Mode Selection Keys - Garmin GTX 330 Pilot's Manual

The GTX 330 transponder is powered on by pressing the STBY, ALT or ON keys, or by a remote avionics master
switch (if applicable). After power on, a start-up page is displayed while the unit performs a self test.

Mode Selection Keys

OFF  Powers off the GTX 330. Pressing STBY, ON, or
ALT key powers on the transponder displaying the last
active identification code.
STBY  Selects the standby mode. When in standby mode,
the transponder will not reply to any interrogations.
ON  In this mode, the transponder replies to interrogations, as
indicated by the Reply Symbol (
ALT  In ALT mode, the transponder replies to identification and altitude interrogations
as indicated by the Reply Symbol (
standard pressure altitude received from an external altitude source, which is not adjusted
for barometric pressure. The ALT mode may be selected in aircraft not equipped with an
optional altitude encoder; however, the reply signal will not include altitude information.
Any time the function ON or ALT is selected the transponder becomes an active part of
the Air Traffic Control Radar Beacon System (ATCRBS). The transponder also responds to
interrogations from TCAS equipped aircraft.
NOTE: If Automatic Airborne Determination senses the aircraft is on the
ground, the GTX 330 will not respond to ATCRBS or Mode S All-Call inter-
rogations regardless of the operating mode.
). Replies do not include altitude information.
). Replies to altitude interrogations include the