Pitot and Static Sources — Page 18, Lesson 24

Let’s pick up right where the static vent story left off, because the passage you’ve just moved into is all about the static vent’s weaknesses and how high-speed aircraft fight back. First, a quick recap of the setup so the new material lands. The static line runs from the pressure instruments to a small circular hole in a plate — that hole is the static vent. A similar vent sits on the opposite side of the fuselage, and the two are interconnected so static pressure is transmitted to the instruments from both sides. That interconnection is the key trick: because the two vents are linked, errors produced by yawing are largely eliminated. If the aircraft yaws, one vent sees slightly high pressure and the other slightly low, and the interconnection averages them out. Now, the passage lists the advantages of the static vent, and I want you to hold these as the baseline. First, the airflow in the region of the vents is less turbulent, so the static pressure measured is more accurate. Second, errors produced when side-slipping or yawing are reduced. And third — this is the duplication point — having static vents on either side of the fuselage reduces errors due to side-slip or yawing. That’s what they call cross balancing of static vents. So the whole design philosophy is: two vents, interconnected, to cancel out the asymmetric errors. But here’s the catch, and this is where the passage turns. At high Mach numbers, the shock waves associated with flight can produce significant errors in the pressure sensed by a static vent. The airflow around the vent gets disturbed by those shock waves, and the pressure it reads is no longer a clean representation of ambient static pressure. So modern high-speed aircraft may be fitted with a more sophisticated combined pitot/static pressure head. That’s the high-speed probe. The goal is to keep position error within acceptable limits. Position error, by the way, is the error caused by the location of the probe or vent on the airframe — the local airflow at that spot doesn’t perfectly match free-stream static pressure. Now, the choice of location for a probe or vent is dependent upon the aerodynamics of the aircraft. There’s no universal spot. 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 of those positions is chosen to sit in relatively undisturbed airflow for that particular airframe. Then we get to manoeuvre-induced error, and this is a distinct failure mode from position error. Manoeuvre-induced errors are caused by short-term fluctuations of pressure at the static vents, plus delays in the associated pipelines transmitting pressure changes to the instruments. So there are two contributors: the pressure at the vent itself is fluctuating, and the pipework takes time to pass those changes along to the instruments. Here’s a crucial point — even servo altimeters and air data computer systems suffer from this type of error, because they utilize the same static vents as the simple pressure instruments. So upgrading the instrument doesn’t fix it; the error is born at the vent and in the pipeline, which are shared. What causes the pressure fluctuations? Change in angle of attack, and turbulence due to lowering or raising flaps and landing gear. Those are the prime causes of the error-producing changes in airflow over the static vents. So when you change the aircraft’s configuration or pitch, you disturb the airflow right where the vent sits. How does it show up? Most commonly, manoeuvre-induced error appears as a marked lag in pressure instrument indications. The instrument doesn’t respond instantly; it lags behind the true pressure change. And when is it worst? 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 — and the passage calls that go-around — and flight in rough air are particularly vulnerable. Now the final, sobering 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 predict whether it will read high or low. Because of that, pressure instruments cannot be relied upon to indicate accurate instantaneous values or accurate rates of change. So during a go-around or in turbulence, the altimeter, the vertical speed indicator, the airspeed indicator — they may all be lagging and off, and you have no way to know by how much or in which direction. That’s the full picture of manoeuvre-induced error: it’s a lag, it’s worst in pitch changes, it’s unpredictable in size and sense, and it affects even the most modern systems because they share the same static vents.