Continuing Ed

Treat Fuel Quantity Gauges With Skepticism

A crack Army Reserve helicopter pilot got checked out in a Mooney. On his first trip, he nearly ran out of gas long before reaching his intended destination. The pilot, who until then had flown only turbine helicopters, had not been taught how to lean the mixture in a piston-powered airplane. In cruise flight he left the mixture in the full-rich takeoff position, which meant the engine burned 25 to 30 percent more gas than if he had leaned. It was an unforgettable lesson in fuel management.

The term fuel management sounds pretty officious when applied to a light, single-engine aircraft. What's to manage? Take off with the tanks full, and land before they are empty. Sounds simple, but there's more to it than first meets the eye. In fact, when we sign up for primary flight instruction we also enroll in Aircraft Fuel Management 101. And, like the business school frosh destined for an MBA, we need to acquire more sophisticated fuel management skills as we progress into more sophisticated aircraft.

Fuel school includes classes in three basic subjects: the fuel system, aircraft performance, and weight. Systems class begins with the preflight inspection - physically looking in the tanks to confirm that there is enough fuel for the mission, and draining the sumps to check for water contamination and ensure that the tanks do indeed contain avgas and not jet fuel.

The systems class also covers the fuel selector, fuel quantity gauges, and the auxiliary fuel pump if the airplane is so equipped. The simplest fuel selector has a Both position, meaning that the engine draws fuel from both the left and right tanks at the same time. However, you can correct an imbalance - when one tank has more fuel than the opposite tank - by moving the selector to the fuller tank until the balance is restored.

If the selector does not have a Both position, basic management technique calls for taking off and landing with the selector set to the fuller tank. In flight, use the selector to maintain an approximate balance between the left and right tanks.

The lesson to learn about fuel quantity gauges is to treat them with skepticism. Experience in the airplane will tell you if the gauges are anywhere close to being accurate. Experience also teaches that the best fuel quantity gauge is a clock. Once you learn the fuel consumption habits of a particular airplane in climb, cruise, and descent, you can use a clock to accurately determine fuel used and fuel remaining - provided that you know how much fuel was in the tanks when you started the engine.

Proper use of an auxiliary electric pump is another fuel management issue. A simple high-wing single may employ gravity as an infallible alternative to an aux fuel pump. Its low-wing counterpart, however, needs the help of an aux pump in the event that the engine-driven fuel pump fails. Normally, an aux fuel pump is switched on for engine start, takeoff, initial climb, when switching tanks in cruise flight, and during approach and landing, but there may be other situations when it is needed. Check your management textbook - the pilot's operating handbook - to find out what those may be for the airplane that you fly.

Performance-based fuel management begins with leaning the mixture. Leaning is done to achieve a fuel/air ratio that results in optimum combustion, horsepower, and fuel economy for a given density altitude and throttle setting. Leaning may not be much of an issue on local training flights, but it's a basic fuel management task on cross-countries, especially when cruising at altitudes above 3,000 feet and when taking off in high-density-altitude conditions. The POH is the basic text for learning how to lean. A flight instructor experienced in the airplane ought to be able to provide useful insight on various leaning techniques and their effect on performance and fuel consumption.

Weight-based fuel management comes into play when the weight of people and bags - the payload - combined with the weight of the fuel exceeds the allowable useful load of the airplane. In that case, you have two choices: take off with less than full fuel, or reduce the payload. Obviously, it pays to do the fuel weight and payload planning before going to the airport. It's a hassle to have to offload fuel or, worse, pick someone or someone's bags to leave behind on the airport ramp.

Fuel management gets more interesting as you work your way up the ladder in terms of aircraft complexity, weight, and performance. For example, if the airplane has been modified with optional auxiliary tanks, you'll need to carefully study the flight manual supplement to learn how to manage the fuel. The aux tanks may deliver fuel to the main tanks or the engine-or both at the same time. Obviously it's important to know which, and how and when to select certain tanks.

One popular fuel system option in higher-performance airplanes is a fuel totalizer. When teamed with a GPS, these electronic devices typically provide all kinds of useful fuel management information - real-time fuel flow, fuel used, fuel remaining, gallons remaining at destination, endurance, and so on.

The Achilles heel of most units, however, is beginning fuel quantity. It's up to the pilot to program the device with the exact amount of fuel in the tanks. If you don't get that figure correct it means that all quantity-based calculations performed by the totalizer will be inaccurate. Garbage in, garbage out. Avoid managing garbage by studying the flight manual supplement for the fuel totalizer and paying close attention when the airplane is fueled.

The worst that can happen to a failing business manager is to get the old heave-ho from the boss. A business school student who doesn't cut it gets a failing grade. The potential consequence for a pilot who mismanages the on-board fuel supply is far more serious than a simple firing or failing grade. Do the homework, pay attention in class, and you'll pass with flying colors every time.

Mark Twombly is a writer and editor who has been flying for 35 years. He is co-owner of a Piper Twin Comanche and recently obtained his commercial multiengine rating.