On Sep 26, 9:43*am, 5Z wrote:
On Sep 26, 7:52*am, Peter Purdie wrote:

Transponders give Flight Level (assuming they are working properly), so
will give the altitude ATC are intereted in. *That's how they know what
FL you are at when not talking to them.

The controller see your transponder altitude CORRECTED for the local
pressure. *So don't give them the number off the transponder display
unless the current pressure is 29.92

That's why the altimeter must be calibrated when you calibrate the
transponder. *You must be able to set the altimeter to the current
(not what it was several hours or hundreds of miles away) before
talking to ATC, and that is why they will give you an altimeter
setting on initial contact unless you indicate you have current ATIS
info.

-Tom

Actually this is a common misconception, (one of several reported
recently in the Safety Corner in Soaring magazine). The encoder/
transponder/altimeter system is checked as a part of an install and
you must also do this if the static system is touched during
maintenance. Subsequent checks for aircraft operating VFR are only
required to test the transponder RF/signal properties and do not need
to check the altitude calibration. IFR calibration do. See FAR 91.413
appendix F (RF/signal properties only) and Appendix E (install/static
maintenance and IFR checks).

Back to the original question, I would not want to fly any aircraft
without the ability to reliably (i.e. when the battery dies) know and
be able to report pressure altitude.

Darryl