# How does the static pressure system keep the ambient pressure?

I understand that the compressed airflow around the aircraft effectively lowers the air pressure, and that's the reason why the alternate air switch affects your altimeter ­­— it uses the slightly lower pressure in the cabin as reference — or why fuel is sucked out of the tanks if you forget your fuel caps.

This begs the question, what in the design of the static pressure system makes it more closely match the ambient pressure? Why is it unaffected by the effects affecting the cabin pressure?

• Hi falstro! Do you mean "lowered per Bernoulli's principle or Venturi's effect"? If so: Why isn't "static" pressure speed-dependent?
– mins
Jun 15, 2016 at 18:42
• BTW, I know @JayCarr suggested it, but "begs the question" means something completely different than what most English speakers use it for. Raises the question is more accurate. Jun 16, 2016 at 4:47
• @mins so in other words, someone is trying their darndest to place the static port somewhere where the effects are as small as possible, while never really succeeding. Would be cool to know how you usually go about looking for that spot though. Jun 16, 2016 at 8:40
• @JanHudec that answer is a mile long, I didn't immediately spot what you're referring to and the question asks something completely different. Can we please ease up on the happy close-as-duplicate trigger fingers? :P If you want, collect the relevant parts of that answer, refer to it and post an answer :( Jun 16, 2016 at 9:38
• @JanHudec Not to mention that someone actually might want to explain it a bit more in-depth. Since this question actually asks about it, as opposed to the other question. There might be more answers. And so on. Jun 16, 2016 at 9:43

Test. Calibrate. Repeat.

There is a new method developed by NASA based on GPS tracking. And here's the same thing but as a video (also NASA).

New Method for Pitot-Static Calibration

A precise, time- and cost-effective method based on global positioning system technology using output error optimization (...) enables near real-time monitoring of error in airspeed measurements, which can be used to alert pilots when airspeed instruments are inaccurate or failing.

Webpage also mentions the current methods that include the trailing cone (the one you see on modern airliners while they are being tested), tower fly-by's, and pacer airplanes.

Trailing cones were first developed and tested in the 1950s and 1960s as a simple means of calibrating the static pressure error of an aircraft's pitot-static system.

So apart from it being time consuming, it should be relatively easy. If it says I'm doing 100 KIAS but I know I'm only doing 96, just calibrate the 100 to be 96. Probably needs a screwdriver and a certified mechanic. But doable. Same for the altimeter and so on. Altimeter would be easy to calibrate on ground. Put in the current QNH, check the reading against the actual (from an airport diagram) and take out your screwdriver (not literally).

The pitot-static system doesn't know the right pressure, it just needs calibration. Which also answers why the cabin atmosphere won't be as precise, it wasn't calibrated for that.

• While interesting, the NASA methods seem to target calibrating ASIs, not the placement of the static port or other means of combating airspeed dependence in the static pressure system (and thus altimeters and VSIs). The trailing cones are interesting though. Jun 17, 2016 at 9:56
• Digital avionics with pitot and static inputs can incorporate Static Source Error Correction, which adjusts the calculated static pressure value based on the preadjusted airspeed, which is associated with some pressure offset by the airplane manufacturer Sep 28, 2017 at 17:49