Baby, Let's Cruise

The cruising phase of a flight--the straight and level portion between the climb and the initial descent--is statistically the safest and longest of the entire flight. The cruising altitude is determined by meteorological, regulatory, and air traffic control factors. The most important consideration is a safe, smooth ride for the passengers. After determining the altitude that will afford a smooth ride, the winds are considered. An altitude with the strongest tailwind will maximize ground speed and minimize flight time and cruise fuel. The wind at cruise altitudes often exceeds 100 knots; strong rivers of wind called jet streams can have speeds of more than 200 knots.

The colder the air temperature, the more efficiently turbojet engines can operate. The coldest air is found at the atmospheric level of the tropopause. Here, the ambient air temperature decreases by about three degrees every 1,000 feet and stops dropping at around 70 degrees below zero. The altitude of the tropopause varies with the latitude and the season. At the North and South Poles, the tropopause is as low as 30,000 feet. It slopes upward to 60,000 feet at the equator. Due to the low atmospheric pressure at this altitude, the air density is less than one-third of that at sea level. This reduced density allows the aircraft to fly much faster than it could if the air was as thick as it is at sea level. Because the air at cruise altitude is so cold and dry, the water in the jet exhaust creates ice crystals, which leave behind the familiar streaks in the sky called condensation trails or contrails.

The direction of the flight will also be a consideration in the cruising altitude. Eastbound aircraft fly at odd thousands, such as 33,000 and 35,000 feet. Westbound aircraft fly at even thousands, such as 34,000 and 36,000 feet. This staggering of altitudes provides at least 1,000 feet of vertical separation between two converging aircraft.

The final consideration in selecting altitude is air traffic. Air traffic controllers use radar and computers to coordinate the flight paths of the various aircraft sharing the same airspace.

Two important performance airspeeds are those for maximum endurance (most time) and maximum range (most distance). Both these speeds are found at a constant angle of attack (as is the stall angle of attack), regardless of aircraft weight or density altitude. The angle of attack is determined by the ratio of coefficient of lift to coefficient of drag. There is only one angle of attack that yields the ratio for maximum endurance and only one other for maximum range.

The Cl/Cd ratio for maximum range and endurance in a turbojet is different than that in a reciprocating propeller because these two types of engines react differently when burning fuel. A turbojet produces thrust proportional to the rate of fuel burned. A reciprocating engine pushes a crankshaft through a given number of rotations in a given time depending on RPM. One will remember from basic Physics that force through a distance divided by time represents power. A reciprocating aircraft produces power proportional to rate of fuel burned. It can be confusing that a turbine engine produces thrust (pounds) and a reciprocating engine produces power (550 foot pounds per second = one horsepower), given that they are both burning the same thing (fuel). Look at it this way: if you put electricity into an air conditioner, you get cold air; if you put electricity into a stereo, you get music.

So, for now, when thinking turbojet, think thrust, and when thinking reciprocating aircraft, think power. What about a turboprop? It is somewhere in between because it produces both thrust and power. Also, a high bypass turbofan starts to look more and more like a turboprop as the bypass increases. To keep this simple, we will just explore the differences between reciprocating airplanes and pure turbojets.

Because a reciprocating airplane burns fuel in relation to the power produced, the point of maximum endurance is found at the airspeed that requires the least amount of power. Maximum endurance would yield the highest time aloft, so it is found where the rate at which fuel is burned is the lowest at the point of lowest power required versus airspeed curve. A practical use of maximum endurance would be while holding.

The speed for maximum range is found where the tangent line from the origin intersects the power required versus airspeed curve. This is the best airspeed because it represents the best ratio of fuel burn rate to rate over the ground, or, put another way, best ratio of fuel burn to distance.

Turning to the turbojet, because it burns fuel in relation to the thrust produced, and thrust in level flight is equal to drag, the lowest point on a drag versus airspeed curve will result in the lowest fuel flow and maximum endurance. Again, this speed is only of practical consideration in holding.

A more important speed is maximum range. As before, by taking the tangent line from the origin to the drag versus airspeed curve, you find the speed that gives the best ratio of fuel burn to distance.

As mentioned earlier, the angle of attack for an aircraft's maximum endurance is constant, as is the angle of attack for maximum range, as is stall. As density altitude increases, the angle of attack stays constant, as does the equivalent airspeed; however, the true airspeed increases. As weight changes, the angle of attack stays constant, but the airspeed required for this angle of attack changes with the square root of the weight ratio. (Airspeed 2 = Airspeed 1 multiplied by the square root of the ratio of Weight 2 to Weight 1.) So, in an extreme example, if an aircraft quadrupled its weight, the speed required for maximum endurance and maximum range would double because the square root of 4 is 2. Because an aircraft decreases in weight as fuel is burned, the speed would have to decrease as weight decreases to maintain the optimum angle of attack if altitude is kept constant.

It should be noted that we have discussed how speed changes with weight and density altitude, not how the maximum range and the maximum endurance change. It is correct to assume that as weight decreases, the maximum range and maximum endurance increase.

Most turbojet aircraft do not fly at the speed that would give them maximum range, even though this would use the least fuel. Jet transport often use a speed called long range cruise--a speed that is approximately 5% faster than maximum range cruise--at an expense of 1% of fuel efficiency. The tradeoff for time is considered worth it, traffic permitting.

The upper cruise speed is generally limited by the speed of sound. As the temperature decreases with altitude, so does the speed of sound. The laws of aerodynamics change drastically above the speed of sound. Transport category aircraft must remain below this speed limit because they have not been designed to fly at supersonic speeds. The aircraft's speed in relationship to the speed of sound is called its mach number. Flying at exactly the speed of sound is called mach one. Most jet transport fly around 85 percent of the speed of sound, or mach .85. The air traveling over the top of the wings is accelerated, which means that even if the aircraft is traveling at only mach .85, the air over the wings may be exceeding the speed of sound. Aircraft wings are swept backward to trick the wings into feeling that air is traveling over them more slowly than it actually is.

Many factors are weighed in the selection of a cruise altitude. Besides performance considerations, there are terrain, weather, airspace, hemispheric cruise rules, and oxygen requirements. A pilot must understand each of these to exercise sound judgment in choosing cruise altitude.

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