Pitot/Static/Transponder Problem

Ron Wanttaja wrote:
On Sun, 05 Aug 2007 10:27:25 -0700, quietguy wrote:


Building to the plans doesn't guarantee accurate static pressure
readings; the system is quite sensitive to tiny plane-to-plane
variations. And the official calibration check with the airplane
stationary wouldn't catch an in-flight static-pressure error.


Yes, but: If the encoder and the altimeter are using the same static source,
both should be in error the same amount. Kyle's problem is a split indication;
the encoder is sensing a different altitude than the altimeter.

This tends to argue that the problem is a leak related to the altimeter itself,
not in the static system. Perhaps the altimeter has a leak that only manifests
itself when it's being vibrated (i.e., the engine running).

My Microair transponder gives me a readout of the altitude that's being sent to
ATC. I can set the A/C altimeter to 29.92 and compare. That's how I discovered
my encoder had gone south two months back; the altitude readout wasn't matching
the altimeter.

Ron Wanttaja

Are your transponder and altimeter physically close to each other?

(How far apart could they be in a Fly Baby??)

In larger planes it's not uncommon for the encoder to be mounted behind
the seats or such. Quite far from the altimeter - with a lot of extra
hose between them.

Just a thought...


Richard

Maybe some friction/hysteresis in the altimeter not showing up during the
altimiter check, but not likely.
Maybe some strange internal pressure/vacuum in the cockpit in flight
combined with a leaking static
system, but you passed the static test you say.
You should try and tie it down to which, if either, is right and wrong.
Does the altimeter indicate field altitude when
you set the Kollsman window to the ATIS altimeter setting? One way to get a
pretty good altitude check if you can fly
the glideslope with accuracy and if the plane is so equiped is to pass over
the outer maker with the GS needle dead nuts centered and compare the
crossing altitude with what is
indicated on the approach plate.
"Kyle Boatright" wrote in message
...
I was using Flight Following yesterday and was reminded of a problem that
my RV has displayed for a long time: The transponder and altimeter do not
agree. In general, ATC sees my altitude as about 200' lower than what is
shown on my altimeter.

A couple of facts:

- The transponder/encoder always pass their certification checks.

- The pitot/static system is installed per plans.

Any thoughts?

KB

"Kyle Boatright" wrote in message
...
I was using Flight Following yesterday and was reminded of a problem that
my RV has displayed for a long time: The transponder and altimeter do not
agree. In general, ATC sees my altitude as about 200' lower than what is
shown on my altimeter.

A couple of facts:

- The transponder/encoder always pass their certification checks.

- The pitot/static system is installed per plans.

Any thoughts?

KB

An update on this thread... There are no apparent loose hoses, etc between
the altimeter and encoder which would explain a discrepancy between the two.
Also, the altimeter is 30' off (it reads low) on the ground which is well
within limits. This 30' low reading is consistent in flight (to the best of
my ability to gauge it during a low pass).

Presumably, this means the encoder needs to be adjusted. Other thoughts?

Oh, and by the way, climbing under the panel of an RV-6 to check this stuff
ain't no fun...

KB

Kyle Boatright wrote:
"Kyle Boatright" wrote in message
...
I was using Flight Following yesterday and was reminded of a problem that
my RV has displayed for a long time: The transponder and altimeter do not
agree. In general, ATC sees my altitude as about 200' lower than what is
shown on my altimeter.

A couple of facts:

- The transponder/encoder always pass their certification checks.

- The pitot/static system is installed per plans.

Any thoughts?

KB

An update on this thread... There are no apparent loose hoses, etc between
the altimeter and encoder which would explain a discrepancy between the two.
Also, the altimeter is 30' off (it reads low) on the ground which is well
within limits. This 30' low reading is consistent in flight (to the best of
my ability to gauge it during a low pass).

Presumably, this means the encoder needs to be adjusted. Other thoughts?

Oh, and by the way, climbing under the panel of an RV-6 to check this stuff
ain't no fun...

KB

At a minimum you need to have your altimeter and encoder
correlated/adjusted by somebody rated to do that. In the mean time, turn
off the alt reporting at the transponder.
The adjustment can't be done without the proper equipment, sorry.
The next question is why did the encoder drift off by so much and will
it do it again? Since it's drifted that much, it might be worthwhile to
get a new encoder. They're pretty cheap.

The altimeter can only be legally adjusted by a certified repair
station. It's a good idea to make sure the altimeter is accurate when a
new encoder is installed.

Dave
www.craigmileaviation.com

"Dave" wrote in message
...

The altimeter can only be legally adjusted by a certified repair station.
It's a good idea to make sure the altimeter is accurate when a new encoder
is installed.

Hmmm ........ Lessee. I'm just working off the cuff here, but if an
altimeter isn't even a required piece of equipment, why would the above
statement be true? In an experimental aircraft, couldn't I use a barometer
and a wet string if I wanted?

Rich (ducking and hiding) S.

On Sat, 11 Aug 2007 15:47:46 -0700, "Rich S."
wrote:

"Dave" wrote in message
...

The altimeter can only be legally adjusted by a certified repair station.
It's a good idea to make sure the altimeter is accurate when a new encoder
is installed.

Hmmm ........ Lessee. I'm just working off the cuff here, but if an
altimeter isn't even a required piece of equipment, why would the above
statement be true? In an experimental aircraft, couldn't I use a barometer
and a wet string if I wanted?

Actually, an altimeter *is* required for VFR. It's just that a "sensitive
altimeter adjustable for barometric pressure" isn't required unless you're going
IFR. That keeps all the one-armed Cub altimeters legal....

Ron Wanttaja

"Ron Wanttaja" wrote in message
...

Actually, an altimeter *is* required for VFR. It's just that a "sensitive
altimeter adjustable for barometric pressure" isn't required unless you're
going
IFR. That keeps all the one-armed Cub altimeters legal....

So the barometer and wet string is okay?

Rich

On Sun, 5 Aug 2007 12:40:36 -0400, "Kyle Boatright"
wrote:

I was using Flight Following yesterday and was reminded of a problem that my
RV has displayed for a long time: The transponder and altimeter do not
agree. In general, ATC sees my altitude as about 200' lower than what is
shown on my altimeter.

A couple of facts:

- The transponder/encoder always pass their certification checks.

- The pitot/static system is installed per plans.

Any thoughts?

Try it at a controled airport. Get an altimeter check while
stationary. Make a low pass and get another to see how they vary.


KB