Transponder modes
 

Hello guys; just a simple question;

On transponder panel, there are 3 deferent modes which I do not understand the differences exactly. These are ALT OFF, ALT ON and XPONDER. Actually ALT OFF and ON are pretty clear. One of them sends altitude information and the other does not. But how about the XPONDER mode?

Markings differ by manufacturer, but usually:

OFF = Off, not operating at all
STBY = Standby (powered up but not transmitting)
ON = On, but no altitude reporting. (Mode A / ALT RPTG OFF)
ALT / XPDR / XPNDR = On, with altitude reporting (Mode C)

TST = Test mode

TCAS equipped planes will have additional modes (TA, TA/RA, TFC)

Some planes have AUTO mode (switches between ground & air modes). In ground mode (GND), only responds to Mode S.

Ofter there are additional switches to select altitude source, left vs right transponder, etc.

XPONDER mode may be an auto mode. Comes on automatically airborne and shuts off at touchdown, plus if mode S then on even on the ground at airports with ASDE or similar.

A Boeing FCOM says XPNDR has the transponder enabled and in flight the altitude reporting is enabled.

Thank you guys all, appreciated. But still have 1 important question; some of 737's has both ALT OFF and XPONDER. If XPONDER sends altitude information in the air and does not send on the ground (that's what I understand of your answers), in that case why still we have both ALT OFF and XPONDER? We wouldn't need ALT OFF mode, would we?

Hi Manicured garden,
If your transponder "ALT" readout was in error (which has caused nuisance TCAS events in the past), then ATC will ask you to turn ALT off with the phrase:
"Stop Altitude Squawk."

Transponder Requirements - AOPA

Goldenrivett, thanks. Now its all clear.
;)

Take a Gues...
 

If you are a pilot, check with your OPS department - Not an unreasonable question and yes, you should know.
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