Inaccurate airspeed indicator

[Wyatt Emmerich]

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?

[Richard Kaplan]

"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

[Bob Gardner]

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?
>
>
>
>

[Stan Gosnell]

"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]

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
>

[William W. Plummer]

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?
>
>
>
>

[Maule Driver]

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
news:DTjfc.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.
>

[Robert Moore]

"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

[John R. Copeland]

"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---

[Stan Gosnell]

"Wyatt Emmerich" > wrote in
:

> 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

[Jim Knoyle]

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

"John R. Copeland" > wrote in message
...

"Robert Moore" > wrote in message
. 6...
> "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

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---

Us old-timers got to see the 'system' evolve from a knuckle rap
to an external strap on vibrator and eventually the built in. :-)
Looking at a 777 diagram, it seems to have AnalogDigitalModules
located right at the pitot tubes or static port pairs and no need to
rap or vibrate anything. Ain't modern technology wonderful?

JK

[C J Campbell]

Shifting winds and tailwinds should have no effect on your airspeed
indicator.

The airspeed indicator is a static instrument. Dirt or something could have
been blown into there during the static check.

[Tarver Engineering]

"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.

[Robert Moore]

"C J Campbell" > wrote

> Shifting winds and tailwinds should have no effect on your airspeed
> indicator.

As opposed to "indicated airspeed"? How does one tell the difference?

Bob Moore

[Dale]

In article >,
"C J Campbell" > wrote:

> Shifting winds and tailwinds should have no effect on your airspeed
> indicator.

They would while in the early part of the takeoff roll...it will take
"longer" to notice the airspeed alive when you have a tailwind...GS
would have to exceed the tailwind before you'd get any indication.

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

[Michael]

"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

[C J Campbell]

"Michael" > wrote in message
om...
> "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.

Frankly, Michael, you seem to hang out with a rough crowd. You keep saying
that the flight instructors are all incompetent, as are all the pilots, the
FAA, the mechanics, and everybody else. Are you the only person in the whole
world who is able to do his job? If so, then we should all take out a big
insurance policy on you. We will need it if anything happens to you and the
world stops turning as a result. :-)

[Michael]

"C J Campbell" > wrote
> Frankly, Michael, you seem to hang out with a rough crowd. You keep saying
> that the flight instructors are all incompetent, as are all the pilots, the
> FAA, the mechanics, and everybody else.

That's fascinating. You have google available - please cite one place
where I said that. If the word "all" is not included, it's not a
cite.

Don't have one? Didn't think so.

> Are you the only person in the whole
> world who is able to do his job?

Nah. Lots are. For example, note the list of highly qualified
10,000+ hour instructors who trained me. BTW, none of them include
non-pilot time in that total.

There are even competent people in the FAA, much as it pains me to say
it. I've only ever met one, but I have many friends in the airline
business who claim to have met many, and I have no reason to doubt
their veracity.

But there is something special about GA. It's not really an industry
- it's more like a disease. Most of us involved in it know full well
that it's not a viable way to make a living. Most, but not all.

How do you make a small fortune in GA? Start with a large one, and
know when to quit. It's a cliche, but it didn't get to be a cliche by
being wrong. Once in a while someone manages to find a specialized
niche and do well with it (a few of them post here - Jim Weir and Paul
Sanchez are good examples), but most people who decide to make a
living in GA fall into the otherwise unemployable category.

I've known many excellent A&P mechanics. Few of them were interested
in doing general retail GA work for a living. One ran a specialty
restoration shop, one was a retired director of maintenance for a
major airline, one was an airline pilot who ran an FBO one the side,
and a few were owners who got the rating to be able to work on their
own stuff and help friends. Most of the really good A&P mechanics who
actually want to make a living of it wind up with the airlines or the
local automobile dealership. More money, fewer hassles. Can't blame
them, really. Very few owners in GA will pay what it costs to do
things right, and most of the ones who understand what doing it right
means in the first place are going to do it themselves. The ones who
are left, well, a handful are there because they really like messing
around with little airplanes, but most are not otherwise employable.

What does doing it right mean, anyway? Realize that most of these
planes are old, and have systems that have been patched and modified
many times, sometimes incompetently, and were often poorly designed in
the first place. As a result, they are fragile, and must be handled
with care. Care means time. If a guy advertises a
pitot/static/transponder check for under $200, you know something is
wrong. Think about it - he has an investment in equipment and labor
at least as great as an automotive safety/emissions inspection
station. Those guys inspect your car in 20 minutes and charge $40 to
do it (this is Houston-specific, but I doubt it's much different in
other major cities). That means the guy had to be able to pull and
check your transponder, encoder, ASI and altimeter and do your
paperwork in well under two hours to make it all work out. Is there
time to do all that CAREFULLY in less than two hours? Of course not.
So he either has to 'find' a problem and charge you to fix it, or he
has to rush the job and most likely break something. But if he was
up-front and charged what it really cost to do it right, he would lose
business to the lower-dollar competitors. Pilots are notoriously
cheap. Someone who is competent and honest will soon get disgusted
with this and leave the industry, so mostly we have the people who
didn't get disgusted and stayed.

I've known many excellent CFI's. These included airline and military
pilots, aerobatic competitors, and even some non-professional pilots
who had flown for years, accumulated many hundreds of hours of
experience flying all over the country, and had a desire to pass on
their knowledge and experience. Few of them were interested in doing
retail instruction, and none were interested in instructing full-time.
They would teach because they enjoyed teaching, when they felt like
it. Of course with that kind of attitude, they weren't too popular
with FBO's that were trying to run a business (not a hobby operation).
Most active instructors are building time for the airlines, and are
not otherwise employable in aviation. Most of them leave as soon as
they can get any other aviation job. The ones who display a
willingness to learn and a reasonable clue level get hired early
(connections also help) so a high-hours full-time CFI is actually a
red flag. You have to ask yourself WHY he is still a full-time CFI.
It could be skeletons in the closet (I knew a good instructor who
couldn't get hired until the incident where he buzzed boats on a lake
went off his record) or immigration issues, but more likely it's
because the professional pilots doing the hiring can spot him as a
loser from a mile away and don't want him in their fleet.

Interestingly, I have known VERY few bad/incompetent glider
instructors and even the very worst I've encountered were above
average for power instructors in terms of skill, knowledge, and
experience. There is a simple reason - glider instruction is not a
stepping stone to the airlines. As a result, the average glider
instructor is an accomplished professional in his field. That field
might be aviation, or it might not.

Michael

[running with scissors]

"Tarver Engineering" > wrote in message >...
> "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 :) :)



pathetic splaps boy. your *one* sample of trying to bolster your
credibility is a snip from the orginal post from the orginal thread,
which everyone knows and your feeble attempt at gaining any degree of
credibility leaves you looking like an even greater ******.

very poor splaps boy, very poor. your dissertation on "negative cabin
pressurization" has more inginuity.

[running with scissors]

"C J Campbell" > wrote in message >...
> "Michael" > wrote in message
> om...
> > "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.
>
> Frankly, Michael, you seem to hang out with a rough crowd. You keep saying
> that the flight instructors are all incompetent, as are all the pilots, the
> FAA, the mechanics, and everybody else. Are you the only person in the whole
> world who is able to do his job? If so, then we should all take out a big
> insurance policy on you. We will need it if anything happens to you and the
> world stops turning as a result. :-)

you are forgetting Tarver to whom, Honeywell/Bendix/King, Boeing, the
FAA, the NTSB, Antonov Design Bureau, Collins, NASA and the Wright
Brothers all owe a debt of gratitude.