Originally Posted by
Cynical Sid
And from that very Wikipedia article ' An individual aircraft is given a SELCAL code upon application to the SELCAL code registrar, '.
That means that when an owner requests a SELCAL code for an aircraft, the a/c will get one of the 10920 available selcal codes assigned to it. Maintenance then programs this selcal code into the RECEIVER of the a/c. When someone then transmits the assigned SELCAL code ( = an audible tone (actually two tones, each lasting about a second, with a very short silence in between them.) on the frequency the receiver is tuned into (HF or VHF), the onboard selcal device will know that someone tries to call it, even when the crew is not listening out on that frequency. In the Airbus the pilots will then hear a buzzer (The same as when the cabin is trying to call the flightcrew on the interphone) accompanied with a light illuminating the corresponding transmitter selection button on his ACP. The crew can then reply by voice on this VHF or HF frequency.
Can you imagine what it would be like if all aircraft would transmit its own 2-second selcal code every time the PTT is pushed?
Now install the selcal app, generate a few selcal codes to hear how those sound and get back to your MS Flight Sim.