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PPL » Aircraft Tech Knowledge (A) » Engine Instruments

12.34 Engine Instruments

12.34.2 Describe the function and principle of operation of the following instruments:

(a) tachometers (rpm) gauges (centrifugal and drag cup);

  • The measurement of engine speed is of considerable importance, since together with such parameters as manifold pressure, torque pressure and exhaust gas temperature, it permits an accurate control over the performance of the engine to be maintained.


  • With reciprocating engines the speed measured is that of the crankshaft, serving as a usefull indication of the thrust being produced.


  • The indicating instruments are normally referred to as tachometers.


Click here: Centrifugal Tacho


Centrifugal Tachometer


Tachometers (rpm) gauges measure the rotational speed of an aircraft's engine in revolutions per minute (RPM). Centrifugal tachometers measure RPM by using centrifugal force. The instrument has a rotating shaft that is driven by the engine. As the shaft rotates, it causes the instrument's pointer to move, indicating the engine's RPM. Drag cup tachometers measure RPM by using air resistance. The instrument has a rotating shaft with a metal cup attached to it. As the shaft rotates, air flows over the cup, creating a drag force that resists the rotation of the shaft. The amount of drag is proportional to the engine's RPM, and the instrument's pointer indicates the RPM value. Both types of tachometers are important for monitoring engine performance and ensuring that the engine is operating within the manufacturer's recommended RPM range.

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