Pitot and Static Sources — Page 18, Lesson 19

Right, let's get into the Pitot and Static Sources. This is the very foundation of how your airspeed indicator, altimeter, and vertical speed indicator get their information, so we need to be precise. We have two distinct sources of pressure. First, the static head. A static head is a tube with its forward end sealed, but with holes or slots cut into the sides. The key here is that these slots do not face into the airflow. Because they're on the side, they are, in theory, sensing only the static pressure — that's the ambient pressure of the air around the aircraft, unaffected by its motion. In reality, there's a suction effect as air flows past those holes, so the sensed static pressure will be slightly lower than true static when the aircraft is moving. This is a real-world error we design around. This static pressure supplies the static 'line' to the pressure instruments. Now, the second source is the pitot head. This faces directly into the airflow. Its purpose is to capture the pitot pressure, which is the pressure of the air brought to rest. The pitot tube must be positioned outside the boundary layer — that's the layer of disturbed, slower-moving air right next to the fuselage skin. So, it's usually a head on a strut if mounted on the side of the fuselage, or it can be a tube placed on the nose, ahead of the fuselage. The opening must be designed to be parallel to the airflow in the normal flight attitude, so it's pointing straight into the relative wind. Now, there are two ways the pitot pressure can be handled. The air can be brought to rest in the pitot probe against a stagnation wall — that's simply a pressure-measuring flat surface inside the probe — and then that pressure is transmitted up the pitot pipelines to the ASI and the Machmeter. Alternatively, and this is more usual in elementary aircraft, the pitot pressure can be passed directly up into the pitot pipelines to the instruments, without that stagnation wall. Now, let's talk about why this matters so much. Measurement of dynamic pressure is essential to safe flight. Dynamic pressure is the difference between pitot pressure and static pressure — it's the pressure due to the aircraft's motion. At too low a speed, the aircraft will stall. At too high a speed, it will be overstressed. So the ASI, the Airspeed Indicator, is critical to flight safety. Because it's so critical, it cannot be allowed to block because of ice. So an electric anti-icing heater coil is usually incorporated into the pitot head. Any errors due to the heating effect may be reduced by design and calibration. Now, here's the clever bit about water. If water is drained, ice should not be able to form. So drain holes are provided in the pitot head. These will, of course, cause some loss of pressure, but this can be suitably calibrated and allowed for in the design. So the system is designed knowing that there's a small leak, and the instruments are calibrated to account for it. Now, I want to show you how these two sources can be combined. The static and pitot sources may be combined in one pressure head. In this design, the static tube surrounds the pitot tube, with separate pressure lines leading to the pressure instruments. So you have one physical unit on the outside of the aircraft, but two completely separate pressure lines running to the instruments inside. That figure shows you the combined pressure head. You can see the pitot opening facing forward, and the static holes on the side of the surrounding tube. The key takeaway is that even though they're in one physical unit, the pitot and static pressures are kept completely separate in their own lines to the instruments. So, to summarise what we've covered: we have a static head sensing ambient pressure, a pitot head sensing ram pressure, and the difference between them gives us dynamic pressure, which drives the ASI. The pitot head needs anti-icing and drain holes, and both sources can be separate or combined in a single pressure head. That's the core of the pitot-static system.