Pitot and Static Sources — Page 18, Lesson 22

I want to walk you through the pitot and static sources now — this is the heart of how your airspeed, altitude, and vertical speed instruments get their information. Let's start with the static source itself. The static source — whether it's a simple hole in the fuselage or a combined probe — must have its opening at right angles to the airflow. That's the critical requirement. If the opening is perpendicular to the airflow, it senses only static pressure, with no component of dynamic pressure. Static pressure is the pressure of the air around you, the ambient pressure. Dynamic pressure is the pressure caused by the air ramming into the aircraft because of forward motion. If the opening isn't at right angles, you'd pick up some of that dynamic pressure, and your reading would be wrong. Now, some static sensors — especially those in the combined pitot/static probe — may be fitted with electric heating. That's to prevent ice from blocking the opening, which would be catastrophic for your instruments. Let me show you what a combined pitot/static pressure head looks like. That's Figure 2.2 — the combined head. The pitot opening faces forward into the airflow, and the static openings are on the side, at right angles to the flow. Now, here's the problem we run into. I mentioned earlier that if the aircraft has forward motion, the static pressure sensed will be slightly lower than if the aircraft is stationary. That's due to suction — the airflow over the head creates a slight vacuum effect. As speed increases, the turbulent airflow in the region of the pitot/static heads makes this error greater. Turbulence will also affect the pitot reading, not just the static. This error is called Position Error — or alternatively, 'pressure' error. Let me be precise about that: it's the difference between the pressure you actually sense and the true static pressure, caused by the position of the sensor in the airflow. Now, when does position error get worse? At large angles of attack — which are usually associated with lower airspeeds — the pressure head is inclined at an angle to the airstream, so position error is usually greater. Think of a nose-high attitude on approach: the head is tilted, the airflow isn't hitting it cleanly, and the error grows. Here's something important for your flight manual: flight manuals may list different values of position error for different flap settings. So the error isn't a single number — it changes with configuration. Let me break down what position error depends on. It depends mainly on three things: the positioning of the pressure head, the airspeed, and the aircraft attitude. Those are the primary drivers. Now, here's a subtle but crucial point. The turbulence produced in the airstream by the pressure head itself affects the value of static pressure sensed — rather than the pitot pressure — because the turbulence is downstream of the pitot opening. Let me unpack that. The pitot opening faces forward, into the clean airflow. The static openings are on the side, downstream. So the turbulence the head creates wraps around and disturbs the static openings, not the forward-facing pitot opening. That's why the static reading suffers more. This is shown diagrammatically in Figure 2.3. That figure shows how turbulence affects the value of static pressure. So, because of this problem, the Static Vent was introduced as a source of static pressure instead of the static head. The pitot pressure is then sensed by a simple pitot head. A static vent separated from the pitot head is shown in Figure 2.4. The idea is to separate the two sensors so the turbulence from the pitot head doesn't corrupt the static reading. Now, here's the key to finding a good static vent location. There is usually some place on the airframe — usually on the side of the fuselage — where true, or nearly true, static pressure is obtained over the whole speed range of the aircraft. A flat metal plate is fitted at this position. That plate is the static vent. The flat plate helps keep the airflow attached and smooth over the opening, so the pressure sensed is as close to true static as possible across the entire speed range. So let me tie this together. You have two separate pressure sources: the pitot head, facing forward, sensing ram air pressure — that's pitot pressure, the sum of static and dynamic. And the static vent, on the side of the fuselage, sensing ambient static pressure. The difference between them gives you dynamic pressure, which is what drives your airspeed indicator. And the static pressure alone drives your altimeter and vertical speed indicator. That's the foundation. The static source must be at right angles to the airflow, position error is the enemy we're fighting, and the static vent on the fuselage is the solution.