engine monitoring instruments

The present day piston engine instruments used in the typical general aviation airplane are not precision laboratory instruments.

Nevertheless, the purpose of this brief presentation is a practical approach to interpreting the readings of your engine instruments in accomplishing a safe and efficient flight. If, for example, you were to observe an irregular reading of one engine instrument, it calls for a cross-check on all other instruments, and not relying on one instrument as a basis for a decision affecting flight.

Since the engine is dependent on fuel, the gasoline gauge is considered a related engine instrument. If pilots are going to attempt to stretch their flight range close to limits, they should be aware of the errors in the gages vs. the actual usable fuel. Some modern single engine aircraft have had the fuel gauge show several gallons remaining, when in reality the tank was empty. Others have indicated a specific number of gallons when filled, but actually the tank held several gallons less than indicated.

Therefore, in planning for each flight, remember that general aviation engine instruments are not precision laboratory types, so cross-check, and give yourself an extra margin for safety.

More aircraft are being fitted with digital readouts. Some digital equipment can automatically monitor the engine functions and will sound an alarm if anything goes outside preset limits. Digital instruments can be harder and slower to read than the traditional analogue type as the brain must take additional steps before interpreting them. An instrument reading of whatever type is only as good as its sender unit.

The instruments of all aircraft operate within a 'normal' range and the operator rapidly becomes adjusted to seeing these. The pilot should be vigilant that engine functions remain within these ranges. An abnormal indication showing on any instrument is immediate cause for concern.


The ammeter indicates whether the engine alternator and circuit are operational. If a continuous discharge is shown during normal flight, it is prudent to land as soon as possible as it would suggest that the battery will shortly become too depleted to operate normal aircraft systems. Remember that aircraft engines will continue to operate as their ignition spark is created by magnetos which are independent of the aircraft electrical system.

Aircraft are usually also fitted with a low voltage warning light which is placed in a prominent position. If a problem arises with the charging circuit, it will begin to flicker and then show red. The light may quite normally flicker or show dull red at very low RPM.

cylinder temperature gauge (CHT)

Most engines are fitted with a CHT on one cylinder only. Every engine design has a cylinder that runs slightly hotter than the rest. If the temperature climbs towards the red line it may indicate a serious problem.

exhaust gas temperature gauge (EGT)

The EGT is the primary instrument to help the regulation of the fuel/air mixture. The red cursor indicates temperature never exceed.

oil pressure

This is the primary indicator that all is well with the engine. A cold engine will always show a very high oil pressure. Once hot, the pressure should remain in the green arc under normal RPMs. A low pressure may indicate a serious problem with the oil pump or engine bearings. The acceptable ranges of pressure are stated in the aircraft operating manual. The oil pressure will diminish if the engine becomes overheated as the viscosity of the oil becomes too thin and begins to break down.

oil temperature

The oil temperature gauge is also very a important device for monitoring the wellbeing of the engine. High power should not be used until the temperature has climbed into the operating range as damage can occur to the engine. If the temperature climbs into the red sector, it indicates that a serious problem may have developed.

vacuum gauge

This is usually a very instrument situated at the lower left of the panel. In monitors the condition of the vacuum system which is driven from the engine. The vacuum system drives the gyros for the attitude indicator and direction indicator.

Fuel flow

This instrument measures (usually in US gallons/hours) the fuel flow to the engine. This instrument illustrated above also combines the manifold pressure gauge. The manifold pressure is used on complex aircraft and monitors the power setting of the engine.

Fuel gauge

Notorious for their unreliability are aircraft fuel gauges. Calculate fuel used and if possible never fly unless you can actually see the fuel through the filler cap!

combination instruments

Aircraft instruments come in a profusion of designs and combinations but all fulfil the same common purpose; to give as much notice to the pilot as possible that something is going wrong.