Physical Science - Other

How Airplanes Measure Speed and Altitude



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Speed and altitude are two of the most important things a pilot needs to know during a flight. Failure of these instruments can lead to a "controlled flight into terrain", hypoxia from excessive altitude, a stall or a dangerous over-speed. All of these situations can lead to a very bad outcome.

The airspeed indicator and altimeter are both fed information from a set of pitot-static tubes. Because of their importance, the pilot and co-pilot each have their own pitot tube. The rate of climb indicator also attaches to this system. The pitot tubes face forward to receive the full impact of air pressure, and are located on the nose of the fuselage or on the leading edge of a wing. Each pitot tube has a "static vent", located where the most still atmospheric pressure can be measured accurately.

The altimeter shows altitude by precisely measuring barometric pressure. They operate quite like a barometer. The altimeter cannot record the correct altitude until the local barometric pressure is set. Extremely sensitive pressure responsive aneroids expand or contract with changing barometric pressure.

Altimeters are sometimes vulnerable to hysterisis errors. This can occur when a large and sudden change in altitude is made following extended cruising at a fixed altitude. This lag can usually be corrected by making slow changes in altitude or by maintaining the current altitude for a reasonable period of time. A damaged or improperly installed pitot tube can display dangerously inaccurate altitudes.

Air speed indicators precisely measure and compare the pressures at the static port and the impact pitot tube. Because the pressure difference can be very slight, this instrument must be extremely precise. The face of the instrument is designed so the pilot can check critical information at a glance.

Some instruments automatically calculate the maximum allowable airspeed for any given altitude. An extra diaphragm measures changes in altitude in order to determine this airspeed. A "true" airspeed indicator measures changes in barometric pressure, impact pressures from the pitot tube and air temperature. All airspeeds are measured in knots.

Supersonic aircraft use a Mach indicator, which functions in the same way. Airspeed is displayed in terms of Mach numbers instead of knots. Some indicators combine both airspeed, Mach and limiting Mach to save space.

These indicators are vital to the safe operation of an aircraft. The pilot must trust his instruments day or night and in any weather. The care required to maintain the impact pitot tubes, static ports and instruments cannot be over emphasized.

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