Pitot Pressure vs Airspeed

[Rob58]

A question for the engineering arm of the POA brain trust. I’m trying to understand how an instrument shop calibrates an airspeed indicator and also to find the associated tables of pressures. It seems that this would be very simple if all an airspeed indicator concerned itself with was the pitot ram air pressure, However since the ASI also ports static pressure into the instrument case this become a bit more complex because a differential pressure condition exists. So what is the process for validating that an ASI is accurate?

 

[Ed Haywood]

http://www.rstengineering.com/rst/articles/KP89JUL.pdf

 

[Dana]

However since the ASI also ports static pressure into the instrument case this become a bit more complex because a differential pressure condition exists. So what is the process for validating that an ASI is accurate?

The ASI is measuring differential pressure, yes, but when you're testing on the ground the static pressure at the static port is the same as the static pressure at the instrument being used to calibrate the ASI, so it cancels out and you only have to worry about the pitot pressure. This is "gauge pressure", which is the actual or total pressure minus 29.92" Hg or 14.7 psi (or whatever it happens to be on that day).

 

[Bell206]

So what is the process for validating that an ASI is accurate?

At the shop level they will use the process contained in their CRS manual as there are a number of specifications that can be used. For example, the TSO for airspeed indicators use RCTA and SAE specs. The tables you seek will be listed in those specifications. However, the bench calibration only consists of a check of true speed at sea level on a standard day. Once passed the indicator will get a correction card showing the scale/friction error values which usually are between 3-5 mph/knots. The max I believe is 5 or 6 depending on airspeed range block. The static side test is merely a leak check to ensure the internal diaphragm integrity and that the given airspeed values correspond to the values of a standard day or sea level at 29.92 @ 59 deg F.

Once you get the indicator back from the shop the in flight calibration falls to the OEM whose limits are usually found in the POH or AFM. There may also be a calibration requirement during the installation of the indicator as well. But once beyond the bench check limits most accuracy values are approximate and so stated in general.

Keep in mind this applies to your standard standalone airspeed ind and not air data systems which may have additional requirments.