TCAS does not need any line of sight with ground equipment. It only looks at other Mode C transponders and gives climb or descent commands in coordination with the other TCAS. My question was if they lost Mode C with Legacy and also VHF com maybe they were in one of their blind spots because of line of sight not allowing either to work. As far as the temperature of the transponder equipment I agree most equipment would probably be in the pressurized climate controlled part of the aircraft but skin temperatures and coax to antennas have to be eventually routed to an external antenna which may be affected by cold temperatures. If on their next flight they had the same problem then we would know what caused the failure. If it didn't happen it could be just the conditions not being the same.