Hung Start?

[low n' slow]

Hi all!
Please help me, my memory fails me...
What is the reason for a hung start. I know that it is when the engine does not accelerate up to idle after light up, but what is the reason for this?

Also, if someone would like to explain what happens during a hot start, I would be very thankful.

I did a search but I came up with nothing.
thanks guys and girls/ LnS

[barit1]

Have you read this? (http://www.b737.org.uk/cfm56_soi.htm)

[gas path]

Hung start
Probably the simplist explanation would be that the engine lights off and accelerates upto starter cutout speed but fails to carry on accelerating to idle. The EGT and shaft speed will 'hang' or creep up very very slowly. Pull the fuel out, let the EGT cool and try again. Usually caused low starter duct air pressure or by compressor damage (or fuel schedule wrong on the old 'springs and capsules' FCU's).
Hot start
EGT races away during the start cycle and can result in a damaged turbine. Again low starter duct pressure or a damaged compressor or fuel schedule is a prime example. A good ballpark figure for shaft speed and rising EGT during a start would be EGT temp below N2/N3 speed upto idle. So during a start if the EGT is say 350deg the shaft speed would need to be 35% or more to ensure a good start. 500 deg and rising with only 40% and you have an impending hot start. :=
coool I like the new smilies!.........if they worked!:D

[Colonel Klink]

The most obvious reason is too little fuel scheduled by the FCU. We see this in the sim. all the time. The only way to get any engine to accelerate is to add fuel, so too little and the engine will hang, unable to reach self-sustaining speed and starter cut-out.
With reference to the above post, a low duct pressure means the engine won't turn fast enough by the starter causing a high EGT and possible hot start rather than a hung start.

[Trogdor]

In the Mighty Beech, a hung start can be caused by blocked or inoperative fuel nozzles. Especially the secondary nozzles which begin operating around 40% N1, the exact N1 escapes me at this time. Obviously this is an antiquated system compared to some of the above posts, but I thought I would throw in my two cents.