Airspeed Indicator

The airspeed indicator is an indispensable cock pit instrument. It's one of the few required by the regulations to be operational on all types of flights, which demonstrates its significant role in keeping us safely flying. A quick glance at aviation's speedometer can tell us much about the current state of the aircraft.

The air speed indicator is the only flight instrument that uses the pitot tube. It takes the air pressure that enter the front of the pitot tube (more speed means more pressure) and directs it to the back of the instrument. There, it meets a closed disc or diaphragm that sits inside the back case. That case is sealed, except for a small hole that is directly connected to the static port. As a result, the pressure that enters the pitot tube is compared to the static air pressure.

The differential pressure is the key to the instrument's operation. A gear or series of gears is attached to the diaphragm, which is then connected to a needle that indicates on the face of the instrument. What comes out on the calibrated face is the speed of the airplane through the air.

The different types of airspeed include: Indicated, which reads on the face of the airspeed indicator. Calibrated, which indicates airspeed corrected for inherent instrument errors. Can be obtained from the pilot's operating handbook. True, is the calibrated airspeed corrected for non-standard pressure and temperature; in other words, the actual speed of the aircraft through the air. And ground speed, which is the true air speed corrected for wind.

The air speed indicator is prone to certain errors, usually as a result of problems with debris or ice on the pitot tube or static source. A blocked pitot tube will cause the airspeed to go to zero. A blocked drain hole or pitot tube opening generally will cause the air speed indicator to read its last setting, but then act like an altimeter on any climbs or descents. A blocked static port will cause the airspeed indicator to falsely display an increase in a descent and decrease in a climb. It is very important to become familiar with these symptoms while flying, so you can gauge and adjust to the situation and properly fly at a safe airspeed. Especially when you are about to land, you don't want to come into a landing thinking you're flying the correct air speed when in reality you aren't.