Pitot-Static System Errors - How to measure them

[Kevin Horton]

Every aircraft with an airspeed indicator and an altimeter has errors in those systems. The errors are due to many factors:

• Instrument error,
• Leaks in the plumbing,
• Position errors in the static system, and
• Position errors in the pitot system.

Instrument error in the airspeed indicator (ASI) can easily be measured using a water manometer. Instrument error in the altimeter cannot be easily be measured by the builder, but an avionics shop can test it with a pitot-static test set.

Leaks in the plumbing can be easily checked.

Position errors in the static system are caused by the fact that the air flowing around the aircraft has to accelerate and decelerate as it wends its way over the shape of the airplane. The air pressure changes as it accelerates and decelerates, so the pressure at the static port is quite likely different than the free stream static pressure well away from the aircraft. This difference between the sensed static pressure and the free stream static pressure is position error, and it accounts for most of the error in the ASI (assuming the instrument error is small, all leaks are fixed, and the pitot tube is not too poorly located).

The pitot tube is relatively immune to position errors, as it measures total pressure, not static pressure, and the total pressure isn?t changed as the air moves around the aircraft. If your pitot tube is reasonably well aligned into the airflow, and it is well clear of the prop wash, and it is not in the wake of something else, it should be quite accurate. Pitot tubes should provide reasonable accuracy if the local airflow is aligned within 15 degrees of the pitot tube. So, it might become inaccurate as you approach the stall.

I have put together a guide on how to measure ASI instrument error using a water manometer and how to determine static system position error via flight testing. This guide is located at http://www.kilohotel.com/rv8/rvlinks/ssec.html.

[kai]

Hi Kevin, is that guide still available somewhere? I would appreciate to get a copy of that!

Thanks,

Kai

[GalinHdz]

Originally Posted by kai

Hi Kevin, is that guide still available somewhere? I would appreciate to get a copy of that!

Thanks,

Kai

Here is a corrected link:

http://www.kilohotel.com/rv8/rvlinks/ssec.html

[kai]

Great, thanks!

[wawrzynskivp]

Going to try this, thanks!

[prkaye]

Within Kevin's guide he has a link to the EAA Chapter 1000 website for specific instructions on pitot-static instrument calibration:
http://www.eaa1000.av.org/technicl/instcal/instcal.htm
This link appear to be broken.
Does anybody know where this article, or alternative instructions for doing a pitot-static leak check, can be found?

[Kevin Horton]

Originally Posted by prkaye

Within Kevin's guide he has a link to the EAA Chapter 1000 website for specific instructions on pitot-static instrument calibration:
http://www.eaa1000.av.org/technicl/instcal/instcal.htm
This link appear to be broken.
Does anybody know where this article, or alternative instructions for doing a pitot-static leak check, can be found?

The Internet Archive is useful to find sites that have died or moved. See:
https://web.archive.org/web/20210125...al/instcal.htm

I retrieved a copy of the last version of the page from the Internet Archive, and saved it on my web site. It can be found at https://www.kilohotel.com/rv8/rvlink...libration.html

[Tim Lewis]

Kevin's VERY helpful airspeed/static system error flight test page link in his original 2005 post (above) doesn't seem to work (clicking the VAF link in the 2005 post, that is). Here's a link that works:

https://www.kilohotel.com/rv8/rvlinks/ssec.html