Pitot and Static Sources — Page 18, Lesson 17

Let’s start at the very beginning of how we measure airspeed, because everything in the flight deck depends on getting this right. I want you to picture an aircraft sitting on the ground in perfectly still air. In that condition, the air presses on every part of the aircraft equally. That ambient pressure, the pressure of the surrounding air at rest, is what we call static pressure. It’s the baseline pressure of the atmosphere around you. Now, take that aircraft into flight. It’s still subject to the static pressure at its flight level, but now something new happens. The leading edges of the aircraft, the nose, the wing leading edges, experience an additional pressure. That’s because the air resists the aircraft’s movement through it. This extra pressure is called dynamic pressure, and its value depends on two things: the speed of the aircraft through the air, and the density of the air. Faster, or denser air, means more dynamic pressure. So, at the leading edges, the air is ramming into the aircraft. The total pressure you feel there is the static pressure plus the dynamic pressure. We call this combined total the pitot pressure. So the fundamental relationship is: Pitot equals Static plus Dynamic. And if we rearrange that, Dynamic equals Pitot minus Static. Here’s the crucial catch, and it’s a real engineering problem. You cannot isolate dynamic pressure by direct measurement. It simply cannot be separated from the static pressure that’s always there with it. So the instruments that need dynamic pressure don’t measure it directly. Instead, they measure total, or pitot, pressure, and they also measure static pressure, and then inside the instrument they subtract static from pitot to derive the dynamic pressure. That subtraction happens within the instrument itself. Now, which instruments need these inputs? Let’s look at the list. Some need static only: the Altimeter and the Vertical Speed Indicator, the VSI. Others need both pitot and static: the Airspeed Indicator, the ASI, and the Machmeter. Here’s a practical point. Inside the aircraft, pressure and temperature are seldom the same as outside. So we can’t just sample the cabin air. The pitot and static pressures must be sensed by devices mounted on the outside of the aircraft. That brings us to the pitot/static heads. The device used to sense total pressure is an open-ended tube that runs parallel to the longitudinal axis of the aircraft. That’s the 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 of physics. As the moving airstream enters that open tube, it is brought to rest. When the air is brought to rest, it generates that extra dynamic pressure. That dynamic pressure, added to the static pressure already present in the tube, gives you the required total, or pitot, pressure. So the tube is essentially a device that captures the ram effect of the air and delivers the full pitot pressure to the instruments. So, to tie it all together: static pressure is the ambient baseline, dynamic pressure is the extra from motion, and pitot pressure is the sum of the two. The instruments that need dynamic pressure get both pitot and static, and subtract one from the other internally. That’s the whole foundation of airspeed measurement.