Pitot/Static/Transponder Problem

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

Tell ATC that you aren't 200' low, they're 200' too high (Actually,
I've encountered 200' differences when switched to a new ATC sector -
somebody's not calibrated.)

Seriously, if you set your altimeter to the local AWOS/ATIS/ASOS does it
read the published field elevation? If not, the altimeter needs
calibration. If so, it's not the instrument.

Presumably, it's not the encoder if it passes the bi-annual field check but
do you know if your avionics shop has calibrated their transponder checking
equipment?

When was the last pitot/static check done? Do you have a leak in the static
tubing? If there is a leak in the static system, the altimeter may be
reading cabin pressure which 'could' be higher than a true static.

Oh the fun of chasing gremlins...

Bill Daniels

"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

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.

Have an observer watch the altimeter during takeoff and landing ground
rolls: if it changes from a standstill to takeoff speed and then
changes in the opposite sense during the landing roll (the VSI would
also depart from zero at these times), then you'll know that there's a
static-pressure measurement error built into your installation. The
amount of the spurious altitude change would be about 200 ft times the
ratio of takeoff/landing speed to cruise speed; e.g., if the speed
ratio is about 1/3, then the change would be 60-70 ft.

The static port(s) wouldn't necessarily have to be relocated if this
error is present; you may be able to tweak the readings sufficiently
with a wire-and-tape "trip strip" just ahead of the port (to lower the
static reading) or behind it (to raise the reading). Even some store-
bought airplanes have such cheap-and-easy fixes near their ports.
Just keep experimenting with the height and position of the strip
until the airplane doesn't "climb" or "descend" while trundling along
a level runway.

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

The encoder gives an uncorrected altitude. You altimeter is
corrected when you set it for barometric pressure at the beginning of
flight. ATC receives the signal from the encoder/transponder and
corrects it for the local pressure. If you are some distance from
their station, there could easily be a 0.2" Hg pressure differential
that would result in a 200' differential (one inch Hg per 1000' of
altitude).
So even if you are using the altimeter setting they give you, I
wouldn't expect perfect correlation if there's a pressure gradient
beteen you and them.

Dan

wrote in message
oups.com...

The encoder gives an uncorrected altitude. You altimeter is
corrected when you set it for barometric pressure at the beginning of
flight. ATC receives the signal from the encoder/transponder and
corrects it for the local pressure. If you are some distance from
their station, there could easily be a 0.2" Hg pressure differential
that would result in a 200' differential (one inch Hg per 1000' of
altitude).
So even if you are using the altimeter setting they give you, I
wouldn't expect perfect correlation if there's a pressure gradient
beteen you and them.

Dan

If it was an intermittent problem, your theory would be plausible, but since
I see the same problem virtually every time I deal with ATC, I'm thinking it
is a hardware problem. The only issue is which piece of hardware? ;-)

KB

"Bill Daniels" bildan@comcast-dot-net wrote in message
. ..
Tell ATC that you aren't 200' low, they're 200' too high (Actually,
I've encountered 200' differences when switched to a new ATC sector -
somebody's not calibrated.)

Seriously, if you set your altimeter to the local AWOS/ATIS/ASOS does it
read the published field elevation? If not, the altimeter needs
calibration. If so, it's not the instrument.

The altimeter was reworked a couple of years ago because it got "sticky".
It came back with a yellow tag and a test card indicating it was calibrated
within VFR limits. Since the altitude discrepancy problem existed before
the altimeter rebuild and has persisted since, I'm guessing the Altimeter is
good.


Presumably, it's not the encoder if it passes the bi-annual field check
but do you know if your avionics shop has calibrated their transponder
checking equipment?

No idea, but doesn't the shop have to calibrate its equipment to remain a
certified shop?


When was the last pitot/static check done? Do you have a leak in the
static tubing? If there is a leak in the static system, the altimeter may
be reading cabin pressure which 'could' be higher than a true static.

I've never done a pitot/static check. Is there a DIY procedure for that?


Oh the fun of chasing gremlins...

You got that right!


Bill Daniels

"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

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

"Kyle Boatright" wrote in message
. ..

"Bill Daniels" bildan@comcast-dot-net wrote in message
. ..
Tell ATC that you aren't 200' low, they're 200' too high (Actually,
I've encountered 200' differences when switched to a new ATC sector -
somebody's not calibrated.)

Seriously, if you set your altimeter to the local AWOS/ATIS/ASOS does it
read the published field elevation? If not, the altimeter needs
calibration. If so, it's not the instrument.

