Aircraft Engine Instruments

Engine instruments give the pilot information about the health of the engine and how it is performing during the flight. This information is also required to keep the engine running within its operating limits so that it will remain reliable throughout its lifetime. Engine data monitoring and logging tools create a history of data in which trends can be detected helpful with preventive maintenance and keeping the engine running for years to come.

Some common instruments are: RPM, pressure and temperature gauges; fuel level and flow gauges are sometimes integrated in one unit. In multiengined aircraft there is usually a set of gauges available per engine.

Engine Monitoring

Accurate engine monitoring is very important to keep the engine running within its limits so as to ensure a safe flight. RPM, pressure and temperature limitations may not be exceeded at any time.

Tacho - RPM

Older aircraft have a mechanical cable driven tachometer, inside the instrument are a couple of flyweights to which a pointer is attached indicating RPM. Not very common in newer modern aircraft anymore. Later types have the cable attached to a magnet which rotates inside a aluminum cup. In this cup eddy currents (generated by the rotating magnet) create a magnetic field coinciding with the magnetic field from the magnet. The cup is fixed to a spring and pointer so it can not rotate but it is dragged along by the magnetic field, hence the name: dragcup.

Electrical tacho's have either a DC or AC generator on the engine. The DC generator generates a voltage related to RPM, which drops if the wires are too long. The AC types relate RPM to a frequency and these are immune to wire length or resistance and are more reliable than the DC models.

The latest modern engines have an electric pickup from either a separate coil on the engine (Rotax) or on the magneto. This signal is pulsed so that a digital counter can be used to indicate engine RPM.

Temperature measurement

These instruments use a thermocouple to measure temperatures. A thermocouple consists of two dissimilar metals welded together and they form the sensing probe, the hot junction. The cold junction is at the instrument (a millivolt meter). When the hot junction is heated a small voltage will exists between the hot and cold junction inside the instrument. The scale in calibrated in °C or °F.

Exhaust Gas Temperature (EGT)

Fitted on the hottest cylinder exhaust pipe of some carburetted engines and used for leaning the engine. If done properly, each cylinder should have an EGT indicator. FADEC controlled fuel injected engines can not live without this important measurement, since they regulate the amount of fuel in combination with RPM and MAP to obtain the best mixture for the power requirement without detonation in the cylinders.

Cylinder head Temperature (CHT)

Also a thermocouple probe fitted in the cylinder head. Each cylinder should have one, but its usually installed on the hottest cylinder (which ever that maybe, this doesn't so much depend on the position but on the mixture entering the cylinders). Made from iron or copper constantan (copper and nickel alloy) and able to measure up to 400°C.

Oil and Carburettor Temperature

This is not a thermocouple but either a bourdon tube (older type aircraft) or a variable resistance (ratio meter) type. The bourdon tube is a hollow flattened tube made of brass bend over a certain arc and fixed at one end in the instrument, the other end attached to a pointer. Any pressure applied in the tube will try to straighten it, moving the pointer.
A volatile chemical like methyl chloride is used as fluid in the probe and bourdon tube in a partly liquid / gaseous state. Its pressure will rise when the temperature of the probe rises, thereby moving the pointer.

The electrical resistance sensor changes its resistance related to temperature. The circuit is a ratio meter as these are not sensitive to voltage variations, they measure a current and this current changes only as the resistor senses a change in temperature.

Pressure Measurement

These are either made of a diaphragm or bourdon type sensors. This depends on the pressure they are working with, bourdon types are more suited to handle higher pressures. Reading is direct (with the pressure line running to the cockpit) or indirect with electrical wires and the sensor close to the engine, either on the engine or firewall.

Oil Pressure

The direct reading type is made of a bourdon tube and this is filled with a thin engine oil. The indirect type uses a resistance ratio meter and a firewall mounted sensor (sometimes on the engine, Rotax). This sensor is a diaphragm connected to a rheostat. The ratio meter can be a millivolt meter or a electromagnet type with two coils which attract the pointer and the rheostat is connected with a wire in the middle of the coils so that the magnetic field is regulated with the varying resistance of the rheostat.

Manifold Pressure (MAP)

Engines with a controllable propeller of some sort need the MAP gauge to set the correct power setting with the selected propeller RPM. It reduces fuel consumption and by having a slight oversquare condition, for example 2200 RPM at 25" MAP, engine efficiency is also greatly improved.

Usually an aneroid capsule or diaphragm connected to a pointer with the scale calibrated in pressure, inches of mercury (inHG) or bar. The instrument can be made to show either only boost (pressure above atmospheric) or manifold pressure, from 10 to 30" (inHG) and for turbocharged engines the scale goes higher, up 50" and more.