Larry Dighera wrote:

James Robinson wrote:

(Frank Ch. Eigler) wrote:

James Robinson writes:

TCAS only works with both transponders working. One was not
working.

Are you sure? Even a TCAS-II system installed on a machine with an
inoperative transponder should still provide traffic alerting, no?

Not as I understand it. Here are some quotes from an ARINC
description of TCAS-II:

"A Mode S transponder is required to be
installed and operational for TCAS II to be
operational. If the Mode S transponder fails,
the TCAS Performance Monitor will detect
this failure and automatically place TCAS
into Standby."

The Mode S requirement is suspect, as I have had jet aircraft indicate
that they could see me on their TCAS units while the transponder
installed in the aircraft I was operating was not even squawking Mode
C, but it was squawking.

I think you are reading that incorrectly. The full functionality of
TCAS-II requires that both aircraft have Mode S transponders, since the
RA is worked out using the Mode S data channel. TCAS will "see" other
aircraft that are using mode C transponders, but with reduced
functionality if "Alt" isn't selected. (TA instead of RA, for example)

What the quoted paragraph is saying is that if the transponder on your
TCAS-II equipped aircraft is detected as having failed, and that can
include one of the two antennas, then the system will automatically drop
into standby, and will therefore not be able to either squawk or transmit
the necessary interrogations for system functionality.

"TCAS does not alter or diminish the pilot’s
basic authority and responsibility to ensure
safe flight. Because TCAS does not respond
to aircraft that are not transponder-equipped
or aircraft with a transponder failure, TCAS
alone does not ensure safe separation in
every case."

That statement does not support your assertion, that both aircraft
must have transponders operating for TCAS to provide avoidance
information.

I would guess, that if the transponder in a TCAS equipped aircraft
were turned off, the TCAS unit in that aircraft would still provide
information about other transponder equipped (and squawking) aircraft
in the area, but that wouldn't do those aircraft with operational
transponders any good unless the pilot receiving the TCAS information
took evasive action.

You have to take the two paragraphs together. As I understand it,
"standby" means that the transponders will not be squawking. That also
means that the interrogations necessary for the functionality of the
system won't transmit either. The system does not seem to have a passive
mode that simply listens.