dry motoring cycle

[Torstennnn]

once more jar fcl is bothering me
this time about enigine jet pipe fire on ground -
they suggest a dry motoring cycle!
I don't have a clue what they're talking about

[tom775257]

A dry motoring cycle (or dry cranking) will blow unburnt or burning fuel in the engine/tailpipe out of the back end and prevent damage in the case of a tailpipe fire.

Simply dry cranking is turning the engine on the starter without introducing fuel hence the term 'dry.'

On some aircraft, in the case of a failure to ignite during a start cycle, a dry cranking cycle is completed before another attempt to start is begun to prevent a large buildup of unburnt fuel in the engine.

[Torstennnn]

@tom
so this is the same i do after "hung start" to prevent "hot start" ?

[mini-jumbo]

I'd also expect to do a motoring cycle after a hot start, to reduce the TGT (or equivalent) to avoid cooking the engine.

On a hung start, I would expect the starter to still be engaged, so by shutting off the fuel you would in effect be turning it into a motoring cycle.

On a hung start, I'd leave the starter engaged until TGT below set limit, then kill the starter. With a hot start, I'd shut the fuel off and then do a motoring cycle to reduce the TGT.

[dkz]

Great posts ... but i fail to see the connexion with "jar fcl" (Flight Crew Licensing)

disregard, just a comment

[BizJetJock]

"On a hung start, I would expect the starter to still be engaged, so by shutting off the fuel you would in effect be turning it into a motoring cycle."

If you'd asked me last year I'd have said the same, but have had two cases in the last year of starts hanging after start valve closing - got me scratching head a bit!

[Torstennnn]

@dkz
one question out of 6500 you might be asked

@everybody
thanx for your time

[mini-jumbo]

@BizJetJock

Interesting, I hadn't really considered an engine hanging after the starter disconnects. The starter disconnects once the engine has become self sustaining (40% N2 (ish)on my type). I guess in your example the engine must have gone into a sort of sub-idle condition, from which it wasn't able to accelerate to ground idle, but equally was able to sustain momentum without the starter?

Did it run down once the starter had dissengaged?

[SNS3Guppy]

so this is the same i do after "hung start" to prevent "hot start" ?

You're not turning the engine to prevent a hot start, although that may be a part of the purpose. You're doing it to get air through that engine and normalize it, ensure that remaining fuel is blown out, create an air barrier with proper circulation through the burner cans, ensure that any heat, or residual flame hasn't propogated forward through the burner toward the diffuser and compressor, and take care of any flame or heat that may have moved aft into the turbine section. You're bringing the entire engine core temperature down somewhat and normalizing, or equalizing roughly speaking, the temperatures through the engine. You're also helping prevent coking the bearings and the labrynth seals by continuing to circulate oil and by overall reducing the internal temperatures somewhat. It's also axiomatic that while temporary cooling occurs internally, your engine temperatures will again climb when you stop turning the engine...you may cool interior core components somewhat, but they do heat up again once you stop.

A hung start isn't related to the use of the starter. It's related to engine acceleration, and can take place with, or without the starter engaged. A hung start may be silent, or may be accompanied with various degrees of a compressor stall. The reasons for a hung start vary from stuck acceleration bleeds to insufficient starter pressure or voltage; a slow start with the starter engaged can become hung, but one in which the starter has disengaged but a fuel flow issue, acceleration bleed issue, inlet airflow issue, hot day, tailwind, or other contributing factor, may also hang.

The biggest problem with the hung start is that your engine may be cooking somewhere you can't monitor. Normally when airflow progresses through the engine, you have an indication in your ITT/EGT/TiT to make a comparison with the rest of the system (usually vs. engine RPM) to determine a proper acceleration schedule, and know that the temperatures are as they should be. Even though you can't monitor all the temperatures throughout the engine, if it's accelerating normally, you know that temps you can't monitor, in places that don't have the EGT or ITT probes, for example, are doing just what they should.

When the engine hangs and stops accelerating, the airflow through the engine is no longer normal, and the temperature schedule throughout the engine is no longer normal. You may see a steady temperature in one particular section of the engine (the one where your EGT probes are located, for example, but you now have no good information about what's going on elsewhere. The temperature may be climbing dramatically in your burner cans, for example, and you won't have an indication of that until it may be too late. Airflow is not accelerating on schedule, but other things such as fuel might; where the burned gasses are still being moved through the engine at the same rate, you may have a rapid temperature increase upstream where it shouldn't occur, before it registers on the engine temperature sensors (EGT, ITT, etc)...hence the reason you want to cut off the engine when it hangs and start over...and blow it out using the starter to motor it.

If it hangs after starter disengagement, then you have no choice but to allow the engine speed to decrease to a point where the starter may be re-engaged to motor the engine for clearing and cooling purposes. Some engines are more susceptible than others.