At last Pilots that understand the architecture of the 135 fuel system!

Anyone have a cutaway pic of a transfer pump?

This seems to violate JAR/FAR certification requirements:

“No engine or fuel pump can draw fuel from more than one tank at a time”

The saying goes, 'There's no such thing as a silly question", but I'll try my best :ok:

Even if the NRV was to fail and the pump was turned off or u/s, how does the fuel still flow through the pump? Surely someone at Airbus would have twigged that this wasn't quite right :confused:

Same way it flows through any other centrifugal pump.

But definitely not like the newly discovered "uphill" syphon.

Google it - listed with perpetual motion machines!

Glad to see you're on the ball, as always Sid. :ok:

With my vast experience of goldfish bowl cleaning and having watched 'Finding Nemo' a couple of times, this reverse siphoning effect going on in the 135 fuel tank seems a bit far fetched. But I guess that's the stage at which we find ourselves.

The source has to be higher than destination. We have been told that the transfer pipes run to the bottom of the supply tanks, the same level as the transfer pump inlet, add a couple of obstructions to a smooth flow, such as faulty nrv, non operating pump, a couple of sharp turns or a pocket of air and syphoning simply can't occur either way.

Having said that, anyone know if the diameter of the tubes is constant or are there different sizes of tube in the transfer system?

How many of us have run the tanks dry whilst sitting on the pad before this ad?

I can only report what I saw.

Have I taken the fuel system apart and crawled around in it? No.

Where did the fuel go, I have no idea.

The collective opinion amongst my colleagues is it's syphoning.

Why would it be uphill????? Aircraft is parked level.

The nub of it all IMHO is where does the XFER pump deliver the fuel to? Does it pour into the supply tanks or does it deliver to the bottom of the supply ?

Well I found this slide.

It looks to me like the XFER pumps are delivering the fuel to the bottom of the supply tanks...


Click me for slide

BS, not that I'm doubting your actions, however one solution is that you turned the prime pumps on and not the transfer pumps off!

When I did the check, after turning the transfer pumps off, the main tank contents remained at a figure over 200 kgs, which I assume is as it should be.

Did you get any feedback from Airbus/Bond after the paperwork was sent in, or have you questioned it outside of an internet forum, as this episode seems rather strange.

Is there a collective opinion about where the fuel syphoned out to? :rolleyes:

Therefore it can't be syphoning can it!

I tried this in the cruise, some years ago, to see what would happen with a double Transfer Pump failure as it didn't feature in the EC drills. There was 255kgs left in the main tank when the supply tanks started going down.

airbushelicopters.com
I hesitate too make this post as too many people want to ridicule and scoff.
But here goes one more time, as a very average Pilot, but some would say a pretty good Engineer!:
If you take the time to read page 5 of the above link you will see that EC know very well about the anomaly of fuel transferring from main tank to supply tanks after transfer pumps have been switched off.
There is no magic or witchcraft at work, just simple fluid dynamics.
The flexible transfer pipes are about 15mm dia and are the [U]only[U]path between main and supply tanks once the fuel level is below the fence transfer ports.
If the head of fuel is large enough, then after the transfer pumps are selected off the syphoning effect will continue pulling fuel through (for want of a better word) a freewheeling pump for several minutes.
The 4 submersible pumps are identical apart from the main tank pumps have NRV fitted, the supply tank pumps do not. There is a very good reason for this, the supply tank feed to the engines is non critical in respect of fuel drain back, the transfer pumps however are critical hence the fitting of NRV/check valves.
Now for goodness sake don't take my word for it, go and ask your Chief Engineers!

Not an IT engineer then, the link doesn't work :rolleyes: :suspect:

Surely the reason there isn't a NRV in the supply tank fuel line is because if there was, not only would we have to continuously have the prime pumps on, if it was to stick, the respective engine would cut off! Nothing to do with drain back!

The link works when I click on it Sid!

Thanks PC, it does now. Must have been caught in the rush!
You're missing a / before the U in the second box if you wanted to underline 'only' in your previous post ;)

Let me do a difficult engineering cut/paste;

"This can be explained by the fuel system architecture of the helicopter and is a normal behavior."

How can it possibly be syphoning if it is the same body of fluid?

???? I think this is nonsense; I should think that the main reason for no NRV is, that as long as the respective engine is running the engine driven fuel pumps are maintaining the fuel feed to the engine by means of suction...thats the reason why you switch the prime pumps off after the fuel flow has been well 'established' (engine started). Even if a NRV was installed in the feed line, the engine pumps need to 'suck' just a little harder to overcome the springload of the NRV. No need to do that.
So, NO, you wouldn't need the prime pumps continuously,
and suppose the NRV stuck in closed position....that can only happen in an engine off situation, and you wouldn't be able to start the engine......stuck in the open position.....you wouldn't notice the difference.

And also SS,... i've been reading a lot of posts of you lately, and also quite some replies to your posts, and I believe I've come across the word 'arrogance' more than once. Must be a reason for that:rolleyes:

....and as to why the main tank continues emptying into the supply tanks for some time after shutting off the XFER Pumps...I don't know, but witnessed it on two machines.

Just thinking...The engines sucking in fuel via the inletport of the prime pump.....transfer pumps delivering the fuel from the main tank to the supplytank. Assuming the transfer fuel lines exits situated somewhere on/near the bottom of the supply tank...the fuel flow generated by the transferpumps is way larger than the suction flow to the engine. Is this creating some swirl effect in the supply tank.....when you suddenly stop the supply from the transfer pumps; the swirl will continue for a couple more minutes, creating static pressure drop at the exit of the fuel transfer line, and thus continue pulling fuel from the main tank. The swirl effect slows in time and after some minutes the static pressure drop ceases to exist and the engine starts sucking from the supply tank........
So I think a lot of sucking is involved:O

so if you did a similar check without switching on the transfer pumps at the beginning of the sequence....no swirl created,.... the supply tanks would start emptying rightaway. (assuming the main tank Qty level is below the overflow channels)

Spot on Yellowbird.

So yellowbird, what cracking pressure is needed in a non return valve?
I'm glad you told us why we turn the prime pumps off after engine start, do you know why we may need to turn them on in flight?
My statement merely highlights the questionable action of placing an obstruction such as a nrv in a crucial fuel line if it wasn't needed :ugh:

Yep, because I question things :ok:

Such as, when you say;
Do you consider it arrogant of me (expecting the obvious answer) to highlight an earlier post;

"Phoinix; We had exactly the same; 159 till 139 after XFER pumps OFF, sucking at it for almost 15' before supply tank indication dropped."

My, thats some swirl effect going on!

Virtually none, they are not spring loaded.