Mode S
 

Hi,

why do you still have transponders on Boeing aircrafts where you can enter a 4-number code for Mode A (and C)? Isn't mode S the only mode used nowadays on commercial flights?

Also, how would a mode s tranponder work? Just switch it to standy before take-off, and then to "on" (just like with the A/C modes), but without doing/entering anything else? (since the ICAO 24 bit hex code would always be the same for a specific aircraft..)

I suspect that as you didn't bother to acknowledge any of the replies to the previous two questions you asked, there might not be any forthcoming this time ...

if i correctly remember, mode s in not available everywhere. so if you make a flight from europe to asia, there might be some areas where you will have to enter your A/C code. it is basically a back up.
Greetings

You have to enter the flight number/reg.
One airline in Scotland quite often give us seasonal greetings on mode S but in the low level sectors at Scottish the A/C code is used to correlate the callsign so we see the correct trip number

MSSR mode A+C only here ;)

Never had an aircraft not able to change their mode A......

Nor, predictably, to this one ...

QED.