Plane & Pilot
Tuesday, July 31, 2012

Never Run It Dry


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



According to the AOPA Air Safety Foundation, fuel exhaustion or mismanagement are common causes of accidents, ranking fourth behind landing, takeoff and maneuvering at low altitudes. Bill Cox once ran a tank dry on a Cherokee Six but was able to restart the engine.
Fuel Management
The AOPA Air Safety Foundation reports that fuel exhaustion or mismanagement are all-too-common causes of accidents, generally fourth behind landing accidents, takeoff incidents and maneuvering flight at low altitude. Fuel exhaustion is exactly what it sounds like—running the tanks dry. Fuel mismanagement relates to landing with fuel still on board but inaccessible because of a system problem, because the pilot didn't know he had it available or simply forgot to change tanks.

Fuel management isn't really that tough these days, considering that totalizers keep almost perfect track of fuel burned and remaining. Even modern aircraft fuel gauges are more reliable than they used to be. In fact, managing fuel use was never that difficult to begin with, provided you knew how much you had on board, how much you were burning and when you departed. Assuming there were no leaks, the answer was a simple problem in elementary math. The difficulties arise when you don't know all three of the items above. Trouble is, many pilots are convinced they do know how much fuel is in the tanks when, in fact, they have only a vague idea.

Let's consider fuel capacity. According to the book, I can carry 64 gallons in my Mooney…or can I? I bought my airplane in 1987 and knew it had never been wrecked, so it was reasonable to assume the tanks were not deformed and still in the original shape. Fortunately, I had my Mooney's tanks resealed a few years ago, so I had the perfect opportunity to determine true capacity. Every ounce of fuel had to be drained in order to reseal the tanks, and that meant I was starting from true empty.

Accordingly, I pushed the airplane out to a level ramp, with no apparent list left or right. It was mid-morning, and the temperature was about 60 degrees F, pretty close to standard, so fuel density wasn't a concern. (Some long-distance flyers, in search of maximum range, have their fuel supercooled and pumped aboard at the last possible minute, then climb quickly to high altitude and burn the top off each tank before the avgas can expand and overflow.)

When the fuel truck arrived, I asked the fueler to pump the 100LL slowly so there would be less chance of an air bubble. While he pumped, I shook the wing at the tip to help any air escape. Then, I watched carefully to make certain the level came to the exact bottom of the filler neck.

When the fueler was done, the meter suggested I had taken aboard 33.1 gallons in the left tank and 33.4 gallons in the right, a total of 66.5 gallons, 2.5 more than maximum. According to Mooney, that's all usable, so I could assume that figure for flight planning. I don't. I use the standard 64-gallon capacity instead.

A deformed tank can be more common than you might imagine, and any deformation will almost always rob you of fuel capacity. After a friend with a Comanche 260 died of a heart attack many years ago, his widow asked me to maintain his airplane for her, taking it out for a walk every two months or so. She swore she'd never sell it, as it had been her late husband's beloved toy. Finally, reality intervened, and she asked me to sell it for her.



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