The Airspeed Indicator (ASI) — Page 39, Lesson 47

Let’s start with the Airspeed Indicator, the ASI. This is one of the most important instruments on your panel, and I want you to understand exactly what it measures and how it works. First, the principle. Think of an aircraft sitting on the ground in perfectly still air. In that condition, the only pressure acting on it is the atmospheric pressure, which we call static pressure, and we denote it with the letter S. Now, the moment you start moving through the air, something changes. The leading edges of the aircraft—the nose, the wing leading edges—are now ramming into the air. That impact creates an additional pressure, which we call dynamic pressure. So in flight, the leading edges experience a total pressure that is the sum of the dynamic pressure plus the static pressure. We call this total pressure the pitot pressure, denoted P. So the relationship is simple: Pitot equals Dynamic plus Static. Now, where do we sense these pressures? The pitot head senses the pitot pressure. The static/vent senses the static pressure. Both of these pressures are fed into the airspeed indicator, which is fundamentally a differential pressure gauge. That means it measures the difference between two pressures. Here, it measures the difference between pitot and static, and that difference is exactly the dynamic pressure. Why does dynamic pressure tell us airspeed? Because dynamic pressure is related to airspeed by a very specific formula: Dynamic Pressure = ½ ρV². Let me unpack that. The V here is the true airspeed, which we abbreviate TAS. The ρ is the Greek letter rho, and it represents the density of the surrounding air. So the dynamic pressure depends on the square of the true airspeed and on the air density. The ASI measures airspeed by measuring this dynamic pressure, and it displays the result on a scale that is calibrated, usually in knots. And just so we're clear on the unit: 1 knot is 1 nautical mile per hour. Now let's talk about the construction, because the clever part is how we isolate that dynamic pressure. Dynamic pressure cannot be measured directly. We have to establish it by subtracting static pressure from pitot pressure. So the instrument is built to do exactly that subtraction. Here's the layout. The static pressure is fed into a hermetically-sealed instrument case. Hermetically sealed means it's completely airtight. The pitot pressure, on the other hand, is piped to a thin metal capsule that is capable of expansion and contraction. Now here's the key insight: static pressure is present on both the inside and the outside of the metal walls of that capsule. Because it's on both sides, it cancels out. So the pressure differential between the inside and the outside of the capsule is (Dynamic + Static) minus Static, which leaves you with just Dynamic. The expansion or contraction of the capsule is therefore proportional to the changes in dynamic pressure, which are produced by changes in airspeed. Finally, those capsule movements are transmitted through a temperature-compensated magnifying linkage to the pointer that indicates airspeed on the face of the ASI. The temperature compensation is important because the capsule and linkage materials could expand or contract with temperature changes, and we don't want that to distort our reading. The magnifying linkage amplifies the small capsule movements so the pointer can sweep across the dial properly. So, to tie it all together: the pitot head gives us total pressure, the static vent gives us static pressure, the ASI subtracts one from the other to get dynamic pressure, and dynamic pressure, through that ½ρV² relationship, is our measure of airspeed. That's the complete operating principle of the Airspeed Indicator.