Allison 250 stuck fuel drain valve

[handbag]

Scenario. The Allison 250 fuel drain valve malfunctions and gets stuck in the closed postion and a significant amount of unburned fuel becomes trapped in the combustion chamber. At what N1 speed before normal light off will compression be sufficient to ignite the gas prematurely?

Thanks.

[Miles Gustaph]

The Rolls Royce book says that ignition of fuel will not occur at mas airflow speeds above 12 ft per second and if you put fuel into an engine before there is a satisfactory airflow it will sit around the bottom of the combustion chamber until the engine ignites and give you one hell of a hot start, possibly cooking your turbine, especially on an A250.

If I remember correctly the A250 fuel nozzles have a spring in them that prevents fuel from entering the engine until the pressure inside the engine is proportionate to the mass airflow, you could say that is your compression in which case I would look it up in the maintenance manual but I "think" it's around 35-40 psi, I'm sure someone will correct me there.

Sorry I don't have a flight manual to hand to get you an N1 speed, but I would suggest that premature ignition of collected fuel is unlikely and that as it is in liquid form it will all happen at the same glorious time upon normal ignition.

I hope it helps

[unstable load]

The air pressure from cranking will not do anything to the unburnt fuel. Once it lights off though, things will get rather interesting and could get expensive too.
Miles has it right here...
Sorry I don't have a flight manual to hand to get you an N1 speed, but I would suggest that premature ignition of collected fuel is unlikely and that as it is in liquid form it will all happen at the same glorious time upon normal ignition.

[gwelo shamwari]

Not much of technical person concerning airflow and pressures. But have been around on two instances where the burner can drain was blocked resulting in fuel being trapped. The pilots involved stated that as soon as they hit the starter button there was a rather spectacular climb in the tot... :\

TGZ

[spinwing]

Mmmm ...

As TGZ said above .... you will most likely get some form of lightoff at upwards of 3-4% N1 .... and it will not be enjoyed by the engine as it will be in the form or a unstable "torching" ... it will be "Illuminating" (so to speak) at night however!


:E

[Um... lifting...]

The pressure in the combustion chamber of a C250 is about 1/2 to 1/4 that of a typical compression-ignition (diesel) engine... compression simply ain't gonna do it... especially during start, when it will be lower still.
It's going to be the igniters that light it off... but that will depend upon what kind of fuel it is... something high flashpoint like JP-5... probably nothing will happen until you introduce some fresh fuel through the nozzles... then it might get exciting.
The best part is on shutdown if you don't manage to keep any fuel from dribbling into the can... autoignition temperature of Jet-A is about 210C... always best to be sure the fire is REALLY out before you get out...
Maybe that's why most machines like to see 10-15% N1 before they start drinking fuel... more air for cooling...

[ericferret]

I have seen this scenario once. An engineer friend of mine in a hurry decided to carry out a fuel bleed on a BO 105 without disconnecting the fuel line from the fuel nozzle.
His theory was that the fuel would just drain out of the combustion chamber drain.
Sadly the drain was blocked.

On the next start the engine lit off and blew the now burning residual fuel out of the exhausts and all over the engine cowlings. This resulted in alarm, lots of running about and a substantial amount of burnt paint!!!!!

[EN48]

As a new operator of an RR250, this thread got me thinking enough to ask my shop a few questions. They said that with a properly maintained engine, there is little possibility of a drain valve sticking closed. If the compressor is washed correctly at the intervals recommended by RR, the washing solution also cleans the drain valve of deposits which might cause sticking, and washing allows confirmation that the valve is not stuck closed. Shop owner says he cant recall the last time an engine maintained by his shop required valve replacement, but he said he sees this problem often enough on helicopters that are brought to his shop for the first time. He didnt say that a stuck valve couldnt happen with the recommended wash schedule, just that it is unlikely. :8

[FH1100 Pilot]

EN48:He didnt say that a stuck valve couldnt happen with the recommended wash schedule, just that it is unlikely.
Well, yeah, but not only that - how is all this unburned fuel getting into the combustion chamber anyway? (1) If you shut it down correctly and ensure a positive cut-off, the fuel stops, period. (2) Once the engine is shut down and not rotating, there is no way to squirt fuel through the nozzle unless something is very wrong...which hopefully you would have noticed on the last shutdown (see #1 above).

If you were to somehow get a "puddle" of fuel in the combustion chamber, then it might or might not ignite upon initiation of the start sequence. If it does (but you have not introduced fuel yet), then it might be expelled out the exhuast as ericferret described.

Not that it couldn't ever happen, but in 25 years of flying underneath Allison/RR 250's, I have never seen the situation in which fuel was somehow allowed to pool in the combustion chamber *and* the drain valve was plugged at the same time. But pilots do like to come up with crazy scenarios.

[Oldlae]

You can get a pool of fuel if the igniter or the igniter system fails and the valve is blocked. To ensure that the drain valve is clear trace the drain down to the outlet to identify it and observe that the wash solution drains out after a wash.