'Wet Cranking'

[Obi Offiah]

If I understand correctly, wet cranking is where fuel is introduced into the engine while is turning (under starter power) but not ignited correct?

I've heard this is usually for maintenance purposes but why is it done, under what circumstances?

Thanks
Obi

[Fargoo]

It's one way of purging preservation fluids from an engine thats been in storage.
If you were to go for a normal start rather than doing this you quite often end up with a hot start or even a tailpipe fire.

[Obi Offiah]

Thanks Fargoo

Is this action usually performed by the pilots or the maintenance personnel, i.e would an installed engine ever be wet cranked just prior to a full start.

Obi

[TURIN]

Leak checking of components downstream of the LP cock?

On the other hand Airbus have an oddball system where the engine should not be cranked unless the components are wet IE lubricated.
You have to trip a CB to 'fool' the engine and keep the HP valve closed. so wet cranking still remains 'dry' if you know what I mean.

[Fargoo]

Thanks Fargoo

Is this action usually performed by the pilots or the maintenance personnel, i.e would an installed engine ever be wet cranked just prior to a full start.

Obi

Normally done by maintenance personnel, as Turin mentions it's also done for leak checking sometimes.
Once a wet cycle is done it's normal for us to do a dry cycle (ie spin the engine on the starter with no fuel or ignition) to make sure any remaining fuel hasn't accumulated in the rear of the engine.

[411A]

You are unlikely to find a pilot who will consent to wet cycle an engine...a mx function only, normally.
Now, I have seen the results of an improperly done wet cycle...the torch out the back end was at least sixty feet long.
Went way past the tail, and scared the cr*p out of everyone nearby.
No damage, however.

[Swedish Steve]

It's one way of purging preservation fluids from an engine thats been in storage.

But its much better to purge the preserving oil by opening the LP fuel filter drain and turning on the boost pumps.
Wet cranking is used when a new engine is installed.
RR call it an "oil circulation and fuel drain test". You wet crank the engine until fuel comes out the jet pipe. Then check that the residual fuel drains from the engine via the drains system, and check the MCDs to see that oil has reached them. Then you start the engine for the first time.
To do a leak check on fuel components, you spray chalk dust on them, and run the engine at high power. Then open it up and check. You must use high pressure fuel, the fuel pressure at cranking speed is not enough.
I can't think of any reason to do it at other times than a new installation.

[TURIN]

To do a leak check on fuel components, you spray chalk dust on them, and run the engine at high power. Then open it up and check. You must use high pressure fuel, the fuel pressure at cranking speed is not enough.
I can't think of any reason to do it at other times than a new installation.

Depends on the size of the leak.

Had experience of discovering a significant fuel leak from a GE90 FMU (or is it FCU? HMU? I forget ) during a 'wet cycle'. Mark 1 eyeball was good enough at cranking speed. Running an engine at high power is not always an option during a 2hr turnround.

[Obi Offiah]

Thanks for the responses everyone.
I'd been trying to get some information about wet cranking for sometime and Google searches weren't very helpful. I guess its quite esoteric because its largely a maintenance process. On the other hand there's alot of info about dry cranking, but this is/can be part of the natural starting process of an engine to I thats understandable.

Thanks
Obi

[groundfloor]

If you have tried a normal start and it didn`t you may have had an inadvertant "wet crank". It is VITAL that you check the fuel has drained out, saw a longranger helicopter burn out it`s engine in spectacular fashion after an ignitor box failure was fixed!!

[AeroTech]

Hi,

Turin (post 4), If I am not mistaken Boeing maintenance manuals also state similar cranking procedure (opening spar valve & hp sov, pull the CB...) for dry cranking/motoring.

Steve Swedish, I am not familiar with RR engine but I am wondering how the preservation oil can be purged downstream the LP fuel filter?

Thank you
Regards

[Nepotisim]

I would suggest when Steve refers to preservation oil he would be referring to the fuel system inhibiting fluid which is an oil like substance. Not engine oil that you may be thinking of.

[AeroTech]

Hi,

Nepotisim, thanks for your explanation, but I was refering to preservation oil as stated in my previous post.
I guess I need to explain my question since it seems not clear.

The goal of wet cranking after new engine installation is to purge preservation oil from the entire engine fuel system (through fuel nozzles). If you open LP fuel filter drain as mentioned by Swedish Steve, part of the preservation oil will come out from the drain filter, the rest of the preservation oil will stay inside other engine fuel components (downstream LP fuel filter).

As I said before I am not familiar with RR engines but I assume its engine fuel system is not completely different than other systems (GE or PW...)

Thanks
Regards

[Beeline]

Engines can also use the fuel as a servo(muscle) force on a lot of components driving them to the commanded position via a touque-servo motor.

These components when replaced require a leak check without the engine running so a 'wet-crank' is performed. These servo lines are tapped off before the main fuel shut-off valve. The trick is to get the fuel to the engines without flooding the combustor chamber as the LP valve is usually linked in series with the HP valve from the Master switches.

Pulling a circuit breaker as previously mentioned can fail safe the LP valve open whilst keeping the HP valve closed. The components can be leak checked or in the case of the Airbus CFM the FADEC can perform a full function check.

Airbus engine run check list has all the scenarios I perform on a daily basis including wet and dry cycles, but I do not hold an electronic copy of this manual, maybe somebody could help Obi with this???

[Nepotisim]

OK AeroTech to answer your question. The only way to purge preservation fluid downstream of the LP fuel filter on a RR is to do a wet spin and send it through the fuel nozzles.

[HAWK21M]

Out here we call it WET MOTORING.....The Engine is motored by the Starter,the Ignition CBs are pulled out & fuel levers moved to permit fuel flow.Normally done to purge the preservative fluid or for checking fuel through the FCU.

Its always followed by a DRY MOTORING,before Engine startup to avoid a HOT START.
regds
MEL.

[muduckace]

Usually done post engine change, In reference to leak check an obvious one will be discovered. Also on most jets FF will not register properly due to air in the system that needs to be purged. I commonly perform the process after changing engine fuel components to prevent fluctuations on start that may indicate an abnormal start by indicated fuel flow. The Fuel in the tail cone is no big deal as long as it is followed by a lengthy dry motor to purge it before engine start.

I have remedied FF issues on engines that have sat for a long time with this process.