT/TGT indicates turbine temperature and is a critical limit, especially during start and at take-off power.
- Other indications: fuel flow, oil pressure and temperature, and engine vibration (N1/N2 tracked for imbalance and bearing health). Amber/red limit markings and exceedance recording support trend monitoring.
- Thermocouples (chromel-alumel) measure EGT as an averaged ring of probes; their cold-junction and harness resistance must be correct or the reading errs.
- Power augmentation temporarily boosts thrust: water/water-methanol injection restores take-off thrust lost on hot/high days by cooling and adding mass to the airflow; afterburning (reheat) injects and burns extra fuel in the jet pipe for a large thrust increase at a steep fuel-consumption penalty.
- ⚠ Exam trap: N1/N2 are shown as a PERCENTAGE of a reference RPM (100% is not the physical maximum), and EPR/N1, not throttle angle, is the thrust-setting parameter; afterburning greatly raises thrust but at a heavy SFC penalty.
- EPR
- EPR = P(turbine exit) / P(engine inlet)
Why might water injection be used on take-off from a hot, high-altitude airport?
On a hot, high day the air is thin, so mass flow and thrust drop. Injecting water (or water-methanol) into the compressor or combustor cools the air and adds mass to the flow, restoring the take-off thrust that would otherwise be lost, a short-duration boost used only when needed because of the water supply and thermal-shock considerations.
Indication & augmentation concept map
Indication & Power Augmentation
Indication & power augmentation quiz
Engine Indication & Power Augmentation, quiz
1. An engine is running at 8400 rev/min against a 100% datum of 12 000 rev/min. Its N1 indication is:
70%143%42%2. An engine has a turbine discharge pressure of 210 kPa and a compressor inlet pressure of 100 kPa. The engine pressure ratio (EPR) is:
2.11100.483. EPR (engine pressure ratio) is:
Turbine-exit pressure ÷ engine-inlet pressureFuel ÷ air ratioOil ÷ fuel pressure4. N1 and N2 are displayed as:
A percentage of a datum RPMActual rev/minA temperature5. EGT is measured by:
Thermocouples (chromel-alumel) averaged around a ringA pressure sensor in the jet pipe, since gas temperature and pressure vary togetherA tachometer on the turbine shaft, the temperature being inferred from the rotational speed6. The thrust-setting parameter is usually:
EPR or N1, not throttle angleOil pressureFuel temperature7. Water/water-methanol injection is used to:
Restore take-off thrust on hot/high daysCool the cabinLubricate the bearings8. Afterburning (reheat) increases thrust by:
Burning extra fuel in the jet pipeSlowing the turbineClosing the nozzle