15.14 Turbine engine indication systems

Scope

A level 1, B1 level 2: EGT/ITT; thrust indication (EPR, turbine discharge or jet pipe pressure); oil pressure and temperature; fuel pressure and flow; engine speed; vibration measurement and indication; torque; power.

Primary and secondary parameters

Engine type	Primary (thrust/power setting)	Secondary
High-bypass turbofan	N1 or EPR, EGT, N2	Fuel flow, oil pressure/temperature/quantity, vibration
Turboprop	Torque, propeller rpm (Np), ITT/TGT, Ng	Fuel flow, oil
Turboshaft (helicopter)	Torque, TOT/ITT, Ng (gas generator), Nf/Nr	Fuel, oil, chip lights

Gas temperature (EGT, ITT, TGT, TOT)

• Measured with chromel–alumel thermocouples (type K).
• Several probes around the gas path, wired in parallel → indication is the average temperature; each probe may have several sensing junctions at different immersion depths.
• ITT is measured between HP and LP turbines (closer to the critical turbine entry temperature, which is too hot to measure directly); EGT after the last turbine.
• A thermocouple circuit is self-generating: the indicator needs no power for analogue gauges; modern systems digitise in the EEC/engine interface unit, with cold-junction compensation.
• Leads have a calibrated resistance — do not alter; check loop resistance and insulation; a high-resistance joint makes the indication read low (dangerous — engine may be over-temperature).

Worked example: Four probes read 640, 650, 655 and 675 °C. A parallel-connected harness gives approximately the average: (640 + 650 + 655 + 675) ÷ 4 = 655 °C. If one probe goes open circuit, the average of the remaining three (assuming the 675 °C probe fails) becomes 648 °C — a small change, which is why a single failure is hard to spot without individual-probe checks.

EGT limits: start limit, take-off limit (time-limited), maximum continuous. Exceedances are recorded and may require inspection (borescope) depending on magnitude and duration.