Indicating and Monitoring Systems
Domain 8 - Indicating and Monitoring Systems
The exam tests sensor types, the indication chain, exceedance recording and trend monitoring.
Core rules
- Primary indications are N1 (fan or LP spool), N2 (HP spool), EGT or ITT, fuel flow, oil pressure and temperature, vibration, EPR or thrust, and torque on turboprop and turboshaft engines.
- Sensors include thermocouples for EGT and TGT, RTDs for oil temperature, magnetic or optical tachometers, pressure transducers and torque sensors. EGT uses several probes averaged; one failed probe gives a bias and often a disagree message.
- Signal chain: sensor, harness, engine interface unit, then EICAS or EFIS display. A fault can lie in any of these, so cross-check before condemning a component.
- Trend monitoring compares current values with a baseline at the same N1, altitude and temperature. Gradual change means deterioration; a step change means a fault or a maintenance event.
- Exceedance recording captures over-limit events for maintenance action. EGT margin is calculated at takeoff thrust against the flat-rated limit, corrected for bleeds and anti-ice.
Worked example
EGT reads 20 °C low on one engine at matched N1, while fuel flow and vibration are normal. A 20 °C step on one engine with unchanged performance is a probe or harness fault, not a real turbine improvement. Swap the probe to confirm before quoting an EGT margin.
Traps
- Using a single EGT probe value for a margin calculation.
- Treating a trend change as normal because the engine remains within limits.
- Dismissing a disagree message as a display glitch without checking the harness.