Cruising the Skies

Statistically, the safest and longest phase of an entire flight is cruising, the straight and level portion between climb and initial descent. Selecting the appropriate cruise altitude requires meteorological, regulatory, and air traffic control considerations. The most important consideration, of course, is a safe, smooth ride for passengers. Once altitudes that will afford a smooth ride are determined, the winds are considered. Finding an altitude with the strongest tailwind to maximize ground speed and minimize both flight time and cruise fuel is desirable. The wind at cruise altitudes often exceeds 100 knots. Strong rivers of wind called jet streams can have speeds of more than 200 knots.

Turbojet engines operate more efficiently as the surrounding air temperature decreases. The ambient air temperature decreases by about three degrees for every thousand feet. The coldest air, a chilly 70 degrees below zero, is found at the atmospheric level of the tropopause, the level at which the temperature stops dropping. 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 and slopes upward to 60,000 feet at the equator. Because of 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 at sea level. Because the air at cruise altitude is so cold and dry, the water in the jet exhaust creates ice crystals, leaving behind the familiar streaks called condensation trails or contrails.

The direction of the flight is also a consideration in determining the cruising altitude. Eastbound aircraft fly at elevations of odd-numbered thousands of feet such as 33,000 and 35,000, while those bound west fly at elevations such as 34,000 and 36,000 feet. This staggering of altitudes provides at least 1,000 feet vertical separation between two converging aircraft.

Air traffic is the final consideration in altitude selection. Air traffic controllers coordinate the flight paths of the various aircraft sharing the same airspace and use radar and computers to ensure their safe separation.

The cruise speed of the aircraft 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. Because they have not been designed to fly at supersonic speeds, transport aircraft must remain below this limit. 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 at around 85 percent of the speed of sound or mach .85. The air traveling over the top of the wings is accelerated so that even if the aircraft is traveling at only mach .85, the air over the wings can be approaching the critical mach number. The wings of the aircraft are swept backward to trick the wings into sensing that air is traveling over them more slowly than it actually is.

There are many considerations in the selection of a cruise altitude. For most passengers, though, the most important of these is what movie will be shown!

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