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

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.

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

In the B-707, the altimeter had a built-in altimeter-tapper at about
two cps. This was required equipment in the event that the electronic
altimeter correction failed or was turned off. Turning the correction
off, turned on the tapper.

Bob Moore

"Robert Moore" wrote in message =
. 6...
"Maule Driver" wrote=20
That's glider stuff. You tap it instead of allowing engine =
vibration to
do it.
=20
In the B-707, the altimeter had a built-in altimeter-tapper at about
two cps. This was required equipment in the event that the electronic
altimeter correction failed or was turned off. Turning the correction
off, turned on the tapper.
=20
Bob Moore

The drum-and-pointer altimeter in my C340 has an internal vibrator.
I don't know its frequency, but it's much higher than 2 cps.
When it's not vibrating, there's a big hang-up every 1000 feet,
at the point where the drum reading indexes to its next value.
Engine vibration is insufficient to keep it free.
---JRC---

"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