Fuel flow meters provide a 10 second warning to empty tank

[humptybump]

A number of pilots either suggest running a tank empty "for the experience" or even as a general practice. I've never done it until today. It seemed a perfect opportunity. I was at the end of a long cross country and in familiar territory and up high and "the other tank" had about 12 gallons. So I kept a close eye on the float sensor round fuel gauge. When it hit the pin, I noted the fuel flow meter's "used" number. Then a flew and waited. After another full gallon was reported, the fuel flow meter flow rate started goin up quickly to 35-55gph this was my 10 second warning because after than, it started to get quiet as I switched tanks and picked back up after about 3 seconds.

I don't plan to make a habit of running a tank empty but this test now provides a lot of useful information.

I know know there is about a gallon in the tank when the needle hits the pin.
The fuel flow meter gives me a very convincing indication that is is getting air, not fuel.
I know what to expect if one tank stops feeding the engine and I switch tanks.
I also get to find out exactly how much useable fuel there is in one of the tanks (thus far, I've just trusted the builder's documentation).

[Walt]

Just an FYI if you run fuel injection with "Vans/AFP" fuel pump:

From the AFP website:

WARNING

Airflow Performance and Bendix/Precision fuel injection systems are non-returning systems. In the event that a tank is run dry in flight, an air lock will be formed on the out let of the pumps. It is possible that the boost pump will not pick up fuel, as the boost pump cannot create enough air pressure to over come the flow divider opening pressure, thus displacing the air and resume pumping fuel. It is not recommended to run a fuel tank dry in flight without adequate testing and proper documentation of the procedure for this operation.

[humptybump]

Thanks Walt. I should have added som details about my test like it was on an O320 with a carburetor.

[rv7charlie]

Some alternative engine guys are providing a 'bleed' path in the fuel system to eliminate the air lock risk. It's a very small orifice, similar to what most do for remote mounted oil/fuel pressure sensors. Most automotive systems have a return line to the tank, but the same thing could be achieved by looping the bleed line from the pressure side back to the pump input (this is what's done from the regulator on a certain high dollar FI boost pump assembly).

Charlie

[APACHE 56]

I wonder if you found yourself inadvertently running a tank dry then getting the dreaded airlock could you actuate the purge valve to get fuel flowing again?

[dlloyd3]

The purge valve is down stream of where the air lock occurs so that would not be an aid in resuming flow. On my first tank fill, I could not get the AFP pump to self prime. I had to put a little pressure in the tank via the vent line to make it happen. I suspect if running one fuel tank dry, the engine driven pump would pull enough vacuum to draw fuel from the other tank through the AFP pump. Haven't had the nerve to try it out. Anyone?

[YellerDaisy]

Originally Posted by humptybump

I also get to find out exactly how much useable fuel there is in one of the tanks (thus far, I've just trusted the builder's documentation).

Hi Glen,

I've done the same thing on both of my tanks to validate the fuel gauges, for the "experience", and confirm that it is a 'recoverable' state.

The "experience", for me, was to know what it looks/feels like should it ever happen accidentally. In addition to what you reported, the fuel pressure (on my EIS) also drops. So, fuel flow rate goes high, fuel pressure drops, and EIS warnings start flashing before the engine shows any sign of quitting (in my airplane). I think I would have to be asleep to miss it.

[JonJay]

It is a good idea to do this type of testing at altitude over a friendly field. I did this test before knowing about the AFP warning. Mine did not have an issue and came right back to life but I was over the field prepared for the what if's.

[Wayne Gillispie]

On my 30 gal tanks was to run a tank down to 5 gal using the float reading. Compare that to my FF transducer derived reading. Then drain remainder. My red cube is the most accurate, showing 1/2 gallon less than actual. My floats show 1 gal more than actual at 5 gal reading. I won't intentionally run below 6 gal per side using the floats.