What Am I? Pitot Tube

As kids, we all loved to put an arm out of the car window on a nice day and feel that pressure against our hand as mom or dad drove us to school or soccer practice. That pressure against your hand is dynamic pressure, which combined with static pressure (sitting still) is ram pressure. The funny little flange sticking out from the airplane’s wing is called the pitot tube. The airspeed indicator measures the difference between ram pressure from the pitot tube and static pressure from the static source to tell how fast—or slow—the airplane is flying. Borrowing an analogy from Boldmethod.com (see “Five Questions: Alex Udris,” on p. 21), think of the airspeed indicator as a scale comparing static pressure from the aircraft’s static ports to the ram pressure from the pitot tube.

Different aircraft sport different pitot tube styles, and the static port may be on the aircraft or a part of the pitot tube. The main opening of the pitot tube faces the windstream and allows ram pressure to enter the pressure chamber. A small hole in the back allows moisture to drain.

A blocked pitot tube leads to errors in airspeed indications. If the pitot tube is blocked but the drain hole is clear, the pressure in the pitot tube drops to ambient outside pressure—the same as at the static port—and the airspeed indicator drops to zero. If both the pitot tube opening and the drain hole are blocked, the pressure in the pitot tube is trapped. Since the airspeed indicator reads the difference in ram and static pressure, the drop in static pressure from a climb will cause airspeed readings to increase (as long as the static port is clear). A descent will cause the airspeed indicator to read lower.

Those instrument errors should make you think twice about ignoring the pitot tube in your preflight. Blockage can be caused by something as dramatic as ice that develops in flight or insignificant as a spider—and let’s not even think of leaving the pitot tube cover on. In airliners, major accidents have been attributed to pilots’ responses to erroneous airspeed readings because of a blocked pitot tube (see “Cautionary Tales,” below). In most aircraft, heat is available for the pitot tube; checking pitot heat is another piece of your preflight that you should remember, regardless of the weather or outside air temperature (see “How It Works: OAT Gauge,” June 2017 Flight Training).