Mode C Indication

The least significant step that your encoder can report is
100' increments. If you are at 8350' per your correctly set
altimeter, the encoder/transponder is going to be transmitting
83 or 84. In an ideally calibrated encoder, it is adjusted
to switch at the 50' points, so while the aircraft is between
8250' and 8350', the encoder will report 83; while the aircraft
is between 8350 and 8450, it will report 84, and so on.
(Ignore all the blather about altimeter setting for a moment).

The encoder is typically an analog sensor going through an OpAmp
ampiflier, and then to an analog to digital converter. As with all
analog circuitry, it is subject to aging and drift. Over time, the
switching points can drift relative to the absolute altitude. The fix
is to adjust gain and zero pots in the encoder so that it tracks the
absolute altitude. At the same time, you also want your altimeter
to also track the absolute altitude. There is no provision for
"setting" the encoder to prevailing atmospheric pressure...

Because of the low resolution of the encoder, and potential discreptancy
of the encoder, the FAA treats what your encoder is reporting with a
grain of salt. They always ask "say altitude" so that they can
establish if your encoder is lying or not. They understand the
limitations of the system, so if you say 5700', and they look at the
scope and see 56, 57, or 58, they will be happy.

If they see 55 (or lower), or 59 (or higher), they will question if
you have your altimeter set correctly. If you confirm that you do,
then they will conclude that your encoder is outside the allowable
limits, and either tell you to stop alt sqwak, or tell you to get it
fixed.

Note that an incorrect altimeter setting, an inaccutate altimeter, or an
inaccurate encoder would all produce discreptancies. The only thing
you can do effect directly is the altimeter setting. Adjusting the
encoder can only be done during a static check. You could also fly
a hundred feet higher or lower than your assigned altitude to bias
the readings so that they agree... (a really bad idea). If your
altimeter is more than twenty years old, it needs overhaul and/or
replacement.

MikeM
Skylane '1MM
Pacer '00Z



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