T-6B fuel system question

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addaon

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I was looking over this document which describes the T-6B fuel system. Attached image is from slide 3.

The system uses two ejector pumps in each wing tank, one each for the fore and aft pickups These ejector pumps feed to a header, which uses a series (EDIT: parallel, not series) combination of an ejector pump and an (automated) electric boost pump to feed in to the engine pumps; high pressure fuel return goes to the header. All approximately conventional for a header-tank system from my understanding.

But while the diagram shows interconnected vent lines on the wing tanks, it doesn't show any venting on the header. Is this just an oversight in the diagram? My understanding is that maneuvering to very high or low pitch is likely to unport one of the wing tank ejector pump inlets, and I believe that in this case the pump will happily pump air to the header tank, yes? This is fine for transient operation since the header tank basically acts as a fuel/air separator... but that only makes sense if there's a way for the air to escape from the header tank.

Q1) In the specific case of the T-6B, what am I missing?
Q2) In the more general design case, is the basic approach here to just T in the header vent with the main tank vents?

1661103512944.png
 
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cvairwerks

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There is no external venting on turbine or turbojet fuel systems. Once the engine has been started, the tanks are continually pressurized. The pressure is low, but always positive. For example, on the F-16, servo air is fed to the tanks and maintains a certain tank pressure as long as the engine is running.
 

addaon

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That was my impression for the big guys, but (a) the diagram doesn't show any pressurization mechanism -- where would the air come from the on the T-6? and (b) even so, doesn't there need to be a way to remove air from the header tank and return it to the wing tanks, if not to the outside? The goal is to keep the header full of fuel, not fuel-air mix... unless the inlets from the transfer pumps are high on the tank and air will flow "backwards" through them?
 

TFF

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No dedicated vent. The low pressure switch will activate and pump whatever it can; air, fuel, foam. It will draw out the air. It’s the only thing that will catch up the engine except time. Assuming you start fuel back to the header. You can put the plane in position to consume air. The flight manual will have limitations. You will be a glider until it clears.
 

addaon

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Interesting. Am I correct that the pumps will be feeding air into the header whenever one of the wing tank ports is unported, or is there something that prevents that? I just figured that for a trainer it's gotta spend some amount of time upside down and in other unusual attitudes... and there's a flop tube in the header tank and time limits on inversion, so that's obviously expected. If each time you're inverted some amount of air gets pumped to the header... seems odd to me.
 

TFF

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It would send air in. The question is would the side with the fuel win the pumped fluid battle. The motive flow is pushing from both all the time. I would think the heavier fluid would win. There is a limit and probably at above low fuel warning there is a no aerobatics limit.
 

John Halpenny

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It would send air in. The question is would the side with the fuel win the pumped fluid battle. The motive flow is pushing from both all the time. I would think the heavier fluid would win. There is a limit and probably at above low fuel warning there is a no aerobatics limit.
The header tank has a flop tube and, presumably, 15 seconds of fuel. The wing tank jet pumps would try to pump air, but would they allow a reverse flow of fuel out of the header? If not, the header tank fuel would stay there.
 

TFF

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Usually it can’t reverse flow out just by placement of the tubes. Don’t know if there is a flapper or one way valve also. Jet engine boost pumps usually are very high volume. I would say triple engine needs, not just a few extra psi like a piston engine.
 

wsimpso1

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But while the diagram shows interconnected vent lines on the wing tanks, it doesn't show any venting on the header. Is this just an oversight in the diagram? My understanding is that maneuvering to very high or low pitch is likely to unport one of the wing tank ejector pump inlets, and I believe that in this case the pump will happily pump air to the header tank, yes? This is fine for transient operation since the header tank basically acts as a fuel/air separator... but that only makes sense if there's a way for the air to escape from the header tank.

View attachment 128989
Some major things appear to be missing or otherwise be wrong with the diagram. There is no air inlet to the wing tanks, and no way to purge air from the collector tank. Let’s analyze this stuff a little.

Air has to enter the wing tanks to draw much fuel from them, so the diagram is missing those lines.

Air that is picked up during maneuvering and pumped from wing to collector tank has no way to get out and air would accumulate in the collector tank, so an overflow line is also missing from the diagram… the overflow line might be a return to the wing tanks or vent to atmosphere.

Each of these has to be done correctly too. The jet pumps are fed fuel by return from the engine, which is greater than max fuel burn rate:

A return line to one wing tank will drive fuel to that tank with only the interconnect keeping any fuel in the other, until the non-return wing goes empty. If the tank accepting overflow is vented, it’s vent has to be configured to allow air out but not fuel - see below. Other failure modes are possible too;

A return to both tanks will do the same thing as a single return, only more slowly because flows will be somewhat different back to each wing and drive imbalance and emptying of one wing tank while overfilling the other;

An overflow line from the collector tank to purge air.

Any line on a tank that can be fed air can also be overfilled with fuel. An overflow vent line to the atmosphere would need to be configured to allow air into the tank, to allow air out, and to prevent liquid fuel from going out. I can already imagine a spring loaded check valve set to close when hit with liquid going out but will stay open when flowing air in either direction. Race car hardware exists to allow air into the tank but prevent fuel tank drainage when the car is upside down. Sounds like it should work.

We need a more complete diagram to use this as a good example…

Billski
 

addaon

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Billski, you've hit exactly on my original question -- cvairwerks suggested that this is a pressurized fuel system, not an externally vented one, but I don't know enough to know one way or the other. Perhaps a better diagram will turn up.

I was mostly digging into this as an example of a low-wing with active balancing ("both" setting).
 

TFF

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While it doesn’t show the port to the atmosphere, it has to have vent lines. Bottom of wing about inline with the over wing cap. The diagram is a pilot systems training aid not a full system description.
 

wsimpso1

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While it doesn’t show the port to the atmosphere, it has to have vent lines. Bottom of wing about inline with the over wing cap. The diagram is a pilot systems training aid not a full system description.
And so is worthless to us and of questionable value to a pilot. The pilot can not do anything resembling a complete version of looking at this diagram and play the "what if" game with in flight failures. In some cases, the incomplete diagram will lead the pilot to the wrong actions and then the chief pilot and board of review will blame the pilot for the error.

For instance, if tanks are not vented but are pressure inerted, fuel flow interuptions may be resolved differently. If air purge of the collector is to atmospheric it might be handled different than if to one wing tank or to borh.... dumbing down has so many pitfalls.
 

TFF

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Those style diagrams are aviation industry standards. It depends on whoever commissioned the drawings on what they think is important. Would the maintenance training manual be different or would the text say something, I don’t know. Pilot walk around checklist and maintenance daily ready list would have to check if it’s clear.

Professional aviation training manuals are written to train monkeys. Pilots, mechanics, ground handler are written as need to know not full systems. Just are. Maintenance and parts manual would have how to fix such and sometimes have a synopsis.
 

TFF

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It can spark an idea, but you already have to know something about how things work. You can’t just copy.
 

Tiger Tim

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That fuel system looks a lot like what’s on the Caravan: vented wing tanks feeding an unvented small reservoir tank which supplies the engine via jet pump. I believe the reservoir in that case is unvented to keep it from overflowing.
 
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