The Airspeed Indicator (ASI) — Page 49, Lesson 55

Let's start with the speeds that are not shown on the ASI dial, because they're part of the same family of limits. We have VLO, which is the maximum Landing Gear Operation speed — that's the fastest you may move the gear up or down. And VLE, the maximum speed Landing Gear Extended — the fastest you may fly with the gear locked down. Neither of these appears on the instrument; they live in the flight manual. Now, the accuracy tolerance. Under CS-25, the certification standard for large aeroplanes, the ASI must be accurate to within plus or minus 3%, or 5 knots, whichever is the greater. So if you're reading 100 knots, 3% is 3 knots, so the tolerance is 5 knots. If you're reading 300 knots, 3% is 9 knots, so the tolerance is 9 knots. It's the larger of the two. Next, the colour coding. Some ASIs have coloured markings on the dial — these are called range markings, and they consist of coloured arcs and radial lines. Let me walk you through each one. The White Arc denotes the flap operating range. It runs from the stall speed at maximum all-up weight in the landing configuration — that means full flap, landing gear down, wings level, power-off — up to VFE, which is the maximum flaps extended speed. So inside the white arc, you're allowed to have flap out. The Green Arc denotes the normal operating speed range. It runs from the stall speed at maximum all-up weight with flaps up and wings level, up to VNO — the normal operating limit speed, also called the 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. That's the key point — the green arc is your everyday, all-weather safe zone. The Yellow Arc denotes the caution range. It extends from VNO, the normal operating limit speed, up to VNE, the never exceed speed. You may operate in the caution range only in smooth air. So the yellow arc is a warning — you're allowed there, but only when the air is smooth. A Red Radial Line denotes VNE, the never exceed speed. That's a hard limit — you must not exceed it under any circumstances. Optionally, for piston-engined light twins, there's a blue radial line. That denotes VYSE — the best rate of climb speed for one engine out, at maximum weight, at mean sea level. So if you lose an engine in a light twin, that blue line tells you the speed to climb at. Now let's look at what happens when the pitot or static sources get blocked. This is critical for understanding ASI behaviour. First, the pitot head. 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. Any change in actual airspeed will not be registered. So the needle stays where it was when the blockage occurred. But here's the subtle part. If you change altitude with a blocked pitot head and a clear static source, the reading changes. During a descent, the IAS will decrease. Why? Because the pressure locked inside the capsule remains constant — that's the pitot pressure, frozen at the moment of blockage — while the static pressure of the air surrounding the capsule increases as you descend. The instrument computes pitot minus static. So with static pressure rising and pitot pressure fixed, the difference shrinks, and you get an under-reading. Conversely, during a climb with a blocked pitot head, the IAS increases — because static pressure falls as you climb, so pitot minus static grows. So the rule to remember: with a blocked pitot source, the ASI under-reads in a descent. That's the takeaway — in a descent, the needle shows less than your true speed. Let me show you the colour arcs on the dial so you can see how they sit together. There you have it — the white arc for flap, green for normal, yellow for caution, the red radial line for VNE, and optionally the blue line for VYSE on light twins. And remember the pitot blockage behaviour: locked reading in level flight, under-reading in a descent, over-reading in a climb.