Menu Sign In Contact FAQ
Banner
Welcome to our forums

ASI Calibration

10 Posts

Just a very simple question please. How is the ASI calibrated and what tolerances are allowed. On another note I am sure I will not be alone in saying that the level of technical know-how and obvious expertise demonstrated on these forums is astonishing. The electronics that people discuss in detail is so lost on me (old pilot!) that I wonder if I should buy a much simpler airplane - (only joking!)

UK, United Kingdom

The ASI checking is done in the same way as the altimeter checks are done: a pipe is attached to the pitot tube and a pressure is applied (for altimeter checks, suction is applied, obviously).

There is a fairly pricey test rig for doing both jobs in one go.

What suprises me is that this works accurately. Presumably the pitot tubes must all be manufactured to deliver an accurate and known relationship between the ram air pressure and the airspeed, otherwise just sticking a tube onto it and pumping that up to some known pressure would not work.

As to accuracy, I don't know for EASA-reg but the FAA altimeter check is FAR 91.411 which references Part 43 Appendix E and the table is in there. On an N-reg this check is done very 2 years and has to be done by a Part 145 Repair Station.

I hope I got that right

Administrator
Shoreham EGKA, United Kingdom

ASI testing can be done with a water manometer. When we did glider C of A renewals we used to check them ourselves. This was in the olden days, probably not allowed any more.

It works because the pressure from a pitot is not to do with pitot design, just basic physics.

Darley Moor, Gamston (UK)

If you intend to check the ASI and ALT on the aircraft be careful to make sure both systems are correctly attached to pitot and static lines otherwise the ASI will go off the stops when testing the ALT. As an observation: it's sometimes difficult to connect to the pitot head because of its shape e.g. Piper blade type. It sometimes easier to check these instruments directly, however this doesn't preclude the possibility of leaks in the lines from the pitot and static ports.

jxk
EGHI, United Kingdom

Learn something every day...

Is it really true there is no possible variation between pitot tubes - even down to the 1kt accuracy level?

Actually, back to the original Q, the definitive way to calibrate an ASI is using the GPS three-headings method. There is loads on google but if you pick a day with light winds, say below 10kt, the formula reduces to flying three headings, 120deg apart, noting the GPS GS for each, taking the average of the three speeds, and that gives you TAS. Then use a standard correction from TAS to IAS, which needs the temperature aloft (so you need a reasonably accurate OAT gauge too) and the altitude.

jxk - what are the UK G-reg requirements for accuracy of the two, and the test frequency?

Administrator
Shoreham EGKA, United Kingdom

Fortunately the pitot head is remarkably insensitive to alignment and potential field errors. So long as the air at the pitot opening has been made stationary relative to the pitot, then the full stagnation pressure rise will be fed to the pitot line. The same can't be said for the static port, which has to be positioned where the free stream flow is undisturbed. In practice, the designer must find a position on the airframe which closely approximates that, but this can't usually be achieved for all aircraft attitudes, speeds and configurations. They usually aim to minimise the error in cruise configuration.

The pitot and static tests on the ground only serve to check the instruments calibration accuracy and for leaks. It can't check for the installation dependent errors that happen in flight.

KHWD- Hayward California; EGTN Enstone Oxfordshire, United States

Manufacturers specifications apply (ASI). Many aircraft manufacturers have also implemented ASI tolerances in their airplane maintenance manuals.
/Sam



Zsolt Szüle
LHTL, Hungary

You do not NEED the fancy kit for calibration of an ASI.
In ancient times when I was gliding, we used to calibrate ASIs using a water Manometer.

I googled and this article came up

http://www.eaa1000.av.org/technicl/instcal/instcal.htm

Darley Moor, Gamston (UK)

However, the three-heading method is dead easy, dead quick and “totally” accurate.

Administrator
Shoreham EGKA, United Kingdom
10 Posts
Sign in to add your message

Back to Top