Pitot and Static Sources — Page 18, Lesson 17

Let's start with the very foundation of this whole chapter, because everything else builds on it. I want you to picture an aircraft sitting on the ground in perfectly still air. In that condition, it's subject to normal atmospheric pressure, and that pressure bears equally on all parts of the aircraft. That ambient pressure is what we call static pressure. That's our first key term — static pressure is simply the pressure of the air around you, the ambient pressure, when the air isn't being disturbed by the aircraft's movement. Now, take that aircraft into flight. It's still subject to the static pressure at its flight level, but now it experiences something extra. The leading edges of the aircraft — the nose, the wing leading edges — feel an additional pressure. That's because the air resists the aircraft's movement. This additional pressure is called dynamic pressure, and here's the important part: its value depends on two things — the speed of the aircraft through the air, and the density of the air. So dynamic pressure isn't a constant; it changes with your speed and with how dense the air is at your altitude. So now, on the leading edges, you have a total pressure, which is the sum of static plus dynamic. And this total pressure has another name — it's also known as pitot pressure. So let me give you that relationship clearly: pitot equals static plus dynamic. That's the fundamental equation of this chapter. Now here's the catch, and it's a crucial one. You cannot isolate dynamic pressure by direct measurement. It's physically impossible to separate dynamic pressure from the static pressure that's always there with it. So how do we get it? The instruments that need dynamic pressure — and we'll see which ones in a moment — they measure total, or pitot, pressure, and they also measure static pressure separately. Then, inside the instrument, they subtract static from pitot to derive dynamic pressure. So rearranging that equation: dynamic equals pitot minus static. The instrument does the subtraction internally. Let me show you this relationship visually, because it's the heart of the whole system. Now, which instruments need these inputs? Let me give you the breakdown, because it's a clean division. There are instruments that need static only, and there are instruments that need both pitot and static. The static-only instruments are the altimeter and the vertical speed indicator, or VSI. The instruments that need both pitot and static are the airspeed indicator, the ASI, and the Machmeter. So remember that pairing: altimeter and VSI are static-only; ASI and Machmeter need both. Now, here's a practical problem. Inside the aircraft, the pressure and temperature are seldom the same as outside. So we can't just sense pitot and static pressures from inside the cabin. The pitot and static pressures must be sensed by devices mounted on the outside of the aircraft. That's why you see those probes sticking out into the airstream. Let me walk you through the pitot head itself. We use an open-ended tube that runs parallel to the longitudinal axis of the aircraft — that's the nose-to-tail axis. This tube senses the total pressure, which again is static plus dynamic. This device is called a pitot tube, and it's mounted inside a pitot head. The open end of the tube faces directly into the moving airstream. The other end leads to the airspeed capsules in the ASI and the Machmeter. Here's the clever bit — the physics of how it works. The moving airstream is brought to rest inside the tube. When the air is brought to rest, that's what generates the extra dynamic pressure. So inside the tube, you have the static pressure that was already there, plus this newly generated dynamic pressure, and together they give you the required total, or pitot, pressure. That's the whole principle of the pitot tube — it's a device that deliberately stops the air so it can measure the total pressure. Let me show you the physical arrangement, because it helps to see how the pitot head sits on the aircraft. And there's a figure in the book that shows this — the static and pitot sources combined. Let me point you to it. So let me pull the whole picture together for you. We have three pressures: static, dynamic, and pitot. Static is the ambient pressure, always there. Dynamic is the extra pressure from the aircraft's motion, and it depends on airspeed and air density. Pitot is the total — static plus dynamic. We can't measure dynamic directly, so the instruments measure pitot and static separately and subtract. The altimeter and VSI need static only. The ASI and Machmeter need both. And all of this is sensed by devices outside the aircraft, because the cabin environment isn't representative. The pitot tube, with its open end facing the airstream, brings the air to rest and thereby generates that dynamic pressure on top of the static, giving us the total pressure we need. That's the complete foundation. Everything else in this chapter — the static vents, the errors, the heating systems — builds on these three pressures and this one equation.