Sub-idle

[tubby linton]

A question for the powerplant experts. Can you explain the sub-idle condition for a jet engine that is not governed by a fadec? The engine is still running in flight but a section of it is below ground idle and this is causing the accesory gearbox to revolve slowly enough for items to trip off. .
Would advancing the thrust lever lead to an engine stalll or failureif there was a reduced fuel flow which is causing the sub-idle condition?

[lomapaseo]

Sub Idle is very bad in high performance engines.

Attempting to increase the turbine speed by adding fuel and pressure in the burner will likely drive the compressor into a non-recoverable stall and that's the end of any response your going to command until you turn off the flame and back pressure entirely.

If you've got the time to restart then it will probably run OK at least at flight idle.

[John Farley]

Tubby

You specifically refer to non FADEC engines.

Such hydromechanical fuel control systems are very complicated with multiple half ball valves. Each of these components can fail - often due to fuel contamination with dirt, swarf etc - and the result in each case will of course be totally type specific.

In my experience such problems can certainly lead to a rundown to a sub idle condition. Opening the throttle will have no effect unless the reason for the rundown coincidentally goes away. Equally a shutdown and relight will not help if the fault causing the rundown still exists.

JF

[tubby linton]

Many thanks to both of you. I cannot find a reference in the fcom which deals with this situation and unless a pilot was aware of the specific values of flight idle for each engine section it would be quite difficult to troubleshoot the problem.and recognise the sub-idle condition.

[rudderrudderrat]

Hi Tubby,
We had several sub idle events on early RB211s during cross bleed starts. If the TL of the supplying engine was not advanced far enough to ensure a health N3 then the HP bleed demand would drag the N3 below self sustaining rpm (about 60%). Once the rpm had decayed a bit further, then advancing the TL had no effect. The engine would continue to decay until flames could be observed from the tailpipe.

[flyboyike]

If there is a sub-idle, there better be a super-idle, too.

[Desert185]

A sub-idle stall while inflight can be caused by a rapid thrust lever movement towards idle. A sharp FE (if installed) will unload the engine(s) by opening bleeds. Sometimes that works. Normally, the engine must be shutdown with the fuel lever and restarted, which "resets" the fuel control (sometimes).

Slow thrust lever movement, particularly in the high-bypass engines is desired. A JT8 they are not.

[lomapaseo]

tubby

it would be quite difficult to troubleshoot the problem.and recognise the sub-idle condition.

Putting the cart before the host in the quote above, recognizing the problem is easy, the engine won't respond to the throttle and the generator trips off.

Troubleshooting is to confirm no throttle response while EGT is still lit. Corrective action is as in posts above, if you have the time and altitude.

[tubby linton]

Lomapaseo the problem was a fuel supply problem which does not appear in the ecam or paper checklist. The aircaft architecture allows for gravity feeding from the affected tank but in this case there was a supply problem. Some fuel must have been getting through but not enough to maintain an idle condition hence the loss of the generator .
The most obvious parameter would have been the reduced fuel flow.