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

I want to walk you through the Airspeed Indicator now — the ASI. We've already covered the basic pitot-static system, so now we're going to look at the speeds that are marked on the instrument, and then what happens when things go wrong with the pitot and static sources. First, there are two speeds that are important but are not shown on the ASI dial itself. You need to know them because they're operational limits, but you won't see them painted on the instrument. The first is VLO — that's the maximum Landing Gear Operation speed, the maximum speed at which you may raise or lower the landing gear. The second is VLE — the maximum speed Landing Gear Extended, the maximum speed at which you may fly with the gear locked down. Remember, VLO is for the operation of the gear, VLE is for flying with it extended. Now, the accuracy of the instrument. Under CS-25, the certification standard, the ASI accuracy tolerance is ±3% or 5 knots, whichever is the greater. So if the instrument is reading, say, 100 knots, the true airspeed could be anywhere from 97 to 103 knots — or if it's reading very low, the tolerance is 5 knots rather than 3%. That's the legal accuracy requirement. Now let's look at the colour coding on the dial. Some ASIs have coloured markings — these are called range markings, and they consist of coloured arcs and radial lines. A radial line is a line that runs from the centre of the dial outwards to the edge, like a spoke on a wheel. 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 the white arc tells you the speed range within which it's safe to have the flaps out. The Green Arc denotes the normal operating speed range. It runs from the stall speed at maximum all-up weight — this time with flaps up and wings level — up to VNO, which stands for 'normal operating limit speed' or 'maximum structural cruising speed'. VNO should not be exceeded except in smooth air. Operations at indicated airspeeds within the green arc should be safe in all conditions, including turbulence. So the green arc is your everyday, all-weather operating envelope. The Yellow Arc denotes the caution range. It extends from VNO — the normal operating limit speed — up to VNE, the never exceed speed. The aircraft should be operated at indicated airspeeds in the caution range only in smooth air. So if you're in the yellow, you're above the normal structural cruising speed, and you must not be there unless the air is smooth. A Red Radial Line denotes VNE, the never exceed speed. That's the absolute limit — you must never exceed this speed under any circumstances. There's one more optional marking, for piston-engined light twins. A blue radial line 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 for the best rate of climb. Now, let's move on to what happens when the pitot and static sources get blocked. This is critical for understanding ASI behaviour in failures. First, the pitot head. If the pitot head becomes blocked, the ASI reading will, in general, remain unchanged. Think about why. The ASI works by comparing pitot pressure against static pressure. If the pitot head is blocked — probably by ice, but possibly by insects — the pressure trapped inside the pitot line is locked in. In level cruise, that means the previous pitot pressure is locked in, and any change in actual airspeed will not be registered. The needle just stays where it was. But here's the interesting part — what happens if you change altitude with a blocked pitot head but a clear static source? Let's think it through. The pressure locked inside the capsule remains constant, because the pitot line is sealed. But the static pressure of the air surrounding the capsule changes with altitude. During a descent, the static pressure increases as you go down. So you have a constant pitot pressure minus a higher static pressure. That means (pitot – static) results in a smaller difference — so the ASI under-reads during a descent. The needle shows a lower speed than you're actually doing. Conversely, during a climb with a blocked pitot head, the static pressure decreases as you go up. So you have a constant pitot pressure minus a lower static pressure — the difference is larger, so the IAS increases during a climb. So the rule to remember: with a blocked pitot source, the ASI under-reads in a descent. That's the key failure mode to know. Let me just make sure you've got the full picture. The white arc is flaps, the green arc is normal operation, the yellow arc is caution, the red radial line is VNE, and optionally the blue radial line is VYSE for light twins. And on the failure side — blocked pitot, the reading locks in level flight, under-reads in a descent, over-reads in a climb. That's the ASI in a nutshell.