Section 15.14 / 15.15

Engine Indication & Power Augmentation

EPR/N1 thrust setting, N1/N2 and EGT indication, plus water injection and afterburning.

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Main ideas

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.

Key formulas

Solved examples

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.

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