Turbine Engine Start Sequence

The order is the whole point. The starter drives the N2 spool up first, and only once there is enough airflow through the engine are ignition and fuel introduced. Light-up follows, EGT rises quickly to a peak and then falls back as the accelerating compressor pushes more air through and dilutes the flame. The starter stays engaged past light-up to help the engine accelerate, cuts out around 50% N2, and the engine then accelerates on its own to idle near 60%.

A hot start is fuel arriving with too little air behind it. Introduce fuel too early, or lose starter air pressure, or start on a very hot day, and the fuel to airflow ratio is too rich, so EGT overshoots the start limit. This is the classic damaging start: the turbine sees temperatures it was never designed for, and an exceedance means an inspection and often a rejected engine, not a note in the tech log.

A hung start is a different failure. The engine lights normally but then stagnates below idle, because the starter cannot supply enough torque to push it through the acceleration and the engine cannot yet make enough of its own. N2 sits stubbornly at some low value with EGT creeping up. It must be shut down, because sitting there is just slowly cooking the turbine. A wet start, sometimes called a no light-up, is when fuel is introduced but ignition fails: N2 keeps rising, fuel accumulates in the combustor and there is no EGT rise at all. The drill is to cut the fuel and motor the engine to blow the residual fuel out, because starting again on top of pooled fuel risks a torching start.

• Try this: Run a normal start and watch EGT. Why does it peak and then come back down without anything being changed?
• Try this: Move fuel introduction from 22% down to 10% N2. What happens to peak EGT, and why is airflow the thing that matters?
• Try this: Turn ignition off and start. What does EGT do, and why must the engine be motored before you try again?