Section 16.10

Engine Indication Systems

RPM and MAP, CHT and oil temperature/pressure, and using EGT to lean the mixture.

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Main ideas

The tachometer shows engine RPM; on a fixed-pitch propeller RPM directly sets power, while on a constant-speed propeller the manifold pressure (MAP) gauge and RPM are set together.
Cylinder head temperature (CHT) and oil temperature guard against overheating; oil PRESSURE is the single most important indication — a loss of oil pressure demands immediate action.
Exhaust gas temperature (EGT) is used to LEAN the mixture: leaning raises EGT to a peak (chemically correct mixture) then it falls; pilots lean to a set number of degrees rich- or lean-of-peak per the POH.
Other indications include fuel pressure/flow and carburettor air temperature (to detect the icing range); markings (green/amber/red arcs) define normal, caution and limit ranges.
On a constant-speed prop, avoid unapproved 'oversquare' settings (high MAP with low RPM) outside the POH, which can over-stress the engine.
⚠ Exam trap: EGT is used to set the MIXTURE (it peaks at the chemically correct ratio, then falls either side); oil PRESSURE loss is the most urgent engine indication; on a constant-speed prop, power = MAP + RPM, not RPM alone.

Solved examples

As a pilot slowly leans the mixture in the cruise, the EGT rises to a maximum and then begins to fall. What does the peak represent?
The EGT peak marks the chemically correct (best-economy) mixture, where combustion is most complete and hottest. Leaning further past the peak makes the mixture too lean and EGT falls (with reduced power); enriching back before the peak gives a cooler, richer 'best-power' mixture. Pilots set a specified number of degrees rich- or lean-of-peak per the POH.

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