There is no greater concern among pilots and airplane owners today than the cost of fuel. Prices vary widely from airport to airport, but $5 is often on the low end and $7 a gallon is not the top. And in many instances jet fuel costs more than avgas, a reversal of traditional pricing. Fuel costs for most airplane owners have doubled in the past year and nobody can predict future trends, but higher prices sure seem more likely than lower.
So now we all want to own and fly the most fuel-efficient airplane, but what is it? There is no single answer because as with all desirable characteristics in an airplane we always must trade one attribute for another. If fuel were really the only driving factor in finding the most efficient aircraft, powered parachutes and motorgliders would win hands down. Or everybody in a jet would cram into a piston-powered airplane because jets can't match the fuel efficiency of a reciprocating engine driving a propeller. What we really want is not the most fuel-efficient airplane possible, but the most thrifty one that suits our mission.
The aviation industry uses a metric called specific range to measure fuel efficiency. Specific range is the number of miles -- normally a fraction except for piston singles -- that an airplane flies through the air per pound of fuel consumed. For example, a piston airplane with a true airspeed of 150 knots while burning 12 gallons per hour (72 pounds) would have a very good specific range of 2.08. A business jet cruising at 440 knots true burning 1,200 pounds per hour (pph) has a specific range of 0.37, good for a jet.
Specific range can be calculated for the cruise condition, as I have done above. But a more useful measure is specific range for the entire trip. When fuel consumed for start, taxi, climb, approach and landing are all measured against the block speed, the specific range will be worse than for cruise.
Though we buy fuel in volume -- by the gallon or liter -- and the tank's capacity is determined by volume, engines burn fuel and air in mass, so measuring fuel consumption in pounds is more useful. At standard temperature of 15° C (59° F) a gallon of avgas weighs about 6 pounds. A gallon of jet-A fuel at standard temperature weighs in at about 6.7 pounds. Avgas density changes very little over the normal temperature range, but jet fuel density can vary by several percent, so a gallon of really cold jet-A is going to take up less space in the tanks than a warm gallon, but the useful work of the fuel will still be measured by the pound.
By using the specific range calculation it's easy to compare airplanes to one another or to measure the efficiency of power settings. For example, let's say that a powerful piston single can cruise at 140 knots while burning 72 pounds per hour (pph), which equals 12 gallons per hour. That yields a specific range of 1.94. If you increase the power and fuel flow to 132 pph and the true airspeed climbs to 180 knots, the specific range is down to 1.36. The airspeed increased by about 22 percent, but the fuel efficiency decreased by about 30 percent.
As you can see, the fuel efficiency of a piston airplane is largely in the hands of its pilot. More speed equals less efficiency. The greatest specific range airspeed for a piston airplane is so slow that few of us would ever contemplate it unless we absolutely had to stretch the fuel all the way back to shore. In general, an indicated airspeed at about the best rate of climb speed is also the most fuel-efficient speed in level cruise. The heavier the airplane, the higher its maximum efficiency speed, so to achieve true maximum range you must continuously slow down as fuel burns off to maintain the same angle of attack and thus drag.
This amazing gain in fuel efficiency at lower speeds was driven home to me flying to Oshkosh this past summer. Controllers told me to pull the indicated airspeed back to 120 knots to stay in trail of slower IFR traffic ahead. My Baron was burning about 30 gph to indicate 170 knots, but it took about 16 gph to hold the 120 indicated. Fuel flow was nearly halved to cut airspeed by less than a third.
Pilots of turbine airplanes actually have less control over the fuel efficiency of their flights because there are so many variables, first among them being air traffic control. Turbine engines are at their least efficient down low where the air is dense. As the airplane climbs and the air thins, the turbine produces less power and thus consumes less fuel, but the drag of the thinning air on the airplane decreases faster than the power from the engine drops, so the airplane speeds up and the fuel flow goes down. There is an optimum altitude for every turbine airplane at its present weight and any level lower than that optimum decreases fuel efficiency. In crowded airspace the pilots of turbine airplanes seldom are cleared for unrestricted climb to the optimum altitude, so the airplane doesn't come close to matching its potential specific range. And on the ground at idle a turbine burns a surprisingly large percentage of its optimum cruise fuel flow, so takeoff delays really cut into fuel efficiency in a jet compared to a piston engine.
