Engine Instrumentation — Page 524, Lesson 613

Let's start with engine rpm measurement. This is the heartbeat of engine monitoring — on a piston engine we measure crankshaft speed, and on a gas turbine we measure compressor speed. The indicator is called a Tachometer, or Tacho for short. There are three basic methods: the Mechanical (Magnetic) Tachometer, the Electrical Generator System — often called the Tacho Genny — and the Inductive Probe System. There are no firm rules about which one to use; engine and aircraft design dictate the best fit. Let me walk you through the Mechanical Tachometer first, since it's the oldest. It's now only found on older piston aircraft. It uses a Flexible Drive Shaft connected to the flight deck Tacho-indicator. The input drive spins a magnet inside the indicator. That magnet rotates inside a copper or aluminium drag-cup, and this induces Eddy Currents in the cup. Those currents oppose the magnetic field of the magnet, creating a torque that turns the drag-cup in the same direction as the permanent magnet. A shaft from the drag-cup moves the pointer. The pointer's motion works against a Hairspring, which controls the drag-cup position and therefore the pointer position. The flexible drive runs at reduced speed, but the indicator shows true speed. It also has compensation devices for temperature changes. So the key idea: magnetic coupling converts engine rotation into pointer movement, with a hairspring providing the restoring force. That's the mechanical tacho. Next we'll look at the electrical generator system and the inductive probe.