The SELCAL system functions in order to allow a ground based station to contact a specific aircraft without the aircraft crew actively monitoring a radio frequency.

When the ground station attempts to contact the aircraft, a "SELCAL" aural warning is heard and also a light illuminates in the SELCAL panel. Let's look at the system a little closer.

SELCAL System Operation

The system operates in the VHF and or HF frequency range without the continuous monitoring of the flight crew. The system consists of a ground based encoder along with the airborne decoder. The persons on the ground attempting to contact the specific aircraft do so by sending a signal consisting of two paired tones out over the radio frequency. Each pair of tones, when transmitted, will only activate one SELCAL decoder unit.

Once the aircraft has been contacted, the crew can communicate with the ground station by transmitting on COMM #2 monitoring frequency.

SELCAL Decoding Unit

The decoding unit can be set to respond to a number of different letter (frequency) combinations. The desired code can be set into the decoder by rotating the dials on the front of the unit. The unit is located in the right (FO's) console. For those of you that are curious, the letters represent the frequencies in the table below. (I had too much time!)

SELCAL Self Test Switch

The SELCAL unit can be tested by toggling the self test switch which is located on the maintenance panel. This panel is located behind the FO's seat. When this toggle switch is moved up, the SELCAL aural warning will be heard and the SELCAL light will illuminate in the pushbutton.

The power for the SELCAL system comes from SHED bus #1. So if the SHED buses are not powered, the SELCAL system will not work. Remember this.