Engine and System Indication Instruments
Tachometer and manifold pressure
The tachometer measures engine or propeller RPM, typically via a mechanical drive or an electrical/electronic sender. The manifold pressure gauge measures absolute pressure in the induction system (inches of mercury or hPa) and is a direct indicator of power for a fixed-pitch or constant-speed piston engine. With the engine stopped, manifold pressure equals ambient pressure; with the engine idling it is low. A rough rule is that manifold pressure should not exceed the RPM/100 figure (e.g. 24 inHg at 2400 RPM) to avoid over-boosting.
Temperature and pressure
- Oil temperature and pressure: oil pressure low or temperature high indicates a serious problem; know the limits and the immediate actions.
- Cylinder head temperature (CHT): high CHT in climb indicates insufficient cooling or too lean a mixture.
- Exhaust gas temperature (EGT): used for mixture leaning; peak EGT is richest best power.
- Fuel pressure/flow: pressure gauges indicate pump and line health; flow gauges indicate actual consumption.
- Fuel quantity: float, capacitance or electronic sensors; all can stick or read incorrectly in uncoordinated flight or with a failed sender.
Traps
Distinguish indicated from actual values. A fuel quantity gauge is not a dipstick check; always verify with a physical check before flight. A manifold pressure gauge reads ambient with the engine off - a common question. A tachometer failure may be a sender or wiring failure rather than an engine failure; cross-check with engine sound and performance.
Worked rule of thumb
For a normally aspirated piston engine, full throttle at sea level gives approximately 29–30 inHg; expect a loss of about 1 inHg per 1000 ft of altitude. At 8000 ft, full throttle gives roughly 21–22 inHg. If the gauge shows less than expected at full throttle, suspect a blockage, leak or gauge error.