Aircraft Tachometers: Mechanical, Electrical, and Electronic RPM Sensing

022-01-05Pablo Asensio Martínez2026-03-271 min

Tachometers measure the rotational speed of the engine, expressed in Revolutions Per Minute (RPM).

Types of Tachometers

1. Mechanical Tachometer

Used on older piston aircraft.

  • Principle: A flexible driveshaft connects the engine gearbox to the instrument. Inside, a rotating permanent magnet induces eddy currents in an aluminum drag cup, creating a magnetic field that rotates the cup and moves the pointer against a spring.
  • Features: Self-powered (no electricity needed). Limited by driveshaft length (max ~2m).

2. Electrical Tachometer

Used on some jet aircraft (e.g., for High Pressure compressor speed).

  • Principle: A three-phase AC tacho-generator on the engine gearbox sends a signal to a synchronous motor in the indicator. This motor spins a magnet/drag cup assembly similar to the mechanical type.
  • Features: Self-powered signal. AC frequency varies with RPM.

3. Electronic Tachometer

Used on modern gas turbines (multi-spool) and EFIS systems.

  • Principle: Uses a speed probe (coil + magnet) or induction probe.
    • Pulse Counting: The probe is placed near the engine fan blades or a notched "phonic wheel". As metal passes the probe, it disturbs the magnetic field, generating electrical pulses.
    • Calculation: The pulse rate is directly proportional to the spool speed.
  • Features: Requires an external power supply. precise.

Note: While all utilize magnetism, electronic systems use it to generate pulses for counting, whereas mechanical/electrical types use it to create torque (drag) to move a pointer.