The altimeter was reworked a couple of years ago because it got "sticky".
It came back with a yellow tag and a test card indicating it was
calibrated within VFR limits. Since the altitude discrepancy problem
existed before the altimeter rebuild and has persisted since, I'm guessing
the Altimeter is good.

It's a good bet that a yellow tagged altimeter is OK but I'd still check it
against field elevation. In fact, it's a good idea to do it each time you
do a pre-flight check. Just set the local altimeter pressure and see if the
altimeter indicates field elevation.

The most likely problem is a leak in the static system. If you're up to it
there's a simple check. Get a large plastic hypo syringe and carefully "T"
it into the static tubing behind the panel using a fish tank tubing "T" and
Tygon tubing - make sure all fittings are leak free. Cover the static ports
with electrical tape to seal them off. SLOWLY inject air with the syringe
until the altimeter reads 1000 feet lower than field elevation. Stop
injecting air and watch the altimeter. If there's a leak, the altimeter
will slowly return to field elevation. The ROC will indicate the rate of
the leak.

Realize that the leak can be inside an instrument. I've seen several ROC
instruments with internal leaks (Usually the glass gasket is bad which can
be determined with a soap solution.) but the ASI or even the altimeter
itself can be the culprit. If there is a leak, look for bubbles with soap
solution until you find where it is.

BTW, there's an old glider pilot trick to sealing Tygon tubing to instrument
nipples. Go to a veternary supply store and buy a bag of the rubber
bands(green O-rings) used to castrate sheep (I'm not kidding!) These are
just the right size to slip over the outside of the Tygon after it is pushed
onto the instrument nipple. Put the O-ring over the tubing before pushing
it onto the instrument nipple then roll it over the nipple. It makes a
reliable gas-tight fit even when the Tygon gets old and yellowed.

Bill Daniels

"Bill Daniels" bildan@comcast-dot-net wrote in message
news

"Kyle Boatright" wrote in message
. ..

"Bill Daniels" bildan@comcast-dot-net wrote in message
. ..
Tell ATC that you aren't 200' low, they're 200' too high
(Actually, I've encountered 200' differences when switched to a new ATC
sector - somebody's not calibrated.)

Seriously, if you set your altimeter to the local AWOS/ATIS/ASOS does it
read the published field elevation? If not, the altimeter needs
calibration. If so, it's not the instrument.

The altimeter was reworked a couple of years ago because it got "sticky".
It came back with a yellow tag and a test card indicating it was
calibrated within VFR limits. Since the altitude discrepancy problem
existed before the altimeter rebuild and has persisted since, I'm
guessing the Altimeter is good.

It's a good bet that a yellow tagged altimeter is OK but I'd still check
it against field elevation. In fact, it's a good idea to do it each time
you do a pre-flight check. Just set the local altimeter pressure and see
if the altimeter indicates field elevation.

I should have mentioned before that the altimeter is close to the field
elevation when on the ground. Well within VFR limits.

One interesting test would be a low altitude pass down the runway at cruise
power and speed. If I'm 50' above the field and the altimeter shows field
level + 50' (or thereabouts) what would that tell me?

A basic question: If there was a problem with the static system, wouldn't
that cause the same discrepancy in both the altimeter and transponder?


The most likely problem is a leak in the static system. If you're up to
it there's a simple check. Get a large plastic hypo syringe and carefully
"T" it into the static tubing behind the panel using a fish tank tubing
"T" and Tygon tubing - make sure all fittings are leak free. Cover the
static ports with electrical tape to seal them off. SLOWLY inject air
with the syringe until the altimeter reads 1000 feet lower than field
elevation. Stop injecting air and watch the altimeter. If there's a
leak, the altimeter will slowly return to field elevation. The ROC will
indicate the rate of the leak.

I was trying to come up with a technique to do this, and this one sounds
like a winner. There is a location in the baggage compartment where it is
easy to get to the static tubing...


Realize that the leak can be inside an instrument. I've seen several ROC
instruments with internal leaks (Usually the glass gasket is bad which can
be determined with a soap solution.) but the ASI or even the altimeter
itself can be the culprit. If there is a leak, look for bubbles with soap
solution until you find where it is.

BTW, there's an old glider pilot trick to sealing Tygon tubing to
instrument nipples. Go to a veternary supply store and buy a bag of the
rubber bands(green O-rings) used to castrate sheep (I'm not kidding!)
These are just the right size to slip over the outside of the Tygon after
it is pushed onto the instrument nipple. Put the O-ring over the tubing
before pushing it onto the instrument nipple then roll it over the nipple.
It makes a reliable gas-tight fit even when the Tygon gets old and
yellowed.

Bill Daniels

Thanks for the ideas.

KB