Airspeed Indicator

The airspeed indicator is a differential pressure gauge. It measures the difference between the air pressure in the pitot tube and the static, relatively undisturbed air surrounding the airplane. A needle displays this difference as airspeed.Aircraft manufactured in the U.S. and Canada after 1976 have airspeed indicators with markings based on indicated airspeed in knots. Older aircraft typically have markings that reflect calibrated airspeed in statute miles per hour.

How It Works

The airspeed indicator is the only instrument connected to both the pitot tube and the static system. Air from the static system fills the case of the airspeed indicator, providing a "base" pressure against an expandable diaphragm. Ram air forced into the pitot tube as the airplane moves fills the diaphragm, which expands as ram air pressure (and speed) increase. A needle connected to the diaphragm rotates as the diaphragm expands. The needle's position on the instrument face indicates airspeed.

The airspeed indicators for the Learjet 45 and Boeing 737-400 include an additional needle with red and white stripes called the "barber pole." A flight data computer takes information about the current altitude, air temperature, and pressure and continuously computes the maximum allowable airspeed as the aircraft climbs and descends. The barber pole shows this speed.

NOTE: The speeds used in Flight Simulator checklists, operating procedures, and in the Aircraft Handbooks section of Help are all indicated airspeeds unless otherwise noted.

TIP: Flight Simulator displays indicated airspeed by default for a more realistic flying experience. As you climb, indicated airspeed decreases while true airspeed increases. The higher you climb, the greater the difference between IAS and TAS. To display true airspeed instead, choose Preferences from the Options menu and select the Display True Airspeed option on the Instrument tab of the Preferences dialog box. To test your ability to fly after one or more instruments fail, you can hide certain instruments or make them unreliable. For more information, see the topic "Setting Instrument Options" in the Using Flight Simulator section of Help.