How Fast Are We Flying?

Knowing how fast an aircraft is traveling is as important, if not more important, than knowing how fast a car is moving. Determining the aircraft's speed, however, is a bit more complex than simply glancing at an automobile's speedometer. Let's examine the various types of airspeed and how they are measured.

Pilots speak of several types of airspeed. The value read directly from the airspeed indicator is called "indicated airspeed" (IAS). To determine the aircraft's actual speed over the ground, two types of pressure must be measured. A pitot tube is positioned on the exterior of the aircraft so that the air molecules of the atmosphere "ram" into it. The faster the aircraft is traveling, the greater this ram pressure will be. As an aircraft climbs, the atmospheric air pressure decreases, as does the ram pressure. To correct for this, the aircraft has a static air pressure port that is also connected to the airspeed indicator. The greater the difference between the ram and static pressures, the greater the indicated airspeed.

When an aircraft slows down and changes its configuration, as it does by lowering its flaps and landing gear, the airflow pattern over the fuselage changes. This change of airflow will affect the pressure in the pitot tube and static port. To account for this, the pilot refers to an "Airspeed Calibration Chart." The "calibrated airspeed" (CAS) is read from this chart.

The air ahead of an aircraft flying faster than 200 knots becomes compressed, increasing the air density and the pressure in the pitot tube. To eliminate the compressibility error, the pilot refers to an "Airspeed Compressibility Chart." The greater the CAS and the higher the altitude, the more the pilot must subtract from the CAS to obtain the "equivalent airspeed" (EAS).

When flying at high altitudes, the pilot must compensate for reduced air density. Imagine the space shuttle in orbit. Even though the orbital speed is more than 17,000 knots, there is virtually no atmosphere to ram into the pitot tube. The indicated airspeed would be almost zero. By knowing the air density, the pilot can calculate the actual, or "true airspeed" (TAS), at which the aircraft is moving through the airmass. It is to the TAS that the velocity of the wind is applied to determine the speed over the ground. The presence of a tailwind or headwind will increase or decrease the ground speed.

Fast-moving aircraft express their speed in Mach, the Mach number being the ratio of the TAS to the speed of sound. Mach 0.5 would be half the speed of sound. Similarly, Mach 2 would represent twice the speed of sound, and so on.

Keeping track of all of the different types of airspeed is a bit complicated. Pilots do have one thing going their way -- there are not nearly as many speed limits in the air. The next time a police officer stops you and asks if you know how fast you were driving, you may want to respond by asking, "Do you mean indicated, calibrated, equivalent, true, groundspeed, or Mach?" That should just about guarantee that you'll get a ticket!

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