Never Run It Dry

On the trip back, we made a stop in Fort Lauderdale to clear customs, turn in our survival gear and close our international flight plan, then relaunched for the short hop diagonally across the state to Venice on the Gulf Coast.

My parents were luxuriating in the back of the big Six as we cruised 6,500 feet above the swamp when the engine suddenly quit cold. The immediate silence got everyone's attention, especially mine. I was the number-one son, and mom trusted me implicitly in any airplane. I didn't want to dispel that trust by doing something stupid, though it seemed I already had.

Of course, I had let one of the Cherokee Six's four tanks run dry, and the engine had shut down in a heartbeat, without a telltale tick of the fuel flow or any other forewarning. As calmly as I could, I turned on the fuel pump, then, feigning a casual motion, reached down and switched to a tank with some fuel in it. I turned to Mom and Bob in the back seat, summoned what I hoped would be a reassuring smile and said, "Sorry about that. It's no big problem. I just ran a tank dry. The engine will pick up in a few seconds."

I turned back forward, expecting power to return at any moment. I waited and waited. Nothing happened. We were gliding down toward Lake Okeechobee, and I was beginning to wonder if we were about to discover firsthand that the lake was only five feet deep as I had read.

Finally, after perhaps 20 seconds that seemed more like 20 minutes, I heard some expectant coughs from the Lycoming before it came slowly back online. We continued to Venice, and Mom's only comment after we landed was, "Does that happen often?"

Fortunately, if you're smart enough to plan ahead, it need never happen even once. I obviously wasn't and didn't, so it did.

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.

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.

At 150 knots cruise and perhaps 15% less during the first hour, I expect to fly about 127 nm in the first hour and another 428 nm during the remaining 2.8 hours for a total no-wind range of 555 nm (plus the aforementioned one-hour reserve). Also, remember that I've built in a deliberate three-gallon hedge, at least another 15 minutes of endurance. Add all that to the extra 2.5 gallons in my airplane's strangely enlarged tanks, and there's more like a "free" 30 minutes of additional reserve.

Trying to pin down range precisely assumes you know your airplane well, can count on a given true airspeed and that you'll be flying in calm air with minimum turbulence. It also assumes the winds will be calm (rarely the case).

I could increase range slightly by using a reduced power setting, but Mooneys seem to gain very little range at lower power, so I stick with at least 70% nearly all the time.

I've always used two simple rules to handle the effects of wind. These were taught to me by a highly experienced ferry pilot. Donn flew a little of everything across the Atlantic to Europe in the halcyon days of the '70s and '80s.

Donn's rules were simple: Reduce any forecast tailwind by a third and add 25% to any headwind. The most likely consequence of such a policy is that you'll arrive at your destination a few minutes early and with more fuel than planned. Works for me.

Actual fuel management is usually fairly simple on most general-aviation airplanes with one fuel tank in each wing and a simple selector that has left, right and off positions. I'll generally depart on one tank, usually the left if I'm alone in the airplane, ascend to my chosen altitude, set up the airplane for cruise; then, switch to the opposite tank for an hour and alternate as necessary.

Another peripheral concern if you have two tanks is to make certain the selector works properly. If you run a tank dry, reach down to switch tanks and the selector refuses to move or comes off in your hand, you have a real problem. That's one reason I always switch before any tank gets below about three gallons, providing me a 15-minute hedge to find some place to park.

After my experience in Florida (and a few others before it), I never run a tank dry. I'm not a complete idiot (some parts are missing), but I'm fairly certain Murphy's law will have a tough time catching up with me. I can lose some fuel to overboarding or an air bubble, the wind can become stronger than forecast, consumption can increase, I can suffer rerouting by ATC, the airplane can pick up some ice---slowing my speed---or I can be forced to deviate around weather, and I can still make it to my destination or a reasonable alternate. At least, that's what I keep telling myself.