Pitot and Static Sources — Page 18, Lesson 24

I want to pick up right where we left off — we were looking at the static vent, the small circular hole in a plate on the side of the fuselage that feeds static pressure to your instruments. Now I want to walk you through the advantages of that static vent arrangement, because it's a classic exam point. The first advantage is that the airflow in the region of the vents is less turbulent, so the static pressure you measure is more accurate. That's the whole point — you want a clean, undisturbed sample of the ambient pressure. Second, errors produced when you're side-slipping or yawing are reduced. And here's the key design feature: there's a similar vent positioned on the opposite side of the fuselage, and the two are interconnected. So you have a vent on the left and a vent on the right, joined together, transmitting static pressure to the instruments. Because they're cross-connected, errors produced by yawing are largely eliminated. If you yaw, one vent sees a slightly higher pressure and the other sees a slightly lower pressure — by averaging them through the interconnection, the error cancels out. This is what we call cross balancing of static vents. So to summarise the advantages: less turbulent airflow and more accurate static pressure; reduced errors when side-slipping or yawing; and the duplication of vents on either side of the fuselage reduces those side-slip and yaw errors through cross balancing. Now, there's a limit to how good a simple static vent can be. At high Mach numbers, the shock waves associated with flight can produce significant errors in the pressure sensed by a static vent. So modern high-speed aircraft are fitted with a more sophisticated combined pitot/static pressure head — that's a single probe that senses both pitot and static pressure — in order to keep position error within acceptable limits. The choice of location for a probe or vent depends on the aerodynamics of the aircraft. Typical locations are: ahead of a wing tip, under a wing, ahead of the vertical stabilizer tip, at the side of the fuselage nose section, and ahead of the fuselage nose section. Each location is chosen to put the probe in airflow that's as clean as possible for that particular airframe. Now let's talk about manoeuvre-induced error. This is caused by short-term fluctuations of pressure at the static vents, and delays in the associated pipelines transmitting those pressure changes to the instruments. So you have two things going on: the pressure at the vent is fluctuating, and even when it changes, the pipeline takes time to pass that change along to the instrument. Here's an important point: even servo altimeters and air data computer systems suffer from this type of error, because they utilise the same static vents as the simple pressure instruments. So no matter how sophisticated the instrument is, if it's fed from the same static source, it inherits the same error. What causes the error-producing changes in airflow over the static vents? The prime causes are change in angle of attack, and turbulence due to lowering or raising flaps and landing gear. So whenever you change the aircraft's attitude or configuration, you disturb the airflow over the vents. Most commonly, manoeuvre-induced error appears as a marked lag in pressure instrument indications. The instruments just don't keep up. The errors are usually more significant during changes of pitch attitude than during yawing or rolling movements. So the worst effects are at the start of the climb or descent, and on levelling out — those are the moments of maximum pitch change. Overshooting — which is referred to as go-around — and flight in rough air are particularly vulnerable. So think of a go-around: you're pitching up hard, changing configuration, possibly in turbulence — that's the worst case. And here's the critical operational point: the errors are unpredictable both in size and in sense. That means you can't predict how big the error will be, and you can't even predict whether it will read high or low. So pressure instruments cannot be relied upon to indicate accurate instantaneous values or accurate rates of change. That's why, in a dynamic manoeuvre, you treat your altimeter and vertical speed indications with caution. That figure shows you a high-speed pitot/static probe — the combined head I mentioned, which is the more sophisticated solution for high Mach flight.