14.2 Electric/electronic engine indication systems

Scope

Level 2 for B2/B2L: EGT/ITT; CHT, coolant temperature, engine speed; thrust indication (EPR, turbine discharge / jet pipe pressure); vibration; oil pressure and temperature; fuel pressure, temperature and flow; manifold pressure; torque. This is the most "avionic" part of Module 14.

Temperature indication

EGT / ITT / TGT

• Measured by thermocouples — chromel–alumel (type K) for turbine gas temperatures.
• Several probes are installed around the exhaust or between turbine stages and connected in parallel, so the indication is the average temperature; a single failed probe has little effect.
• A thermocouple's voltage depends on the difference between the hot junction and the cold (reference) junction, so the indicator or signal conditioner includes cold-junction compensation.
• Leads are of thermocouple material and calibrated resistance — never replace with copper wire or shorten them.
• Names: EGT (exhaust gas temperature), ITT (inter-turbine temperature), TGT (turbine gas temperature), TOT (turbine outlet temperature).

CHT, oil and coolant temperature

• CHT (piston engines): thermocouple (often iron–constantan) under a spark plug or in a bayonet well in the hottest cylinder.
• Oil, fuel, coolant, carburettor air temperatures: resistance temperature bulbs (RTD, nickel/platinum) read by Wheatstone bridge or ratiometer indicators, or by an engine interface unit.

Engine speed

Sensor	Principle
Tachometer generator	Engine-driven small three-phase AC generator; frequency proportional to speed drives a synchronous motor in the indicator (drag-cup) — indication independent of voltage
Magnetic (variable-reluctance) pickup	Senses passing gear teeth or fan blades; pulse frequency ∝ speed; no drive needed. Common for N1 (fan blades) and N2
Hall-effect / optical sensors	Digital pulse output
Magneto P-lead / electronic	Piston engine tachometers on light aircraft

Turbine speeds are shown in % rpm (N1, N2, N3); piston and propeller speeds in rpm.