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Larry,
The pitot-static check only evaluates whether the instruments themselves function properly and whether there are leaks in the lines between the test gear and the instruments. Errors caused by the location of the pitot and static probes on the aircraft and in the airflow can not be checked on the ground. In TC'd light airplanes 14 CFR 23.1323 requires that the error be known and and not exceed 3% of calibrated airspeed or 5 knots, whichever is greater (during certain phases of flight). The error, shown in the form of a plot or table of calibrated vs indicated airspeed is found in the POH of TC'd airplanes.
As far as I know, this is yet another case where we experimental builders are left to our own devices to calibrate (or not) as we wish. This is one prime reason to question the fantastic claims of high airspeed that
some proud builders make.
To calibrate, I'd suggest running some GPS legs and use the spreadsheet shown on this page (http://www.ntps.edu/HTML/Downloads/) to determine true airspeed. You'll simply pick an airspeed and maintain altitude and heading and read the GPS groundspeed. Then, holding the exact same airspeed, repeat at two significantly different headings and punch the numbers into the spreadsheet. Don't forget to record pressure altitude and temperature. Then, using the measured true airspeed (what comes out of the Excel spreadsheet) and the temperature and altitude, one can calculate calibrated airspeed. The 'position error' is simply the difference between this calibrated airspeed and the indicated airspeed read during the level flight points corrected for instrumentation error (as determined by the pitot-static checks). This should be done for a few representative airspeeds in level flight (100, 150,
180, etc) since the error isn't usually constant.
If you need any help with this, lemme know.
Cheers,
- Kyrilian
--- On Sun, 6/22/08, Larry Henney <LHenney@charter.net> wrote:
From: Larry Henney <LHenney@charter.net>
Subject: [LML] IAS Error/ Trouble shooting tips?
To: lml@lancaironline.net
Date: Sunday, June 22, 2008, 10:37 AM
Gents,
I flew formation
with a 320 today that had an IAS 20 kts above mine in welded wing. The
weird part was that the a/c had an IFR pitot/static transponder cert less than a
month ago. Additionally, the internal static port was found to be loose
and the shop reattached it and certified the installation. Meanwhile the
airspeed indicator was sent out for check out and was yellow tagged with no
problem found.
What is wrong with
my buddies airspeed system? Inquiring minds need to
know.
TIA for helping us
mitigate trouble shooting.
Larry
Henney
N360LH
IAS within a couple
kts of a dozen other planes. |
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