Section 14.2

Engine Indicating Systems

Speed (N1/N2), temperature (EGT), thrust (EPR), fuel flow, oil and vibration sensing, and EICAS/ECAM display.

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Main ideas

Engine SPEED (N1/N2) is sensed by a tacho-generator or, more commonly, a phonic-wheel/variable-reluctance speed probe: a toothed wheel passing a magnetic pickup generates a frequency proportional to RPM. It needs no external power — it makes its own signal — and is shown as a PERCENTAGE of a datum RPM.
Gas TEMPERATURE (EGT/ITT/TGT) is measured by a ring of chromel-alumel thermocouples wired in PARALLEL so the display reads the average of several probes; accuracy depends on cold-junction compensation and the correct harness/lead resistance.
THRUST is indicated by engine pressure ratio (EPR = turbine-exit ÷ engine-inlet pressure) from pressure probes, or by N1 (fan speed) on N1-rated engines — this is the thrust-setting parameter, not throttle angle.
FUEL FLOW is measured by a flowmeter (motor-driven or mass-flow type); OIL pressure and temperature and engine VIBRATION (accelerometers, tracked at N1 and N2) complete the health picture.
On modern aircraft these feed a centralised EICAS/ECAM display with green/amber/red limit markings, automatic exceedance recording and warnings — the indicating channel is a core B2 responsibility.
⚠ Exam trap: EGT thermocouples are chromel-alumel wired in PARALLEL to average several probes; a variable-reluctance/phonic-wheel speed probe generates its own signal (no external supply needed), and N1/N2 are shown as a PERCENTAGE of a reference RPM.

Key formulas

Solved examples

Why are the several EGT thermocouples around the turbine connected in parallel rather than in series?
Connecting the thermocouples in parallel makes the indicated voltage the AVERAGE of all the probes' outputs, giving a representative mean gas temperature around the ring rather than a single local hot spot. It also provides redundancy — if one probe fails open, the others still give a usable (slightly biased) average, whereas a series string would be broken by a single open probe.

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