Fill 'Er Up?

Why do GA pilots top off the tanks?

December 1, 2005

It's a rare day that a pilot of a light general aviation airplane doesn't head for a destination with a full tank of fuel. The only barrier to this practice is almost always going to be some kind of performance consideration, such as a short runway, a full airplane, a hot day, or a combination thereof. If you fly a one-hour flight, you will probably still have close to three hours of usable fuel on board when you land.

Contrast this with the airlines. If you have ever been on an airline flight that's had to divert, you have probably been made all too aware of the fact that the amount of fuel in the tanks for holding is often a very small amount relative to the rest of the load. Why is this? After all, it costs money to have to take the passengers someplace other than their intended destinations.

The answer is not as simple as it seems, but it provides some insight into why we do things differently in our smaller airplanes. For those who own their airplanes, they are taught from their earliest lessons to store the airplane with full fuel tanks in case water should get into the tank, either from condensation or from rain. If the airplane is always kept in a hangar, rain (or snow) water is not as much of an issue. That isn't to say that keeping the tanks full is not a good idea, because it is. It's justifiable by the simple fact that most airplanes don't fly every day. Business jets fly more often, and airliners fly every day.

Second on the list of differences is that airliners are usually weight-limited for landing. While a Cessna 172 has a maximum takeoff and a maximum landing weight, which are each about 2,400 pounds (depending on the model), if it takes off at max weight, it will land at less than max weight by way of the fuel burned in the pattern. Even then, most light airplanes are certified to land at their maximum takeoff weight. The only time landing overweight should be an issue is if the airplane is properly — and legally — overloaded for an FAA-approved ferry flight. In that case, an auxiliary fuel tank will probably be installed, and if an emergency should develop before enough fuel has been burned, a landing may have to be made at more than the published maximum takeoff weight. The airplane should be inspected after the landing.

Larger airplanes have much larger discrepancies between takeoff and landing weights, and for an airliner, it isn't unusual for the maximum takeoff weight to be determined by the maximum landing weight. That means the takeoff weight is determined by adding the aircraft's landing weight and the anticipated fuel burn together. From that is subtracted the aircraft weight and the weight of the payload. The difference is the amount of fuel that can be carried. Most of the time, it will not be a full load, especially on shorter flights.

If after takeoff the flight develops an emergency and must land, there are two options. The first is to land overweight on a suitable runway and then subject the airframe to an overweight-landing inspection. The second option, and one that is generally only available to larger aircraft, is to dump fuel. Both options are expensive.

Are the fuel gauges of a jet more accurate than a Cessna Skyhawk's? You better believe it. Smaller airplanes, especially older smaller airplanes, have notoriously inaccurate fuel gauges. You can count on the gauge being accurate only when it is reading empty; that's the only time it is required to be accurate. Sobering thought. In addition, it's also difficult to accurately gauge how much fuel is in a partially used tank before starting a flight. Best solution for these problems? Carry lots of gas. So, we do, even for a short flight when said practice may not be necessary. Is this because we don't trust our ability to use a watch to determine how much fuel is left at a known burn rate?

The final reason that larger airplanes don't carry any more fuel is cost. Fuel is expensive to carry because of its weight. A percentage of the fuel on board, usually in the range of 3 to 4 percent, is used just to carry the fuel. On an airplane with 100,000 pounds of fuel, that means as much as 4,000 pounds of fuel is on board just to transport the weight of the fuel that is on board. Start doing the math on the number of flights and the number of airplanes per day in an airline's fleet, and...well, that's a lot of fuel. Sometimes, though, since airlines and corporations are able to negotiate better rates at their own hubs or bases, they will try to avoid buying any more fuel than necessary at an outstation. That "home field" discount is often substantial, especially when you look at airlines the size of American in Dallas or Continental in Houston, so if the passenger/cargo loads work out right, the flight dispatchers can often tanker some extra fuel from the hub (or from the home field for a business jet) for the return flight, instead of buying the more expensive fuel at the outstation. This practice can save quite a bit of money. Airlines have figured out how to balance the overall equation in their favor through years of experience with their fleets.

But even this practice is being reduced to some degree in times of economic hardship for the airlines. Fuel is more expensive than ever, and now more companies are filing flight plans for higher altitudes. In order to do this, some flights need to carry less fuel in order to have any hope of being able to climb to the higher flight levels, especially in the summer. The Canadair Regional Jet, which my company flies, is known to have lousy climb performance when equipped with older-model engines, so to get the higher altitudes, we need to be flying either longer legs or shorter legs with less fuel, especially during the summer. Still, a few thousand feet can mean substantial fuel savings, especially if descents are started as late as possible.

Reading the pilot's operating handbook for our general aviation airplanes, we can garner a pretty good idea of our fuel burn under known conditions. Throw in our minimum reserve fuel, plus the fuel we'll burn en route, and it stands to reason we should not need to carry full tanks on a flight that won't require a full tank. Even though, with practice, the average private pilot can get from here to there pretty much on the flight-planned route and on time, we still carry as much fuel as we can squeeze into the tanks. Is it necessary? No. We certainly don't do this in a car. Who fills up a car for every trip? Nobody. But pilots do it all the time.

Why do we do it? Aside from the reasons stated above with regard to storing an airplane with full tanks, there often may not be a reason to do it. Even in flight school operations it is typical to see a trainer leave the blocks with a full tank, fly for an hour, maybe two, land, and get refueled and then fly for an hour. It could just as easily have flown without refueling. So why do we do it?

The most common response is weather. If we are flying a trip when weather might be an issue, it's better to have extra fuel in order to deviate or divert to an alternate. That's true, but that doesn't stop so many pilots from loading up on days when poor weather simply is not going to be a factor. In a twin it might make sense because if an engine fails, you can fly farther for a suitable airport on one engine with extra fuel...assuming the temperatures aren't so high that just maintaining altitude is going to be a problem. In a single, you could argue the opposite: Carry as little as possible in case of a fire.

Back to square one: Why do general aviation pilots refuel to full tanks when it isn't operationally necessary? Is it a good idea? Well, it's our money to spend, it is an ingrained habit that is hard to break, and as the old saying goes, you can't have too much fuel unless the airplane is on fire. But, simply put, it just makes us (or our CFI) feel better.

What more reason do we need?

Chip Wright, of Hebron, Kentucky, is a Canadair Regional Jet captain.