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9.3. TEXT
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!
