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.
