The SELCAL system provides notification to the flight crew when a ground station desires contact on one of the communication transceivers. SELCAL reduces pilot diversions and workload by eliminating the need for continuous monitoring of communications frequencies.
A ground station calls a particular airplane by transmitting a set of four audio tones over a
communications transceiver. The SELCAL decoder receives the audio from the particular transceiver and compares the tones to an assigned code. When the tones and the code are the same, a chime sounds and a call light on the pilots' call panel comes on, corresponding to the communications transceiver which received the signal. The decoder and call switch are manually reset after each call.
The SELCAL system consists of a SELCAL decoder, a coding receptacle, HF, and VHF call switches on the pilots' call panel. System electrical power comes from the left DC bus, through a circuit breaker on overhead panel P11.
There is one blue call switch on the pilots' call panel for each transceiver connected to the
SELCAL decoder. Each call switch alerts the flight crew of a ground-to-air call received by that transceiver. Pushing the call switch, or pushing the PTT button on the associated transceiver, resets the light and decoder.
Following pictures are during light test.
Power and audio inputs are wired directly to the decoder. Thus, the only operational control is for the reset function. The system is reset by pushing the associated call switch or bypushing the PTT button for the associated transceiver.
The decoder monitors audio from the VHF and HF transceivers. When the received code matches the code programmed by the coding receptacle, the decoder recognizes it as a call to that airplane.
The decoder then turns on the call switch associated with the transceiver which received the call, and also sounds a chime in the flight compartment. The call switch and SELCAL decoder are then manually reset to await the next call.
This panel is on the overhead panel, P5. The cabin interphone system connects to the pilots' call panel through lighted call switches (FWD, MID, AFT, ALERT). The blue lights in the buttons come on to show incoming attendant calls and go off when pushed.
The CABIN CALL switches are also used to signal the related attendant station. To make an alert call the ALERT call switch signals all stations at the same time.
The FLT INT switch connects the cabin interphone and flight interphone amplifiers in parallel to give the flight crew access to the cabin interphone system through the audio selector panels.
The GND CALL switch signals between the flight compartment and nose Landing gear area.