Starting, Operating and Engine Instrumentation
Domain focus: This section covers the start sequence, normal operating checks and the instruments used to monitor the engine, including how to recognise abnormal starts.
Start sequence
- The starter motores the HP rotor; at a defined N2 the igniters energise and fuel is introduced. Light-off should occur within a few seconds, followed by acceleration to idle and a stabilisation check.
- Start parameters include N1, N2, EGT/ITT, fuel flow, oil pressure and start time. A hot start is a rapid EGT rise above limit; a hung start is stalled acceleration; a wet start is fuel without light-off; a false start is no light-off and no EGT rise.
Instrumentation
- N1 and N2 indicate rotor speeds; EGT/ITT indicates turbine temperature; fuel flow indicates fuel burn; torque or ITT may indicate power. Trend monitoring compares values over time to detect deterioration.
- Limits are published in the flight manual or maintenance manual. Exceedances require recording and inspection, even if the engine appears normal.
Worked example
On start, EGT rises quickly toward the limit while N2 stays below idle. The reasoning: fuel is entering faster than airflow is accelerating. Likely causes are an over-rich fuel schedule, a bleed valve stuck open, a weak starter or a restricted compressor. The correct action is to abort the start, close the fuel, continue motoring to purge, and investigate before another attempt. Continuing risks turbine overtemperature and damage.
Traps
Do not treat EGT as the same as turbine blade metal temperature; do not ignore start time limits; do not confuse N1 and N2; do not reset an exceedance without approval; do not continue a hot start to see if it recovers.