The Airspeed Indicator (ASI) — Page 1, Lesson 7

I want to walk you through the Airspeed Indicator — the ASI — one of the most fundamental flight instruments you'll rely on every single time you fly. Let's start with some speeds that are important but, as the book says, are not shown on the ASI dial itself. These are VLo and VLE. VLo is the maximum Landing Gear Operation speed — that's the speed limit for moving the landing gear either up or down. VLE is the maximum speed with the Landing Gear Extended — once the gear is locked down, you must not exceed this speed. Now, let's talk about accuracy. Under the certification standard CS-25, the ASI must be accurate to within plus or minus 3 percent, or 5 knots, whichever is greater. So if you're flying at 100 knots indicated, the instrument could legally read anywhere from 95 to 105 knots and still be within tolerance. Next, I want to show you the colour coding on the ASI dial. Some ASIs have coloured markings on the dial — these are called range markings, and they consist of coloured arcs and radial lines. Let's start with the White Arc. The white arc denotes the flap operating range. It runs from VSo — that's the stall speed at maximum all-up weight in the landing configuration, meaning full flap, landing gear down, wings level, power off — all the way up to VFe, which is the maximum flap extended speed. So if you're in the white arc, you can safely operate your flaps. Next is the Green Arc. The green arc denotes the normal operating speed range. It runs from Vs1 — the stall speed at maximum all-up weight with flaps up and wings level — up to VNo. VNo stands for 'normal operating limit speed' or 'maximum structural cruising speed'. You should not exceed VNo except in smooth air. Operations at indicated airspeeds inside the green arc should be safe in all conditions, including turbulence. Then we have the Yellow Arc. The yellow arc denotes the caution range. It extends from VNo — the normal operating limit speed — up to VNe, which is the never exceed speed. You should only operate the aircraft at indicated airspeeds in the yellow arc when the air is smooth. That's important — no turbulence when you're in the yellow. Finally, a Red Radial Line marks VNe — the never exceed speed. You must never fly faster than this speed under any circumstances. Optionally, for piston-engined light twins, you may also see a Blue Radial Line. That denotes VYSE — the best rate of climb speed with one engine out, at maximum weight, at mean sea level. Now let's move on to what happens when the pitot or static sources get blocked. If the pitot head becomes blocked, the ASI reading will, in general, remain unchanged. In level cruise, a blockage — probably ice, but possibly insects — will lock in the previous pitot pressure, and any change in actual airspeed will not be registered. Here's the key behaviour: if you change altitude with a blocked pitot head but a clear static source, the indicated airspeed will decrease during a descent. Why? Because the pressure locked inside the capsule remains constant, while the static pressure of the air surrounding the capsule increases. That difference causes the ASI to under-read. Conversely, the indicated airspeed increases during a climb with a blocked pitot head. So to summarise: with a blocked pitot source, the ASI under-reads in a descent.