Engine Indicating Systems
Speed (N1/N2), temperature (EGT), thrust (EPR), fuel flow, oil and vibration sensing, and EICAS/ECAM display.
Engine indicating systems, summary notes
- 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.
- EPR
- EPR = P(turbine exit) / P(engine inlet)
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.
Engine indicating concept map
Engine Indicating Systems
Engine indicating systems quiz
Engine Indicating Systems, quiz
1. EGT is measured by:
Chromel-alumel thermocouplesA pressure probeA tacho-generator2. The EGT thermocouples around the turbine are wired:
In parallel, to average several probesIn seriesIndividually to separate gauges3. A variable-reluctance (phonic-wheel) speed probe:
Generates its own signal, needs no external powerRequires a 28 V supply to workMeasures temperature4. N1 and N2 are displayed as:
A percentage of a datum RPMActual rev/minA temperature5. EPR is the ratio of:
Turbine-exit pressure to engine-inlet pressureFuel to airOil to fuel pressure6. Engine vibration is sensed by:
Accelerometers, tracked at N1 and N2ThermocouplesPitot probes7. Accurate EGT indication depends on correct:
Cold-junction compensation and harness/lead resistanceThe grade of fuel in use, since the combustion temperature varies with its energy contentThe cabin altitude, which sets the reference pressure the indicating system works against8. Fuel flow is measured by a:
Flowmeter (motor-driven or mass-flow type)ThermocoupleTachometer