Originally Posted by
Piper19
But why is a 747 in mode A? I should say they never fly less than C? And Tcas equipped aircraft even have mode S? Or am I wrong here.
Mode A is the basic mode, the four digit code. Every transponder transmits mode A. In some countries the phraseology is: "Airliner 123, squawk 3553 mode A" for any aircraft transmitting in any mode.
In addition to mode A most transponders can transmit mode C, which means the current altitude (actually the current flight level) of the aircraft will show up on the ATC radar screen.
Mode S is where the transponder is able to transmit selected data to the ground unit, like airspeed, selected altitude and so on.
I stand to be corrected but mode S is not a requirement for TCAS, just mode C.
I am not familiar with the ergonomics of the B747 flightdeck, but all I can think of is that the crew accidentally changed the transponder code during the approach, and ATC told them to change it back.
Regards,
Ziggy