Inaccurate airspeed indicator

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?
Question #2: How do you check accuracy of an airspeed indicator? Question
#3: How long should you go before replacing an airspeed indicator?

"Wyatt Emmerich" wrote in message
...


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

Debris in your pitot system could cause inaccuracy of the airspeed
indicator, as could a leak in the system.

While your airspeed indicator could be malfunctioning, it is more likely to
be a problem with the pitot system.

Are the VSI or altimeter also affected? If so, the problem could be the
static system. Do you have an alternate static source on your airplane
which you could try?

Airspeed indicators are not generally replaced unless they fail; often 40+
year old airplanes still have their original airspeed indicator.


--------------------
Richard Kaplan, CFII

www.flyimc.com

Kinda hard to imagine how a tailwind would affect the airspeed indicator. I
think you are due for a visit to the radio shop's instrument department.

Bob Gardner

"Wyatt Emmerich" wrote in message
...
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?
Question #2: How do you check accuracy of an airspeed indicator? Question
#3: How long should you go before replacing an airspeed indicator?

"Wyatt Emmerich" wrote in
:

Question #1: How would a transponder/static check
screw up my airspeed? Was this just coincidence?

Connections were likely loosened during the check, and weren't tightened
correctly. Perhaps a bug or some dirt were lodged in the system and were
loosened during the check. Have it rechecked before flying again.

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

Here is where familiarity with your aircraft is essential. If you know it
always gets a certain airspeed at a certain power setting and weight, you
can get a good idea. You can't check exactly, though, and that's why you
need to have a shop you trust check it annually.

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

Have a shop check your system. They can tell you if the indicator is bad,
but that's rare. More often it's the pitot-static system piping.

--
Regards,

Stan

The altimeter readings were all correct on the flight. The transponder
reported accurately and center never had a problem with my altitude. So it
seems to me, the static system must be correct.


"Stan Gosnell" wrote in message
...
"Wyatt Emmerich" wrote in
:

Question #1: How would a transponder/static check
screw up my airspeed? Was this just coincidence?

Connections were likely loosened during the check, and weren't tightened
correctly. Perhaps a bug or some dirt were lodged in the system and were
loosened during the check. Have it rechecked before flying again.

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

Here is where familiarity with your aircraft is essential. If you know it
always gets a certain airspeed at a certain power setting and weight, you
can get a good idea. You can't check exactly, though, and that's why you
need to have a shop you trust check it annually.

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

Have a shop check your system. They can tell you if the indicator is bad,
but that's rare. More often it's the pitot-static system piping.

--
Regards,

Stan

"Wyatt Emmerich" wrote in
news
The altimeter readings were all correct on the flight. The transponder
reported accurately and center never had a problem with my altitude.
So it seems to me, the static system must be correct.

A loose connection in the ram air system could give erroneous airspeed
readings while giving correct altimeter readings, as could a partial
obstruction. I've seen water in the pitot system give this error also,
which is really a partial obstruction.

--
Regards,

Stan

"Stan Gosnell" wrote in message
...
"Wyatt Emmerich" wrote in
news
The altimeter readings were all correct on the flight. The transponder
reported accurately and center never had a problem with my altitude.
So it seems to me, the static system must be correct.

A loose connection in the ram air system could give erroneous airspeed
readings while giving correct altimeter readings, as could a partial
obstruction. I've seen water in the pitot system give this error also,
which is really a partial obstruction.


Just a comment:
We were required to do a leak test after opening a pitot or static
line for whatever reason. Eventually water drain fittings were
removed from this required list but that seemed to me at the time
to be a mistake since those o-rings were especially prone to
rotting out. O-ring quality improvement seemed to solve that.

JK (expecting a mud wasp comment from tarver)

"Jim Knoyle" wrote in message
...

Just a comment:
"Tarver Engineering" wrote:


"Gord Beaman" wrote in message
.. .
"Jim Knoyle" wrote:

Hang on here a second now Jim, you still need two samples. As Dan
says you need 'static pressure' to read the altitude from and you
need 'pitot pressure' (ram air pressure) as well as the static
pressure to derive the airspeed reading from. Sounds like you're
saying that you can read 'both' from just the 'ram air pressure'
alone. Or did I misunderstand you?

Jim has finally figued out what a pitot tube is, but somehow he still wants
to be correct in his archive troll. It is a great paradox.


I know...ain't life a bitch John

--

-Gord.

My ASES instructor had a habit of tapping on the altimeter from time to
time. He said you had to do that in the old days to get an accurate
reading.

"Wyatt Emmerich" wrote in message
...
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?
Question #2: How do you check accuracy of an airspeed indicator? Question
#3: How long should you go before replacing an airspeed indicator?

That's glider stuff. You tap it instead of allowing engine vibration to do
it.

If the engine is running, you shouldn't need to touch it.

"William W. Plummer" wrote in message
newsTjfc.38063$_K3.163660@attbi_s53...
My ASES instructor had a habit of tapping on the altimeter from time to
time. He said you had to do that in the old days to get an accurate
reading.