AS9120B, ISO 9001:2015, and FAA AC 0056B ACCREDITED

The most important Aircraft Piston Engine Instruments

When rookie pilots commence their aviation journey, they usually start with piloting a Cessna 172. Most ab-initio trainers like the Cessna 172 have fixed pitch propellers where the engine speed is controlled by the throttle lever. Advancing the throttle will increase the speed of the engine and propeller, whereas retracting the throttle causes the engine and propeller to slow. Generally, the engine speed is monitored through the tachometer, while the health of the engine is monitored by the oil temperature and pressure, the cylinder head temperature, and the exhaust gas temperature. Below, we will outline some of the most common engine instruments found in light piston aircraft and common faults that pilots encounter during and after start up.


A tachometer measures the speed of the engine in revolutions per minute (rpm) and is color-coded to indicate the operating limitations of the engine. The normal operating limit is always colored green, while maximum operating speeds are colored red. A yellow band may be featured on some instruments, which indicates a precautionary range. Here, the engine should only be operated for a short period before resuming operation in the green band.

Manifold Pressure

The manifold pressure gauge provides a measure of the absolute pressure of the fuel-air mixture within the intake manifold of the engine, and it uses this measurement to provide an indication of the power being produced. Moreover, the amount of power an engine produces is directly connected to the mass flow rate of the fuel-air mixture being sucked into the cylinders. Usually calibrated in inches of mercury, the manifold pressure gauge always reads a value less than the atmospheric pressure as the engine draws in air and fuel into the cylinders during operation. In light aircraft, fuel in the tanks is at atmospheric pressure, so in order for fuel to make its way into the engine, it must be sucked in through the fuel lines and into the carburetor or injectors.

Oil Pressure and Temperature

Sufficient engine lubrication is achieved by an oil system within the aircraft that consists of a storage of oil in the base of the engine or at the exterior of the engine. An oil pressure gauge measures the pressure of the oil at some point in the system upstream of the oil pump, and provides a way of gauging whether the oil pump is working properly. The oil is pumped through the engine under pressure in order to lubricate the engine’s moving parts, and the failure of this lubrication system can lead to complete engine failure. For this reason, it is vital to monitor the temperature and the pressure of the lubrication system throughout any flight.

Cylinder Head Temperature

Temperature variations within the engine are measured by a thermocouple situated in the hottest cylinder head and are displayed on the Cylinder Head Temperature (CHT) gauge in the cockpit. This gauge provides a direct indication of the temperature inside the engine, which must remain within a particular band during all stages of flight. In general, most light aircraft engines are air cooled, and the effectiveness of the cooling arrangement in a particular aircraft is monitored by referring to the CHT gauge.

Fuel Pressure

Fuel storage tanks are usually in the wings and use either a pumping or gravity-fed mechanism to supply fuel to the engine during operation. In most aircraft, there is a backup pump system present in the event that the primary pump fails. So, if the fuel pressure drops, the backup pump can be used to increase the pressure. It is critical that the pilot monitors the fuel pressure during flight and keeps it within the normal operating range at all times.

Exhaust Gas Temperature

The temperature of the gas leaving through the exhaust manifold after combustion provides a means for the pilot to adjust the fuel-air ratio to an optimum setting. More than that, it varies according to the ratio of the fuel-air entering the cylinders, and this is particularly true when the mixture is leaned such that the exhaust gas temperature (EGT) increases to a maximum before a sudden drop-off. During cruise speeds, the mixture is leaned using the EGT gauge by reducing the mixture to a point where the EGT peaks. With a more efficient combustion process, the EGT is hotter when dealing with leaner mixtures, and enriching the mixture will reduce the temperature according to specifications detailed in the aircraft flight manual.

Common Faults During an Engine Start

Starting an engine properly has a major impact on the aircraft’s overall operation, and some of the most common faults to look for include:

  • Improperly priming an engine before a start
  • Pumping the throttle prior to, or during a start, which can lead to the accumulation of fuel in the venturi
  • Flooding the engine with too much fuel
  • Because there is a thin film of accumulated oil on the cylinder walls when starting an engine that has been out of operation for a while, this results in a hammer-like effect on the piston which can bend the connecting rod or rupture a cylinder head

Common Faults After Engine Start

Once the engine has been started, there are a few common faults to look for:

  • Not checking the oil pressure within 30 seconds of start to ensure it rises
  • Failing to look at the color of the smoke seen in the exhaust gasses as an indication of a problem, where blue smoke is caused by oil seeping into the combustion chamber and black smoke means that the mixture is too rich or excess unburnt fuel is making its way into the exhaust
  • Rough running due to fuel starvation, blockages between the fuel tank and the intake of the engine, or the fuel pump not providing adequate pressure
  • Magneto faults
  • Excessive carbon build up at the firing end of the spark plugs
  • Carburetor icing


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