I was looking at a friends Lancair 4PT the other day and what I found really scared me.

Seems that Lancair is telling builders to use the belly tank as a header tank, and there is

a vent that has an solenoid (electric) that is closed when the master is switched on.

The theory seems to be that when the plane is refueled, the master is off and therefore solenoid open and any

air in the system can escape at that time.

 

First, it is ridiculas to use a belly tank as a primary tank on one of these little planes.

This means that the belly will always be the tank to have the last bit of fuel in the plane.
It is completely dangerous to do this simply because of a potential gear up landing  intentional or not...... 

 

Second, it seems to me that when the plane is low on fuel, clearly there is the potential

of air getting into the belly tank when either wing gets low. 
These planes can climb really aggressivly. When this happens, the fuel rolls aft, and

now the pump can pull air.  

 

It is completely non-standard to gravity feed fuel in a low wing aircraft.

On a turbine aircraft,,, No way.

 

I don't even want to think about what would happen if this little electic solenoid would

fail... If it didn't close, the electic fuel pump will have a much easier time pulling air than fuel...

 

I can't imagine that there haven't been failures of this sort already.

 

I am suggesting to him that there be a way to purge air from the fuel system all the time.

Also, he should use the fuel in the belly first, and use it completely. A simple selector valve

would take care of this. Some selectors even have a return line built into them for injected piston engines that need

it. You should have a return to a vented wing.

Another way would be to pump the belly to a wing, as needed.

 

Maybe a small header tank after the electric fuel pump, with a reduced fitting on the top for a return to purge any air in the system.

A reduced fitting will keep the pressure up but also purge any air back to a vent wing.

 

I find it difficult to believe Lancair would endorse such a fuel system as it stands now.

It would never pass an FAA system check. They must have never run this past a FAA system DER.

I certainly hope the insurance company's that underwrite these planes never hear about this.

Seems to me just a flameout waiting to happen.

Please anyone tell me where my mis guidance is.....

 

Bill McDonald

Sys DER

Big airplanes

Bill,

 

Welcome to the list.

 

Let's look at the  whole picture - While low wing aircraft carrying fuel are at risk in any gear up landing, belly fuel doesn't necessarily increase the risk - if it rips open during the slide, the fuel will escape before coming to rest.  Maybe.

 

Another view is that the belly is merely a large sump for the wing tanks, ideally located at the lowest point so that all the wing fuel can be utilized during level coordinated flight.  Properly baffled, the belly should deliver almost all of the fuel aboard and is kept filled via gravity and positive pressure supplied by the wing tank vents - simple, no? 

 

No slosh compartments are necessary in the wings and no selector valves that seem to be a root of so many fuel management accidents.  Of course, this requires that the vents be operational only in the wing tanks in flight since an unbalanced flight pressurized vent in the belly could force the fuel back into the wings, could it not? Even check valves might stop the reverse flow, but the belly vent pressure could be working against the gravity flow.  If the wings had a greater pressure (gravity plus vent) than the belly vent, fuel could be forced out the belly vent - well maybe a check valve could stop that too.  Of course, everybody checks their well baffled belly tank for the minimum fuel required in a slip,skid, climb or descent so that the fuel intake would not become unported.  See, the pump should only pull air if there is not enough fuel in the entire plane.

 

You have interesting ideas.  However, the system ,as originally outlined, should have no problem passing the requirements for an Experimental, Amateur Built airworthiness certificate as determined by an FAA inspector or DAR.  As an experimental aircraft, the builder can obtain good solenoids as opposed to those TSO'd in 1938 - you know, the ones that you expect to fail because of your experience.