TM 55-1520-228-10
Change 11 3-24.1/(3-24.2 blank)
SECTION III. TRANSPONDER AND RADAR
3-55. A TRANSPONDER SET – AN/
APX-72.
3-56.
A DESCRIPTION – TRANSPONDER
SET.
The APX-72 (figure 3-13) provides a radar identification
capability. Five independent coding modes are avail-
able to the pilot. The first three modes may be used
independently or in combination. Mode 1 provides 32
possible code combinations, any one of which may be
selected in flight. Mode 2 provides 4,096 possible code
combinations but only one is available since the selec-
tion dial is not available in flight and must be preset
before flight. Mode 3/A provides 4,096 possible codes,
any of which may be selected in flight. Mode C is used
with the MU-32/A Encoding altimeter. Mode 4, which is
connected to an external computer, can be programmed
prior to flight to display any one of many classified opera-
tional codes for security identification. The effective
range depends on the capability of interrogating radar
and line-of-sight. The code setting can be mechanically
retained. On the instrument Panel and Console – place
the IFF CODE HOLD-OFF switch to the HOLD position.
On the Transponder Set Control Panel – turn CODE
switch to HOLD (only momentary action is required).
The transponder set is mounted on the upper left side of
the instrument panel. The associated antenna is shown
in figure 3-1.
3-57. A CONTROLS AND FUNCTIONS –
TRANSPONDER SET.
The control panel is located on the instrument panel. It
provides remote control of the APX—72 transponder
set. Mode 2 code select switch is on the front panel of
the receiver-transmitter radio. Controls and functions
are shown in figure 3-13.
3-58. A OPERATING PROCEDURES –
TRANSPONDER SET.
a. Transponder Operation.
(1) MASTER control – STBY. Allow approxi-
mately 2 minutes for warmup.
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