Gas turbine engine start malfunction - failure to stabilise at idle

[mnjmc]

Just trying to find some info on the why a gas turbine engine might fail to stabilise at idle on start-up, and what actions a pilot should take if it occurs. Thanks in advance!

[AmarokGTI]

Grr Prune lost my reply.

I’ll go again quickly.

Could be starter staying in starter mode and not automatically switching to generator mode. Can potentially remedy by switching the ignition off momentarily, depending on type. If that fixes it consult MEL. If not, shut down.

Turbine engines expensive tho - if in doubt shutting it down is safe option.

[tdracer]

Just trying to find some info on the why a gas turbine engine might fail to stabilise at idle on start-up, and what actions a pilot should take if it occurs. Thanks in advance!

What exactly do you mean by "fail to stabilize at idle"? Doesn't reach idle, keeps accelerating past idle, rpm starts oscillating around idle speed?
There are a number of ways a start can go wrong - hung start (stops accelerating short of idle), hot start (exceeds start EGT), etc.
In most cases, autostart will take corrective action (assuming the engine is so equipped), if not the normal procedure is to cut fuel, dry motor for a period (typically 30 seconds), then try again (although there are some engine specific techniques that can come into play).
There have been cases where a crew started an engine with the throttle well forward of idle - which caused the engine to keep accelerating above idle. The obvious corrective action is to retard the throttle to idle.

[mnjmc]

What exactly do you mean by "fail to stabilize at idle"? Doesn't reach idle, keeps accelerating past idle, rpm starts oscillating around idle speed?
There are a number of ways a start can go wrong - hung start (stops accelerating short of idle), hot start (exceeds start EGT), etc.
In most cases, autostart will take corrective action (assuming the engine is so equipped), if not the normal procedure is to cut fuel, dry motor for a period (typically 30 seconds), then try again (although there are some engine specific techniques that can come into play).
There have been cases where a crew started an engine with the throttle well forward of idle - which caused the engine to keep accelerating above idle. The obvious corrective action is to retard the throttle to idle. So basically I'm just putting together some notes for KDRs from my ATPL AASA exam. One of the things it is asking about is the cause/remedy of certain start-up malfunctions: hot start, hung start, failure to light off, starter fails to disengage and, lastly, fails to stabilise at idle. I got all of them covered but the "fails to stabilise at idle". Perhaps the reason I can't find any info on it is because it is so vague.

[B2N2]

Just trying to find some info on the why a gas turbine engine might fail to stabilise at idle on start-up, and what actions a pilot should take if it occurs. Thanks in advance!

Why? Many reasons:

Excessive tailwind
Excessive crosswind
Insufficient starter speed (duct air pressure low, bad APU, bad starter cart, too many packs on)
Starter valve cutout too soon
Too much/ too little fuel
Engine contamination (ice)
Engine damage

I’m sure I’ve forgotten another half dozen or so.
Usually the same or similar pilot action:
Fuel OFF and motor to get rid of any excess heat.

[mnjmc]

Thanks for your help guys! Much appreciated!