"Wyatt Emmerich" wrote
I had a my two-year transponder/static system check while visiting my mother
in San Antonio. When I departed, I noticed the airspeed didn't come up like
it should. I attributed this to shifting winds and assumed I had a slight
tailwind. Then in cruise, I did my normal operating performance check. My
IAS was about 30 knots low, but my groundspeed was normal. I tapped on the
airspeed indicator and it gained six knots. Question #1: How would a
transponder/static check screw up my airspeed? Was this just coincidence?

In my experience, taking the airplane to the shop for something and
having it come out with something broken that worked just fine
previously is more the norm than the exception. This is why I advise
people who are not going to be filing IFR to not get a pitot-static
check - not only does it cost money, but usually something winds up
broken.

Most likely, your problem is that the pitot line was opened in order
to attach a calibrated pressure source, and in the process either old
dirt was disturbed or new dirt was introduced. Now it's in the line
or the ASI.

Question #2: How do you check accuracy of an airspeed indicator?

By making up your own calibrated pressure source. Google
rec.aviation.homebuilt - there was a thread about this very thing
there in the last few months. A calibrated source is easy - all it
takes is some nylon tubing, a liquid of known density (like water),
and a ruler.

Question
#3: How long should you go before replacing an airspeed indicator?

Normally, they don't wear out. If you don't let sloppy workmen get
dirt into it, it will last decades.

Michael