Plane & Pilot
Tuesday, July 31, 2012

Never Run It Dry

Keeping track of the time/speed/distance equation is only part of fuel management

The second year I had the Comanche relicensed, however, the mechanic commented there was a significant ding in the left tank. Though I had never run the Comanche dry, I'd noticed that the left tank always seemed to take less fuel than the right. Lesson learned.

Fueling on level ground was another part of my fuel-management education. Once, on a ferry flight to South Africa in a Cessna Caravan back in the 1980s, I parked on a slightly slanted ramp in Libreville, Gabon, and fueled the airplane for the next day's early departure. The following morning when I walked out to the Cessna, there was a huge fuel stain under the downsloping wing. I called the truck back for a top-off, and discovered I had overboarded 30 gallons of Jet A onto the ramp. I departed some $200 poorer but at least that much wiser.

There's a second situation that can result in misfueling. Both struts need to be equally depressed or you could wind up with a fuel imbalance even on level ground.

Okay, so you know the airplane is perfectly level when the truck arrives. There's yet another way to wind up with less fuel than you planned. Some models with high dihedral, the Piper Malibu for one, are prone to trapping air at the tips. The fuel level will be right up to the lip, but you may wind up with less than the maximum 120-gallon capacity (on the Malibu).

Once, in Reykjavik, I went out to a new Mirage one morning, popped the caps, shook the wings to release any trapped air, and the level dropped an inch on one side and two on the other. It required five additional gallons.

Now that you're reasonably certain you have truly full fuel, you can make some intelligent estimates of burn and endurance. I've had so many experienced pilots suggest deducting an automatic 5% from known capacity that I do it all the time. If you normally fly with 80 gallons in full tanks, you might want to consider using 76 gallons as max and do all flight planning based on that number. On my Mooney, I assume I'm starting with 61 rather than 64 gallons.

If your airplane has a combination engine analyzer/fuel totalizer such as the JPI EDM-800, that's the best possible protection against a dumb mistake. For those with a reasonable idea of fuel burn but no totalizer, you can make intelligent estimates that will work nearly as well.

My airplane burns about 11 gph at max cruise, and I subtract that figure as my reserve. That means I have 50 gallons to work with on any trip. As a rule of forefinger, I typically multiply burn at 75% by 1.4 for the first hour (that will vary with weight, temperature, final altitude and turbulence, but it should be close), then determine what will be left for the en route portion. In this case, I'll burn about 16 gallons in the first hour, so I'll have 31 gallons plus reserve remaining. That's about 2.8 hours of endurance.


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