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Aircraft Engine Tacho

Engine Instruments, Monitoring

The engine instruments supplies 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 the specified 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, this is helpful with preventive maintenance and keeping the engine running for years to come.

Some common instruments you will find in the cockpit are: RPM, pressure and temperature gauges; fuel level and flow gauges are sometimes integrated in one unit. In multi-engined aircraft there is usually a set of gauges available per engine stacked next to each other.

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. If you are able: keep a running log of engine parameters noting them on set intervals. Using a smart phone images can be taken quite simple.

Engine Monitoring

For the (co)pilot engine monitoring starts with by maintaining a regular watch over the engine RPM and temperature gauges during the course of the flight. Below we discuss how these instruments operate.

Tacho - RPM

Aircraft Engine Tacho

Older aircraft have a mechanical, cable driven tachometer. Inside the instrument are a couple of flyweights to which a pointer is attached indicating RPM. These are not very common in newer modern aircraft anymore. Later types have the cable attached to a magnet which rotates inside an aluminum cup.

Eddy currents

In this cup the 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: drag cup.

Electrical pickup

Electrical tachos have either a DC or AC generator installed in the engine. The DC generator generates a voltage related to RPM, which drops due to resistance if the wires are too long. The AC types relate RPM to a frequency and these are therefore immune to wire length or resistance and are more reliable than the DC models. The voltage will still drop but the frequency will remain the same.

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 is the sensing probe (or the hot junction) and is usually made from Chromel with Alumel alloys forming the well known K-type thermocouple. The cold junction is at the instrument (which is basically 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 of the instrument is calibrated in either °C or °F.

Extending the cable

Should you wish or need to extend the wiring of the thermocouple please use a wire with the same alloys! If you do not you will introduce another thermocouple at the junction creating a voltage and a misreading on the instrument.

Exhaust Gas Temperature (EGT)


Should be fitted on the hottest cylinder exhaust pipe of some carburetor engines and it is mainly used for leaning the engine. If done properly by the engine installer, 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 per cylinder in combination with RPM and MAP to obtain the best mixture for the power requirement without causing detonation in the cylinders.

Engine CHT Indicator

Cylinder head Temperature (CHT)

This is 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 more on the mixture entering the cylinders). Made from iron or copper constantan (consisting of 55 % copper and 45 % nickel) and its able to measure up to 400 °C.

Oil & Carburetor Temperature

This is usually not a thermocouple but either a bourdon tube (on older type aircraft) or a variable resistance (ratio meter) type. The Engine OIL Temp Indicator 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 is attached to a pointer. Any pressure applied in the tube will try to straighten it, thereby moving the pointer along a calibrated scale.

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.

Some carburetor temperature indicators are equipped with a red light showing the pilot when to apply carb heat, usually below 4°C. This is accompanied with a colored indication on the temperature scale.

Ratio meter

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 difference and this current changes only as the resistor senses a change in temperature.

Written by EAI.

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