Boeing Co 737-476, VH-TJJ

Both pilots incorrectly tuned the Cairns runway 33 localiser on 109.5 MHz instead of the runway 15 localiser on 109.9 MHz and subsequently misidentified the morse-code identifier. Their errors represented inadvertent failure to carry out routine and highly practised tasks. The crew had operated into Cairns the previous night and on that occasion the runway 15 localiser was not operating properly. On the night of the occurrence, although both pilots had the incorrect frequency selected for the runway 15 localiser, they incorrectly assumed the localiser was still experiencing service difficulties. This assumption arose because neither crew member was receiving a glideslope indication on his flight instruments. Also, the weather conditions in Cairns on the night of the occurrence indicated that a complete instrument approach would not be required. This situation resulted in a decreased level of vigilance by both crew-members to the extent that they did not adequately cross-check that the correct localiser frequency had been set.

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.