Range vs. endurance – cruise control for pilots

Do I plan the flight for best range? Do I plan it for best endurance? Does it matter? During checkflight oral examinations, I’ve found that most pilots can correctly differentiate between range and endurance, however, many pilots are unclear about what factors affect range and endurance. Subsequently, they have difficulty deciding which to optimize during the flight. The root of the problem is that the pilot may not ask themselves the right questions during preflight planning. Knowledge of the factors affecting range or endurance may also be lacking. Let’s look at some of these questions.

First, how does one decide on planning for best range as opposed to best endurance? This is normally a matter of distance to fly, fuel available, and winds aloft. But more basically, the pilot needs to address one main question: "Am I trying to get as far as the aircraft will go, or am I trying to stay aloft as long as I can."

If the pilot decides that getting to the destination quickly is more important than saving fuel, then they should select the maximum allowable power setting. However, if the goal is maximize the distance for the given amount of fuel, a power setting less than maximum must be used and the engine properly leaned. Of course, if building flight time is the primary goal, then the power setting for maximum endurance is desirable.

For instrument pilots, the trade-offs between a best range power setting and best endurance setting are clear. If it becomes necessary to circumnavigate bad weather, speed should be reduced to increase the range. If holding becomes necessary, again, airspeed should be reduced through a reduction in power to that specified for maximum endurance. The tradeoff is speed for efficiency; namely lower fuel consumption.

Now that the distinction between range and endurance is clear, let’s consider the specific factors that affect both: configuration, weight, speed, thrust, wind and temperature.

Any increase in drag effectively reduces performance and increases fuel consumption per mile. While flap extension will reduce airspeed, which is a necessity for fuel economy, they are reducing the speed due to a drag increase as opposed to a power reduction. Therefore, flap extension of any amount, while increasing lift, will decrease forward speed and increase fuel consumption per mile. Open cowl flaps not only decrease performance, but add no lift while increasing fuel consumption. Cowl flaps should be closed during cruise flight as long as engine temperatures are normal.

Weight has semi-obvious implications for performance. As weight increases, the power required to fly increases. Any increase in weight will require an increase in angle of attack in order to fly at a given airspeed. In order to fly at a more fuel efficient angle of attack, airspeed must be increased. Increased airspeed requires excess power and, once again, increased fuel consumption. The tradeoff between low airspeed and low angle of attack is hard to conceptualize here, but the fuel consumption per mile used to fly faster at the lower angle of attack will be less than the fuel consumption per mile used to fly slowly and at a higher angle of attack.

The airspeed at which an aircraft is most efficient is normally near the maximum lift/drag (L/D) speed. Performance charts for most aircraft will indicate appropriate speeds in order to achieve efficient flight. But one important point is left out of most charts. Since the stalling speed of the aircraft decreases during flight due to the reduction in weight from fuel burn, the maximum L/D speed will likewise decrease. As the indicated airspeed increases during the flight (due to loss of fuel weight), the pilot is likely to be thrilled and leave the power setting alone. However, the pilot’s folly is that the extra airspeed is actually hurting overall maximum performance more than helping it. In reality, the correct action is not only to reduce power so that the airspeed does not increase, but also to slowly reduce airspeed below the original setting to account for the lower L/D speed. This reduction is probably negligible for a light aircraft given the relatively short distances we normally fly.

Thrust is the basic power output of the reciprocating engine. In general, the thrust required to propel an aircraft at a given airspeed is the same near the ground as it is at high altitude. However, when you fly at higher altitudes, the true airspeed is higher and that same amount of power is now spread out over a longer distance per unit of time. The result is that the higher you fly, the larger the power effect. Of course, in the absence of supercharging or turbocharging, there is also some intermediate altitude limit that maximizes performance due to the loss of air pressure to support engine combustion and subsequent power output. In spite of a slight reduction in endurance for higher altitudes, the maximum range achievable will actually increase slightly as cruise altitude increases.

Many people believe that a tailwind is God’s gift to the pilot. Well, they’re right, but only under one circumstance. The only way to come out ahead when flying with a tailwind is if you plan not to return to the same point of origin with that same tailwind now as a headwind. Ok, I lied! Actually, it is possible to fly as efficiently in the wind scenario described above. When flying in the tailwind condition, it turns out that a TAS airspeed power reduction from the no-wind maximum range TAS of approximately 25 per cent of the wind velocity will maximize fuel efficiency. When returning with the headwind, adding 25 per cent of the wind velocity to the no-wind maximum range TAS will minimize fuel consumption. Sound like too much math? You’re probably right, but some people like to get the maximum efficiency out of their aircraft. Please note however, that it is impossible to get to and from a destination with any kind of crosswind component in the same amount of time as in a no-wind condition. That extra time is created by the requirement for a wind correction angle to stay on course.

Although most people cite little change in performance due to temperature changes other than vertical speed, it actuality, the loss can be dramatic. Reciprocating engines can lose up to five per cent simply due to extreme hot temperatures and another five per cent for high humidity. Chock it all up to the effect of lower density altitude. The engine just can’t produce as much power is those conditions. So in order to achieve the ideal TAS for either the best range or best endurance, more power will be required and the fuel consumption increases.

These are the critical factors when determining how to get the most performance for the least cost. With a good deal of forethought, it is possible to fly light aircraft very efficiently. But I suspect that most of us are not quite that concerned about spending forty or fifty extra dollars if we can get there ten minutes faster. In any event, should you need to get that range or best endurance that your aircraft has to offer, hopefully this months column will help you figure out how to achieve that goal.

Donald Anders Talleur is an assistant chief flight instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Pilot Training and Aviation Research Laboratory Departments. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 50 aviation related papers and articles and is also working on an M.S. degree in Engineering Psychology at the University of Illinois.