Pitot/Static/Transponder Problem

"Kyle Boatright" wrote in message
...

... 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?
...

That seems likely as far as static sytem design is concerned (e.g. static
port location).

If they differ only in flight, then they probably aren't seeing the same
static source due to kinks and leaks. I assume that both are really
connected to the static source (You have looked, right? No hose has fallen
off?)

Have the encoder output compared to the altimeter while sitting on the
ground - if they are the same, you have a plumbing problem. If they are
different, the problem is in the instruments, eh?

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

Capt. Geoffrey Thorpe wrote:
"Kyle Boatright" wrote in message
...
... 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?
...

That seems likely as far as static sytem design is concerned (e.g. static
port location).

If they differ only in flight, then they probably aren't seeing the same
static source due to kinks and leaks. I assume that both are really
connected to the static source (You have looked, right? No hose has fallen
off?)

Have the encoder output compared to the altimeter while sitting on the
ground - if they are the same, you have a plumbing problem. If they are
different, the problem is in the instruments, eh?

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.


Where the static port is located has no bearing on this issue. There
could be a kink/clog like Geoff indicates but it would need to be
between both the instruments. There could also be water in the static
line somewhere between where the altimeter and encoder are.
A leak won't do it since the altimeter and encoder are at the same
potential. They will both just read the same erroneous altitude. Also,
your airspeed would be off quite a bit since it shares the static system.

Either the altimeter or encoder is off. Your allowed 125 foot deviation
between the two to be legal.
You need to get the system checked and recalibrated. The encoder can be
recalibrated on site if it is off, but the altimeter will need to go to
the shop.

My guess if the altimeter reads the correct baro at field elevation,
then the encoder is the problem.
And if it's suddenly off by 200 feet, then it probably has more of an
issue than a calibration would permanently fix.

If your in the Dallas area, let me know and I can give you a hand to
figure this out. I run/own a repair station for doing this sort of thing.

A cheap and dirty way to check for static leaks (that makes a
difference) is to open a window in flight and see if the airspeed and
altimeter jumps around. If the system is tight, there should be no change.

Good luck

Dave

www.craigmileaviation.com

"Dave" wrote in message
. net...

snip
Either the altimeter or encoder is off. Your allowed 125 foot deviation
between the two to be legal.
You need to get the system checked and recalibrated. The encoder can be
recalibrated on site if it is off, but the altimeter will need to go to
the shop.

My guess if the altimeter reads the correct baro at field elevation, then
the encoder is the problem.
And if it's suddenly off by 200 feet, then it probably has more of an
issue than a calibration would permanently fix.

This is a problem that has seemingly existed for years. I'm just getting
around to addressing it.


If your in the Dallas area, let me know and I can give you a hand to
figure this out. I run/own a repair station for doing this sort of thing.

A cheap and dirty way to check for static leaks (that makes a difference)
is to open a window in flight and see if the airspeed and altimeter jumps
around. If the system is tight, there should be no change.

That or open or close the fresh air vents.


Good luck

Dave

www.craigmileaviation.com

I'm not in Dallas, but appreciate the advise.

KB

Kyle Boatright wrote:
"Dave" wrote in message
. net...

My guess if the altimeter reads the correct baro at field elevation, then
the encoder is the problem.
And if it's suddenly off by 200 feet, then it probably has more of an
issue than a calibration would permanently fix.

This is a problem that has seemingly existed for years. I'm just getting
around to addressing it.


That or open or close the fresh air vents.



KB



Kyle,

What type of certification did you get? IFR or VFR?

"Dave" wrote in message
. net...
Kyle Boatright wrote:
"Dave" wrote in message
. net...

My guess if the altimeter reads the correct baro at field elevation,
then the encoder is the problem.
And if it's suddenly off by 200 feet, then it probably has more of an
issue than a calibration would permanently fix.

This is a problem that has seemingly existed for years. I'm just getting
around to addressing it.


That or open or close the fresh air vents.



KB

Kyle,

What type of certification did you get? IFR or VFR?

VFR

What type of certification did you get? IFR or VFR?

VFR



There's no requirement to verify/validate mode c correlation with the
altimeter for the vfr check. (91.413)
It is strictly a stand alone transponder check.

The encoder check is only a requirement for the 91.411 check. I offer
two types of VFR checks, one for only 91.413 and the other with
altimeter and encoder correlation. Some owners want to know whats being
sent out on mode c, and others don't.

Next time you get yours done, ask for the mode c check with it.

Dave

"Dave" wrote in message
. net...
(snip)
Either the altimeter or encoder is off. Your allowed 125 foot deviation
between the two to be legal.
(snip)

Hmmm . . . interesting. Sometimes I fly over a local airport where the
controlled airspace tops out at 2,800'. Over that is Class B airspace with a
floor of 3,000'. So, I fly at 2,900' and figure I'm okay.

If my encoder is 125' different than the altimeter reading, I could be
reporting to either the tower or to ATC that I was infringing on their
airspace. Come to think of it, last time I flew over that tower (at 2,900'),
I requested a transponder check and they reported that I was showing 2,800'.
They didn't seem to get upset.

IIRC, the encoder reports in 100' increments, so I guess mine could be
anywhere from 49' to 149' off the altimeter. (it is VFR certified BTW).

Rich S.

Rich S. wrote:
"Dave" wrote in message
. net...
(snip)
Either the altimeter or encoder is off. Your allowed 125 foot deviation
between the two to be legal.
(snip)

Hmmm . . . interesting. Sometimes I fly over a local airport where the
controlled airspace tops out at 2,800'. Over that is Class B airspace with a
floor of 3,000'. So, I fly at 2,900' and figure I'm okay.

If my encoder is 125' different than the altimeter reading, I could be
reporting to either the tower or to ATC that I was infringing on their
airspace. Come to think of it, last time I flew over that tower (at 2,900'),
I requested a transponder check and they reported that I was showing 2,800'.
They didn't seem to get upset.

IIRC, the encoder reports in 100' increments, so I guess mine could be
anywhere from 49' to 149' off the altimeter. (it is VFR certified BTW).

Rich S.


Yep.
If you think about it if you fly at 2990' where class B airspace is at
3000', you could be squawking 3100 feet and be legal, sort of.
I don't know what altitude they can really bust you for, but you could
be totally legal and bust "B" airspace.

Does anybody have a read on what the ATC regulations are for this sort
of thing?

And one more tidbit, 91.217 says that the altimeter and encoder need to
be within 125' of each other, but no nothing to the accuracy of either
device. So if both your encoder and altimeter are off by 1000', they are
legal.



Dave
www.craigmileaviation.com
Dallas

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