Mode C Indication

Jeff wrote:

In everyone's observation, what is the typical difference between the
indicated altimeter reading with current pressure setting and the altitude
reported by mode c transponder. It seems mine always has a discrepancy of
100', sometimes 200' compared to what ATC shows.

As far as I understand, your mode C will be reporting pressure altitude to
ATC, and their computers will adjust that based on the altimeter setting in
use for the sector. So the difference between the mode C and indicated
altitude can be huge, but I don't think that's what you're asking.

What happens on the ground when you set your altimeter to the station
altimeter setting? How far off is it from the field elevation?


All the best,


David

"David Megginson" wrote in message
news
As far as I understand, your mode C will be reporting pressure altitude to
ATC, and their computers will adjust that based on the altimeter setting
in
use for the sector. So the difference between the mode C and indicated
altitude can be huge, but I don't think that's what you're asking.

You are correct that if you have a transponder which displays its reported
pressure altitude, that can be significantly different than field elevation.
Just turn your altimeter to 29.92 and note the difference between that
pressure altitude and the true altitude with a proper altimeter setting.

When ATC reports your Mode C altitude back to you, their computer has
adjusted the pressure altitude for the proper altimeter setting. The
allowed tolerance is 200 feet; if your altitude is off by more than this
then you should have your altimeter and static system checked.


--------------------
Richard Kaplan, CFII

www.flyimc.com

Altimeter reading at field elevation with station altimeter setting is +/-
20-30', no problem there.

After rereading my first post, I think I misphrased the question. I'm not
necessicarily questioning the actual altitude (pressure) being brodcast by
the mode c transponder as much as i'm wondering about what ATC is telling me
they see.

For example: Depart local field, level off at two thousand five hundred. Fly
that altitude till clear of local airspace. Call up approach for flight
following and give the usual info. ATC calls back with squak code and
altimeter setting. ATC calls back November 1234 alpha radar contact 5 miles
south of brown vor two thousand six hundred (or seven hundred). That's the
descrepency I'm trying to get a baseline on.

Is it usual to have indicated differ from ATC reported by one to two hundred
feet?

Cheers!
Jeff


"David Megginson" wrote in message
news
Jeff wrote:

In everyone's observation, what is the typical difference between the
indicated altimeter reading with current pressure setting and the
altitude
reported by mode c transponder. It seems mine always has a discrepancy
of
100', sometimes 200' compared to what ATC shows.

As far as I understand, your mode C will be reporting pressure altitude to
ATC, and their computers will adjust that based on the altimeter setting
in
use for the sector. So the difference between the mode C and indicated
altitude can be huge, but I don't think that's what you're asking.

What happens on the ground when you set your altimeter to the station
altimeter setting? How far off is it from the field elevation?


All the best,


David

Jeff wrote:

Is it usual to have indicated differ from ATC reported by one to two hundred
feet?

Not with my aircraft. Bendix-King transponder and AmerKing encoder, purchased 1995.

George Patterson
If you don't tell lies, you never have to remember what you said.