PERSPECTIVES FLIGHT LESSON PREFLIGHT HOW IT WORKS BY EMMA QUEDZUWEIT BY DENNIS K JOHNSON AIRSPEED INDICATOR THE NEED FOR SPEED WHETHER YOU'RE FLYING AN F-35 Lightning or Piper J-3 Cub, everyone needs a way to reliably tell their airspeed, and we've come a long way from the early weathervane or spring-resistance types. The first airspeed indicator to use a pitot tube was patented in England in 1909, yet our modern airspeed indicator (ASI) still operates by the same pitot-static system whether you're behind gauges or glass. The ASI calculates airspeed from air pressure changes caused by the aircraft's movement through the air, and inputs from both the pitot tube and static ports are combined to sense these changes. The pitot tube provides the dynamic air pressure-also called ram air-as it's forced through the inlet by forward movement (the faster you fly, the greater the pressure), while the static port detects the static atmospheric pressure. Pressure lines transmit these air pressures to the ASI. In gauge instruments, the static air goes into a chamber while ram air is channeled to a diaphragm. The pressure differential that results between the diaphragm and chamber is translated to the face of the instrument by a gear mechanism to show your indicated airspeed (IAS). Airspeed indicators on glass panels still use pitot and static lines, but a solid-state pressure sensor is used to transmit the data electronically. It is important to know the basics of your ASI functionality and study the types of failures that can occur if different parts of the system get blocked by icing or other debris. emma.quedzuweit@aopa.org Static air line 160 140 Diaphragm Ram air 100 120 80 AIRSPEED KNOTS Pitot tube 16 FLIGHT TRAINING NOVEMBER/DECEMBER 2023 APRIL 2017 STEVE KARP 30 100 80 40 T.A.S. KTS 120 140 120 100 5 5 20 10 160 90 60 30 PRE ALT SS