Why does setting the Legacy 600's transponder to standby mode also deactivate its TCAS? In 2006, a 737 operating as Gol Transportes Aéreos Flight 1907 crashed following a collision with a Legacy 600 business jet; the Legacy's transponder proved to have been in standby mode1 at the time of the collision (due either to a transponder malfunction or to the crew having inadvertently switched it into standby mode), which, obviously, prevented the 737's TCAS from seeing the Legacy and sounding an alarm.

However, setting the Legacy's transponder to standby mode also, apparently, deactivated its own TCAS:


  Considering, according to declarations, that the N600XL pilots did not perceive that the transponder had stopped transmitting and that, as a result, the TCAS was inoperative...  [CENIPA report, page 83.]


This is also, apparently, the result should the transponder fail outright:


  In case there is a Transponder failure, the operation mode and code selected are replaced by dashes in the RMU. An orange message “TCAS FAIL” will appear on both Primary Flight Displays (PFD) precisely in front of the visual field of view of both pilots, as shown by the pictures bellow [sic].  [CENIPA report, page 92.]


I'm not seeing the rationale for this; even if the aircraft's transponder fails or is switched off, there doesn't seem to be any inherent reason why this should require also deactivating the TCAS.  Even if the transponder can't, or won't, transmit, the TCAS should still be able to pick up signals from other aircraft's transponders, allowing it to continue to function, albeit in a receive-only mode.

So why does the Legacy 600's TCAS cease to function if the transponder is standing by or inoperative?



1: In standby mode, the transponder does not respond to any interrogation signals, either from the ground or from other aircraft.
At least under FAA regulations the TCAS system goes into standby (by design) if the transponder fails


  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 transponder performs the normal
  functions to support the ground-based ATC system and can work with
  either an ATCRBS or a Mode S ground sensor. The Mode S transponder is
  also used to provide air-to-air data exchange between TCAS-equipped
  aircraft so that coordinated, complementary RAs can be issued when
  required.


TCAS II systems get their altitude information from the transponder, which is a vital parameter to the system working. 

 

(source)

All modes also require a functioning Mode S transponder 


  
  Stand-by: Power is applied to the TCAS Processor and the Mode S transponder, but TCAS does not issue any interrogations and the
  transponder will reply to only discrete interrogations. The
  transponder still transmits squitters. Note: If the aircraft is on the
  ground and transmitting extended squitters, it is not required to
  transmit short (acquisition) squitters.
  Transponder: The Mode S transponder is fully operational and will reply to all appropriate ground and TCAS interrogations. TCAS
  remains in Stand-by.
  TA Only: The Mode S transponder is fully operational. TCAS will operate normally and issue the appropriate interrogations and perform
  all tracking functions However, TCAS will only issue TAs; RAs will be
  inhibited.
  Automatic or TA/RA: The Mode S transponder is fully operational. TCAS will operate normally and issue the appropriate
  interrogations and perform all tracking functions. TCAS will issue TAs
  and RAs when appropriate.
  


Strictly speaking all of this is only applicable to TCAS II systems but based on the noted case and my reading of the accident report it appears TCAS II was the system in use in this case.