Background Information from NTSB

• A mid-air collision occurred in Brazil on 29 September 2006 in between a Boeing 737-800 and Embraer Legacy 600 business jet.
• The investigation of this accident is ongoing. However, preliminary findings suggest that a critical safety issue exists regarding the loss of functionality of an aircraft’s collision avoidance system (comprising a transponder and Airborne Collision Avoidance System (ACAS)) and associated cockpit warnings to flight crews.
• For a TCAS II equipped aircraft to provide a flight crew with collision avoidance information, the TCAS II unit and the transponder must be turned on, and the transponder cannot be selected to the STANDBY mode.