The concern related to the over transmission of radio broadcasts by the AFRU and PAL system. This would mean that radio broadcasts are over transmitted resulting in pilots not being aware of other aircraft operating in the circuit area.
The reporter expressed a safety concern regarding the AFRU and PAL system in use at [location] airport. The reporter stated that the response is triggered by the activation of the radio. This response will often over-transmit the actual call from the pilot which could then lead to unsafe separation within the circuit.
Generally, the AFRU and PAL response does not trigger until after the initial broadcast has been completed.
The AFRU+PAL at [location] has been programmed and configured to transmit various messages after a radio signal of correct code and/or duration has been received. No transmissions are made upon acceptance of a signal - only after the pilot transmitted signal has ceased.
The program is configured this way to accept:
- Signals of greater than 2 second duration. The AFRU responds with the 'aerodrome name' or a 1000 Hz (beep) transmission after the cessation of the pilot transmission.
- Three signals of correctly coded transmission within 5 seconds. The PAL responds by turning on the lights and transmitting an OK message. If the AFRU transmission occurred when a pilot transmission was still being made, the pilot would never hear the 'aerodrome name' message, nor would he ever hear a 'beep' message. They would always be swamped by the pilot transmissions. This is not the case. The messages are clearly heard after the pilot finishes transmitting.
A possible scenario: Pilot 1 enters the CTAF and selects the airfield lighting. At the finish of his third PTT the lighting is activated and when the circuit current is 'sensed' as okay, the following transmission would be sent; '[location] aerodrome - airfield lighting on'.
Pilot 2 may send a CTAF message of greater than 2 seconds duration stating his intentions. If his message was received by the AFRU before the airfield lighting message is transmitted (to pilot 1), the airfield lighting message would be placed in a queue with the 'aerodrome name' message.
The AFRU+PAL Transmission would then be; '[location] aerodrome' immediately followed by '[location] aerodrome - airfield lighting on' This would be unusual, but the program is written to 'queue' the messages for just such an event. If another pilot was transmitting and stopped during the AFRU+PAL transmission he may hear the remainder of the message only. Also, the AFRU+PAL transmits the carrier signal briefly before attaching the message. If any pilot begins to transmit during this period before hearing the aerodrome name or beep from the previous transmission he will be transmitting at the same time as the AFRU+PAL and may hear the end of a message when he releases the PTT.
The AFRU+PAL responds to PTT transmission at the completion of the signal only.
CASA has reviewed the matter with the operator. The equipment and program were tested under the instruction of a technician and no abnormalities were detected. It may be that the occurrence was isolated. CASA will continue to monitor this matter.