Mode C is reading high....

Hello,

I've noticed during a flight the other day to Columbus that our Mode C
was reading high... by about 250 feet. The altimeter seems to be
indicating fine as on the ground, with the correct altimeter setting,
it shows the correct altitude. As well, flying the ILS, i make a habit
to check the altitude when crossing the FAF and it is correct too.

Any thoughts on what the issue is?

Thanks,

Dico

On 03/11/07 07:49, Dico wrote:
Hello,

I've noticed during a flight the other day to Columbus that our Mode C
was reading high... by about 250 feet. The altimeter seems to be
indicating fine as on the ground, with the correct altimeter setting,
it shows the correct altitude. As well, flying the ILS, i make a habit
to check the altitude when crossing the FAF and it is correct too.

Any thoughts on what the issue is?

The transponder uses its own altitude encoder, not the plane's altimeter.

I think there's a regulation which states that the transponder must be
within 100 feet of actual pressure altitude, so you probably should have
it looked at.


Thanks,

Dico




--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

Not necessarily. There are optical encoding altimeters. They are not
common, but they do exist.

Jim



"Mark Hansen" wrote in message
...


Any thoughts on what the issue is?

The transponder uses its own altitude encoder, not the plane's altimeter.

Dico wrote:
Hello,

I've noticed during a flight the other day to Columbus that our Mode C
was reading high... by about 250 feet. The altimeter seems to be
indicating fine as on the ground, with the correct altimeter setting,
it shows the correct altitude. As well, flying the ILS, i make a habit
to check the altitude when crossing the FAF and it is correct too.

Any thoughts on what the issue is?

Thanks,

Dico

You could have water in the line. This usually causes strange readings.
If it is past the Altimeter then it will not affect it.

Michelle P

Dico wrote:
Hello,

I've noticed during a flight the other day to Columbus that our Mode C
was reading high... by about 250 feet. The altimeter seems to be
indicating fine as on the ground, with the correct altimeter setting,
it shows the correct altitude. As well, flying the ILS, i make a habit
to check the altitude when crossing the FAF and it is correct too.

Any thoughts on what the issue is?

The altitude your mode C reports comes from a separate pressure
transducer, not your flight instruments, so the accuracy/precision of
your mode C altitude is not related to the accuracy/precision of your
flight instrument.

The mode C reported altitude is pressure altitude, so needs to be
corrected for barometric pressure for comparison with your flight
instrument. The altitude shown on ATC's display is corrected to the
local altimeter setting, but the raw data from your transponder is not.

Mode C altitude typically has a resolution of 100 feet, which gives rise
to possible sizable rounding errors. Newer pressure transducers used
with TSO146 GPSs have a resolution of 10 feet.

On Mar 11, 7:49 am, "Dico" wrote:
Hello,

I've noticed during a flight the other day to Columbus that our Mode C
was reading high... by about 250 feet. The altimeter seems to be
indicating fine as on the ground, with the correct altimeter setting,
it shows the correct altitude. As well, flying the ILS, i make a habit
to check the altitude when crossing the FAF and it is correct too.

Any thoughts on what the issue is?

1) the attitude from your transponder is pressure altitude, ATC
corrects for pressure.
2) I learned long ago not to try to debug issues with transponders,
they seem to be well out of the range of non-radio,electronics people.
However, generally adjustments are very inexpensive. In fact, most
places that quote fixed price IFR certs include basic adjustments to
the transponder. Every time I bring my plane in for IFR cert they have
to do some tuning of the unit.

-Robert

......somebody in rec.aviation.ifr would just *love* this one.....

"Robert M. Gary" wrote in message
oups.com...
1) the attitude from your transponder is pressure altitude, ATC
corrects for pressure.
snip
-Robert

Why?



dlevy wrote:
.....somebody in rec.aviation.ifr would just *love* this one.....

"Robert M. Gary" wrote in message
oups.com...
1) the attitude from your transponder is pressure altitude, ATC
corrects for pressure.
snip
-Robert

Transponder attitude is not corrected by ATC. Never has and never will be.
I, as the pilot in command, control transponder attitude. I'll shut the
damn thing off.

"Newps" wrote in message
. ..
Why?



dlevy wrote:
.....somebody in rec.aviation.ifr would just *love* this one.....

"Robert M. Gary" wrote in message
oups.com...
1) the attitude from your transponder is pressure altitude, ATC
corrects for pressure.
snip
-Robert

On Mar 12, 3:38 pm, "dlevy" wrote:
Transponder attitude is not corrected by ATC. Never has and never will be.
I, as the pilot in command, control transponder attitude. I'll shut the
damn thing off.

How do you control the altitude the transponder puts out?
-Robert, CFII