Hung Start

[Daz801-82]

Could anybody out there, please explain to me what is meant by a hung start, is it when the core won`t accelerate and it overtemps?

Cheers Daz

[Firebird]

A hung start is when fuel is applied and exhaust temp is noted but the engine will not accelerate (exhaust gas temp stays low). A hot start is a rapid exhaust gas temp rise (heading for the red line) and usually a higher than normal fuel flow.
Hope that helps.

[Black Labradors MaTE]

As Firbird correctly states 'A hung start is when fuel is applied and exhaust temp is noted but the engine will not accelerate.'
But a bit deeper - it is usually when full ignition of the fuel in the combustion chamber does not take place, normally attributed to an inop ignitor or HEIU (there are two per engine). As all the fuel is not ignited the turbines are slower to rotate which in order is slow to rotate the compressors (HP & (IP) & LP). This manifests its self as low pick up on the N1 and N2 indications in the flight deck. The engine will take longer than normal to reach self sustaining speed - if too long the engine may self abort the start but hopefully the operator will notice it and terminate the start. The starter valve will only rotate the HPC to a nominal percent (usually 40-45%) and this is usually the RPM at which the engine 'hangs' Allowing the hung start to go its distance is bad practice as once unburnt fuel is ignited it can torch back to the NGVs/ HPT causing damage.
A hung start can also be attributed to late opening of the throttle control valve which would be shown as no initial ignition followed by a sharp rise in EGT/JPT/TET.

[Tinwacker]

it is usually when full ignition of the fuel in the combustion chamber does not take place, normally attributed to an inop ignitor or HEIU (there are two per engine).

Since when would there be a partial light off?
Combuster interconnectors or Can Annualar arrangements ensure full light off unless a rare blocked fuel nozzle.

Only one ignitor will be used during the start sequence unless a second start is being attempted.
If the first ignitor used is inop then the engine will remain at Max motoring with a lot of fuel mist seen at the exhaust.

As all the fuel is not ignited the turbines are slower to rotate which in order is slow to rotate the compressors (HP & (IP) & LP).

Really. If fuel is lit then fuel is lit..

The engine will take longer than normal to reach self sustaining speed

My experience you would be waiting all day watching a 40% rpm (Hung start).
If you were to let it....

The starter valve will only rotate the HPC to a nominal percent (usually 40-45%)

The starter motor will turn the HPC via the gearbox, start valve opens to allow air to the air driven starter.

once unburnt fuel is ignited it can torch back to the NGVs/ HPT causing damage.

The NGV's and HPT will take a great deal of temp punishment. You are describing late fuel light off and then monitoring the EGT as routine would be the order. If EGT seen approaching the start limit the fuel would be cut-off and engine continued to be motored to clear all unused fuel prior to a second attempt then using both ignitors.

A hung start can also be attributed to late opening of the throttle control valve which would be shown as no initial ignition followed by a sharp rise in EGT/JPT/TET.

Again Max motoring with ignition 'ON' followed by a sudden but controlled EGT rise due late fuel only.

Confusion with Max motoring with no fuel and/or ignition approx 25-30% and Hung start where fuel has been ignited but HPC slows at approx 40% due to insufficient fuel flow or bleed schedule problem.

Sorry just had a bad day

TW

[Daz801-82]

Thanks very much for the information chaps, I just wondered as I`ve been overhauling RB 211 04 modules for the past 20 yrs, and I`ve never thought to ask the question.


Cheers Daz

[GotTheTshirt]

TW

Since when would there be a partial light off?
Combuster interconnectors or Can Annualar arrangements ensure full light off unless a rare blocked fuel nozzle.

Very true when new :ok:
However the combustor and liner have a very hard life and when they erode you do not get the situation as advertised. Several AD to rework and replace the FCL ( Including one last month !) due to - wait for it !! a batch were made forn the wrong material !!:confused:

The accel shedule is very critical on start and as engine and FCU get time in sevice some of that margin decreases. A tail wind is also not helpful
for clean starts

[muduckace]

The OP had the most important part of the answer in his question. The most basic answer is when the engine is unable to advance from the point of starter assist to idle as the "cores" fail to accelerate each other.

Could be a result of bleed loss, a rich or lean mixture that will not atomize well (rich could be early starter cutout) (lean could be a fuel metering issue). There are also variable stators in the compression section that could be adjusted incorrectly or lagging. Probably a combination of several factors high and hot airfields not helping much.