Mode C Indication

The encoded altitude reported by Mode C is in 100' increments, so in the ideal
world the encoder resolution alone introduces a +/-50' error. Add on top of
that encoder calibration error, altimeter error, pressure differences between
your location and ATC, non-standard pressure lapse rate, temperature, etc, and
you can easily get a couple hundred feet difference. Fortunately, only the
altimeter and encoder errors are unique to your airplane, so any other airplane
near you will have the same systemic errors (outside the aircraft) as you do.

Ron Rosenfeld wrote:

On Tue, 04 May 2004 16:15:26 GMT, "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.

Just wondering if it needs further investigation. Pitot static, altimeter
and transponder inspections are all current.

Thanks!
Jeff


Lot's of room for systemic errors here. Your encoder likely only has a
resolution of 100'. And ATC may not have given you the same altimeter
setting that their computer is using. Add in a 50' altimeter error in the
a/c, and you could easily have a 100' discrepancy.

If your altimeter has been certified for IFR, then you should know what the
error is there. Your encoder should also have been certified.

With my new CNX80, I also had installed an encoder with 10' resolution, and
the CNX 80 can read the output from that encoder. My panel altimeter, and
the output of the encoder, seem to agree once I take into account the
altimeter error from the last certification. Before, when I had the 100'
resolution encoder, I would occasionally be informed by ATC that I was off
by 100', and rarely that I was off by 200'. At the time, I might be off by
30 or 40 ft on my panel altimeter.

Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

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