How does a Pitot tube and airspeed indicator tell a pilot how fast the aircraft is flying?

Along with an altimeter and vertical velocity indicator in the cockpit of a plane is an airspeed indicator (ASI), which registers the speed at which the aircraft is flying. The instrument has two needles, one indicating hundreds, the other tens of knots, but from where do its readings derive?

Outside the plane, usually placed on one of the wings, is a thin cylindrical tube, or Pitot tube, originated by Henry Pitot (1695 1771) to measure the flow velocity of air or water.

Always positioned parallel to the axis of the plane, this tube projects forward into the airflow. As the plane accelerates, an increasing amount of air is driven into the open ended tube, thus raising the air pressure inside.

A second tube, this one sealed at its forward facing end, is also attached to the frame of the plane, sometimes along with the Pitot tube, sometimes in a more protected area. A series of holes along its length provides access of air, but the air pressure inside it is not affected by the plane’s forward motion. This static tube thus allows a reading of the atmospheric pressure outside the plane. Both tubes are connected by pressure lines to an expandable diaphragm inside the airtight case of the ASI.

As the aircraft accelerates, increased air pressure causes the diaphragm to expand; it contracts, of course, as the plane decelerates. The needles of the ASI are attached to this diaphragm and move according to its expansion or contraction.

Through mechanical linkage, then, the differential in air pressure readings of atmospheric air and air driven into the Pitot tube is measured as airspeed.