How much of the fuel I put in is unusable? In other words, can the engine use every drop I put in the plane?  Or does some of that fuel, because of the position of the fuel pickup in the bottom of the sump tank, make that last little bit inaccessible in flight? I had to know.

Given my fuel pickup is in the middle of the sump using that holy hydromat filter, I figured that coming in on a 3 degree glide slope would be a good test. This is less favorable that straight and level and more likely if I was going for a landing zone. If I was onto the bottom of my reserves this was the scenario that would be useful to be sure of.

With fuel running into a bucket my method was to watch the 35psi pressure and when it dropped I would have run out of fuel. Then  I could measure what was remaining.

It turned out that at the 3 degree the fuel stopped draining out and kept recycling. So with whatever was left it was enough to wick up the hyromat and still given me full flow.

I shut down the pump and had to really lower the nose nearly to the ground to get the last bit of fuel out that would be ‘unusable’. This is also the deck angle I need before flight to check for water in the fuel sump daily and after refueling.

This turned out to be only 0.4 liters or 400ml. Now lets say at a very conservative 30 liters an hour this would be about 25 seconds or so of engine time.

Interestingly if the nose gear is all the way ‘up’ when parked, fuel will move from the sump tank to the wing strake forward tips. If there is only 10 liters or so in the sump then it will appear as if there is no fuel in the plane. However as soon as the nose is raised a little, it flows back into the sump. I don’t see this as a problem as 10 liters is not exactly flight fuel and even a slight nose up gets it back to fill the sump.

You know, I just might live dangerously and call all my fuel usable, rather than take off 25 seconds of my calculations with this plane’s endurance. My instructor would correctly say, take off a full minute. If it ever came down to that, I would be too busy at the time to change actions and I might just save another extra calculation for every flight I plan.

What do you think?