Proficient Pilot: How slow can you go?

September 1, 2008

Barry Schiff retired as a captain for TWA in 1998.

The Aeronca Champ in which I learned to fly had a four-cylinder, 65-horsepower engine that sipped only 3.5 gallons of avgas per hour. At 30 cents per gallon, the pug-nose little “Airknocker” burned only a buck’s worth of fuel every 60 minutes. Avgas was a minor expense.

Flash forward 54 years. As I write this, fuel is selling at Signature Aviation in Santa Barbara, California, for more than $7 per gallon. If that is not pricey enough, Signature’s facility in Las Vegas is selling the precious petrol at $8.56 per gallon. Fuel has suddenly and obviously become a major consideration in the cost of operating an airplane and reducing the total amount consumed each year can pay handsome dividends.

Most pilots intuitively know that reduced power and cruise speeds result in less fuel consumed over a given distance. A handy way to calculate fuel savings is by using the specific range of an airplane, a concept that I discussed with respect to turbine operations in “ Turbine Talk: Specific Range” (June AOPA Pilot). This concept applies equally well to airplanes with reciprocating engines. Specific range is a fancy aeronautical term similar to what we refer to as gas mileage when driving an automobile. It is easily determined from the cruise performance charts found in pilot’s operating handbooks.

A Cessna 172S Skyhawk, for example, consumes 10.4 gph at 77-percent power when flown at 6,000 feet on a standard day. This results in a true airspeed of 122 knots. Specific range is found by dividing airspeed by fuel consumption, which in this case is 11.73 miles per gallon. Near the bottom of the speed spectrum, cruising at 45-percent power yields a fuel burn of 6.7 gph and a true airspeed of 94 knots. Specific range, therefore, is 14.03 mpg. Flying at reduced power in this case represents a 20-percent improvement in mileage.

If slogging along at 94 knots is not for you, then perhaps you might want to compromise. The point is that the slower you fly, the less fuel you will consume. Those flying with a heavy hand on the Skyhawk’s throttle, for example, will consume 17.1 gallons during 200 nautical miles of cruise flight. Those willing to enjoy the scenery at a more leisurely pace (and log more flying time in the process) will use only 14.3 gallons to cover the same distance, a saving of 2.8 gallons.

I understand the resistance to fly at reduced power. After all, we fly for speed, to get somewhere as quickly as we can. My personal technique of late is to use normal cruise power when flying into a headwind and reducing power when getting a helping hand from a tailwind. I operate at reduced power settings when the wind is calm.

There are times, though, when we are going nowhere in particular, such as during local flights when we fly simply to wash our wings in the wind or to practice maneuvers or to introduce a friend to our magical world. Pushing the throttle to the firewall at such times is a waste; there is no benefit to expending the additional energy. Retarding the throttle to a sub-cruise power setting can save substantial fuel and reduce the noise level as well.

Those who rent airplanes and pay an hourly rate based on the unrelenting accounting of a Hobbs meter are penalized for saving fuel during cross-country flights. There is no incentive for them to operate efficiently at slower airspeeds. Flight schools and pilots could benefit from a return to charging on the basis of a recording tachometer. Perhaps hourly rental rates can be developed that consider both Hobbs and tachometer readings in a way that encourages pilots to operate more conservatively.

A discussion about saving fuel inevitably leads to questions about maximum range, which is the greatest distance that can be flown per gallon of gas. Maximum endurance, which often is confused with maximum range, is the greatest period of time that an airplane can remain aloft per gallon of gas. The problem with attempting to achieve maximum range or endurance is that these operations require such low airspeeds that most pilots do not have the requisite patience. It requires flying so slowly that airframe manufacturers do not even bother to publish the target airspeeds.

Rules of thumb, however, are available for propeller-driven airplanes. Maximum range, the greatest number of miles available per gallon of avgas, requires approximately the same speed as that used for best glide. (It increases somewhat when flying against a headwind and decreases when under the influence of a tailwind in the same way that the speed for best glide speed varies according to wind conditions.) Because the speed used for maximum range is an indicated airspeed, cruising for maximum range becomes less agonizing with increasing altitude because slow indicated airspeeds translate into increased true airspeeds.

Want to remain aloft for as long as possible? The speed for maximum endurance is typically about 25-percent less than the speed used for maximum range (or best glide). It can be determined during flight by slowly reducing power until finding the minimum power needed to maintain altitude. It results in the minimum fuel flow and is used when loitering without a destination.

Even considering the high price of fuel, however, it is unlikely that many pilots will be sufficiently motivated to fly at such a snail’s pace.