While conducting a flight between Brisbane and Cairns, the crew of Boeing 737 VH-TJJ were cleared to conduct a runway 15 instrument landing system (ILS) approach at Cairns. The clearance included a requirement for the aircraft to track to position UPOLO, 15 NM to the north-east of Cairns aerodrome, then via a 15 NM arc with reference to the Cairns distance measuring equipment (DME) beacon to intercept the runway 15 ILS instrument approach. The co-pilot was the handling pilot for the sector and approximately 20 minutes prior to UPOLO, he conducted the descent and approach crew briefing for the runway 15 ILS at Cairns.
The Cairns runway 15 ILS beacon radiates localiser and glideslope signals that permit aircraft to make precision instrument approaches onto the runway. These signals are radiated on frequency 109.9 MHz, and the frequency also radiates a morse-code identifier for the approach aid. The three-letter morse-code identifier for the runway 15 ILS at Cairns is ICS. The published approach procedure for the runway 15 ILS at Cairns permits descent in instrument meteorological conditions to a height of 311 ft above ground level.
Because the cloud base was reported to be at 2,500 ft at Cairns, both crew members considered it unnecessary to activate the marker beacon audio receiver switch on their respective audio selector panels. Additionally, both crew members did not set the reference altitude markers on their respective altimeters to the minimum descent altitude for instrument approach for runway 15.
Both pilots had been operating with their respective very high frequency (VHF) navigation control panels in the automatic setting mode, with tuning of VHF frequencies being automatically accomplished by the flight management computer. The pilot in command then incorrectly preset the manual frequency selector of his VHF navigation control panel to 109.5 MHz, the frequency for the runway 33 localiser at Cairns, morse-code identifier ICN. The co-pilot noted that 109.5 MHz was preset in the pilot in command's VHF navigation control panel. Assuming that this was the correct frequency for the runway 15 ILS, the co-pilot then preset 109.5 MHz into the manual frequency selector of his own VHF navigation control panel, but left the navigation control panel in the automatic setting mode. Both pilots reported that they incorrectly identified the morse-code ICN signal on frequency 109.5 MHz as ICS, the morse-code identifier for the runway 15 ILS on frequency 109.9 MHz.
The co-pilot programmed the flight management computer to fly the 15 DME arc from UPOLO to intercept the runway 15 ILS. Because the flight management computer was being used to intercept the ILS, the pilot in command selected the Cairns VOR, frequency 113.0 MHz, on the manual selector of his VHF navigation control panel. He then selected the MAP mode on his electronic horizontal situation indicator to monitor the aircraft flight path with raw data gained from reference to the ground-based Cairns VOR navigation aid. With the electronic horizontal situation indicator set to MAP mode, a plan view of the flight progress was displayed. The MAP mode consisted of a fixed aircraft symbol superimposed on a moving map background that could include destination/origin airports, flight plan route, and display of navigation aids in use at the time. The co-pilot had his electronic horizontal situation indicator set to MAP mode to monitor the autopilot's conduct of the 15 DME arc. He also armed the autopilot mode control panel to permit the autopilot to intercept the runway 15 ILS.
Shortly after passing position UPOLO, the co-pilot selected his navigation control panel to the manual setting mode. This was done to permit the presentation of navigation information from the 109.5 MHz approach navigation aid that the co-pilot had preset on his navigation control panel prior to the descent and which he incorrectly assumed was the frequency for the runway 15 ILS. After completing the 15 DME arc from UPOLO, the aircraft approached the localiser at 15 NM and at an altitude of 3,700 ft. The crew verified that the aircraft was on the runway 15 centreline by referring to the relative bearing of the Cairns non-directional beacon. The co-pilot's electronic horizontal situation indicator also indicated that the aircraft was on the extended centreline of Cairns runway 15. This information was displayed on the co-pilot's electronic horizontal situation indicator from the destination aerodrome data stored in the flight management computer. After confirming the aircraft was on the centreline for runway 15, the pilot in command then transferred his navigation control panel to 109.5 MHz. This resulted in both crewmembers having the incorrect approach aid tuned on their respective navigation control panels.
The flight mode annunciators on both crewmembers' electronic attitude direction indicators signified that the autopilot had captured the localiser. However, neither crewmember's electronic attitude direction indicator was displaying a glideslope pointer adjacent to the glideslope deviation scale. The crew sought and received confirmation from the Cairns aerodrome controller that the glidepath was operating normally. In the absence of any cockpit indication of the glideslope, the crew elected to continue the approach using the localiser for track guidance and the Cairns DME for descent guidance, in accordance with the published approach chart for runway 15. After checking the descent point from the approach chart, the co-pilot initiated descent by manual input into the autopilot mode control panel.
Shortly after descent had been initiated, both pilots noticed the aircraft commence a right turn away from the centreline of the localiser, and they elected to discontinue the approach. The pilot in command assumed control of the aircraft, disconnecting the autopilot and initiating a left turn away from the coast. At the same time, the controller, who had been observing the aircraft's approach on his radar screen and had noticed what he considered to be an unsafe flightpath deviation, immediately issued an instruction to the crew to turn left onto a heading of 110 degrees. When the controller was satisfied that the aircraft was clear of terrain, the crew was cleared to descend to 1,500 ft. After the crew reported visual, the controller cleared them to make a visual approach onto runway 15.
On the previous night the crew had operated a service to Cairns. On that occasion, the Cairns runway 15 ILS had been unserviceable and the crew received information about its unserviceability prior to their departure for Cairns. As a result, at the time of the occurrence, the crew suspected that the ILS may again have been experiencing service difficulties. The crew reported that this may have delayed their realisation that the incorrect frequency had been selected.