Staying on the safe side of the gauges
It's laughably easy to avoid running the tanks dry. Here's the not-so-secret, can't-possibly-fail method: Before taxing away from the chocks, be sure you've got enough fuel aboard to make it to your intended landing, plus an adequate reserve to handle unexpected delays or changes in your route.
Pretty simple, huh? Yes, it is, but then again maybe not. It's possible to encounter a scenario where running low on fuel, if not running out, is a looming possibility if you aren't paying attention. I recently flew a long trip in my airplane that called for some creative flight planning. The challenge was not a difficult route, rugged terrain, or deteriorating weather. No, on this flight the issue was how much fuel to have aboard, and when.
I calculated the total flying time to my final destination at about three hours 30 minutes. No fuel problem there--my airplane has enough tank capacity for five hours of nonstop flying. However, much of this flight would be over water. If I did not stop for fuel before my final destination, I'd have to fly for 45 minutes on the return leg before reaching the first available island that had fuel. That's four hours and 15 minutes of flying before refueling, leaving 45 minutes of reserve fuel.
That should be more than enough, except for the what-ifs. Such as, what if the single runway serving the island airport that had fuel was closed for some reason, or the fuel truck was broken down, or the tanker that supplies the airport was late and they had run out? What if the winds were far stronger than forecast, causing me to use more fuel than I planned over the full route? What if a thunderstorm settled over the fuel-stop airport for an hour or more?
So, the what-ifs dictated that I stop on the way down, top off the tanks, and then make the 45-minute hop to my final destination. Except that the runway at the destination airport is short and unpaved, so I wanted to be as light as possible for the landing--and for the takeoff a few days later. Topping off the tanks meant I would be heavy, and I did not want to be heavy.
With all of those factors in play, I sharpened my pencil, constructed various scenarios, and came up with a workable flight plan that would keep me on the safe side of the fuel gauges. I would land at the island fuel stop on the way down and take on an hour's worth of fuel. That would impose only a minor weight penalty for my arrival and departure from the short destination strip, yet give me a generous 90 minutes of reserve fuel for the first leg of the return flight. That would be more than enough to handle any surprises that might pop up on flight back to the same fuel stop, where I'd take on a generous amount for the long flight back to the mainland.
I'd had to strike a balance between a number of competing factors--flying to the destination without refueling so that I would be light for the landing, or filling the tanks and being too heavy. The compromise I arrived at worked out just fine.
So the can't-fail method of never running out of gas--have enough on board, plus an adequate reserve--is not always so easy to figure. You have to weigh all of the factors, some of which may not be obvious at the time. The hard part is deciding how much reserve constitutes adequate.
The FAA says that for VFR flights you must have enough fuel to fly to the destination airport, plus 30 minutes (45 minutes if it is a night flight). But is that really enough? You can eat up 30 minutes in a hurry, even in good weather.
Suppose, for example, that you encountered strong headwinds on your flight, which increased the trip time a significant amount. Let's say you are making a 360-nm trip in a 120-knot airplane that burns, on average, 10 gallons per hour. In a no-wind situation the flight should take three hours and use 30 gallons of gas. However, with an average 20-knot headwind, the trip will take three hours 36 minutes and consume 36 gallons. If you had planned on a three-hour flight and put only 30 minutes of reserve fuel in the tanks, you'd be a gallon shy of making it.
Failure to lean properly has the same effect as headwinds in terms of increased fuel consumption over the length of the trip. It has been estimated that setting the throttle and mixture for best power imposes a 15-percent fuel penalty compared to best-economy setting. Failure to lean at all--leaving the mixture control in the full-rich position while at cruise altitude--can easily increase fuel consumption by 20 percent or more. That's like taking off with 40 gallons instead of the 50 you thought you had.
That hypothetical runway problem I mentioned earlier? It has happened to me. I was flying a colleague to a one-runway general aviation airport in Pennsylvania, and when we arrived overhead I discovered a large "X" marking each end of the runway. It was closed for resurfacing. I had neglected to check notams for the destination airport. Fortunately, I had plenty of gas to make it to another nearby airport, but what if I had only planned on a 30-minute fuel reserve? By the time I reached the alternate airport I'd have been sweating bullets.
Another more subtle but still powerful factor that contributes to fuel exhaustion accidents is the old "get-home-itis" disease, a dangerous malady that afflicts pilots eager to wind up their road trips and get back to their own beds. It can blur judgment to the point that the pilot sidesteps caution--landing short of the destination to take on some fuel--and proceeds into dangerous, risk-taking territory. When home beckons it is possible to convince yourself that, despite what the gauges, the clock, and your gut are telling you, there's enough gas in the tanks to make it.
There's more to fuel planning than simply filling the tanks. You have to take into consideration all of the known factors when figuring the fuel needed for the flight, then add enough reserve to cover the unknowns--plus some.
Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and co-owner of a Piper Aztec.
