On Sat, 07 Jan 2006 09:34:26 -0600, "O. Sami Saydjari"
wrote:

In my Cessna 421C, the pilot-side airspeed indicator reads 6 nts below
the co-pilot-side airspeed indicator. When I pointed this out to a
mechanic, he said that it was "within toleraance." He said it was
possible that one was 3 nts high and the other 3 nts low and so both
could be within tolerance. He added that there is no real internal
adjustment, so there is not much he could do about it anyway.

Seems to me that when you are talking about a possible 6nt difference
and given the need for accurately maintain blue-line speed in case of a
single-engine failure, it is important to know that one's airpseed
indicator is accurate. Isn't there a way to judge true accuracy of
speed indicators (just as there is for altimeters)? Should I be
satisfied with my mechanic's answer and move on?

-Sami
N5554G, Cessna 421C

There are units, similar to those used for altimeter checks, that can be
used to verify ASI accuracy on the ground.

In addition, if you can perform a TAS calculation, and you have a GPS on
board, you could do three runs at 90° angles to each other, and compare
your real TAS to the one obtained from your ASI -- of course that only
gives you the error at one point, but that might be enough. (There are
tools on the web to enable you to calculate your TAS from the three GPS
groundspeed determinations).




Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)