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Expert: Capt Paul - 9/16/2006


Question
Dear Jeeves,

Do you know how a plane measures its airspeed (vertical and horizontal)?


Thank you,

Frans Indroyono

Answer
Simply, an airplane's speed is measured with air pressure.

The Pitot tube, normally on a wing facing forward, has Ram air pressure while the aircraft is moving. A static port on the side of the aircraft lets in the static or normal air pressure.

Both are led to the Airspeed indicator where the static air pressure is subtracted from the ram air pressure to measure the forward speed of the aircraft.

This differential pressure is known as indicated airspeed. It is the speed of the aircraft through the air, but is not necessarily its actual airspeed (True Airspeed or Ground Speed)

Basically, the airspeed system and the altimeter are just pressure gauges.

Cheers

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Capt Paul

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I can answers questions relating to: aircraft and airline operations; aircraft technical matters relating to a number of different aircraft types; flight operations & flight planning; Aviation meteorology; accident prevention and investigation; the Air Traffic control system and how pilots interact with it; and pilot and airline licensing in the Europe, Asia, Canada and the US. I also answer questions about all types of Navigation, Aviation Weather, Aircraft Performance, Procedures and Aviation law (in general).

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Current Airbus Captain with over 23 years as an airline pilot and over 13,500 hours of experience. Also experience as a Flight Operations Manager/Director; Chief Pilot; Instructor; Safety Officer, and training as an Accident/Incident Investigator. Degree in Airline / Aerospace Operations. Minor in Behavioural Psychology and studies in pre-law BS, Aerospace

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