Engine Indication Systems
RPM and MAP, CHT and oil temperature/pressure, and using EGT to lean the mixture.
Engine indication, summary notes
- 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.
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.
Indication concept map
Engine Indication
Engine indication quiz
Engine Indication Systems, quiz
1. Cylinder head temperature is normally measured with:
A thermocouple at the hottest cylinderA pitot tubeA tachometer2. Manifold absolute pressure indicates:
The absolute pressure in the induction manifold, a measure of power being producedThe pressure of the oil delivered to the bearings, which rises and falls with the power the engine is makingThe back pressure in the exhaust system, which is the most direct measure of how hard the engine is working3. On a fixed-pitch propeller aircraft, power is set by:
RPM (the tachometer)MAP aloneEGT4. On a constant-speed propeller aircraft, power is set by:
MAP and RPM togetherRPM aloneOil temperature5. Exhaust gas temperature (EGT) is used primarily to:
Lean the mixtureSet the ignition timingMeasure oil level6. As the mixture is leaned, EGT:
Rises to a peak then fallsOnly fallsStays constant7. The single most urgent engine indication is:
Loss of oil pressureFuel colourCarb air temperature8. Cylinder head temperature (CHT) indicates:
Whether the engine is cooling adequatelyFuel quantityRPM