A Boeing 737-8BK (737), registered VH-VOB, was en route from
Melbourne, Victoria to Adelaide, South Australia on descent from
flight level 240 to 3,000 ft. The copilot was flying the aircraft
under the supervision of the pilot in command. The pilots were
aware of a current notice to airmen (NOTAM) advising that the
instrument landing system (ILS) was radiating intermittently on
test and was not to be used for navigation. The weather at Adelaide
was visual meteorological conditions (VMC) and they intended to
conduct a visual approach to land on runway 23. During the turn
onto final, the pilots observed that the aircraft's flight
instruments indicated that the ILS was operating and after the
approach controller instructed them to make an ILS approach, they
elected to continue with the autopilot selected. About 20 seconds
after ILS glide path (GP) capture by the aircraft's flight
management system, the aircraft commenced to descend rapidly and
its enhanced ground proximity warning system (EGPWS) announced a
SINK RATE caution. The pilot in command immediately assumed control
of the aircraft and arrested the rate of descent. During that
manoeuvre the EGPWS announced a PULL UP warning.
It is normal operator practice for pilots to fly a visual
approach using the ILS if possible. The pilots reported that they
were in VMC, at about 3,000 ft, after capturing the GP. The pilot
in command reported that because they were visual and there were no
terrain concerns he used minimal control inputs during the recovery
from the descent. Information from the aircraft's flight data
recorder indicated that the maximum rate of descent was 6,100 feet
per minute and that it had descended to a radio altitude of 1,180
ft above ground level (2,000 ft above mean sea level) before
resuming the approach profile.
Technicians were scheduled to work on the runway 23 ILS and
there were three NOTAMs issued for the facility. The pilots had the
first two NOTAMs issued on the runway 23 ILS but they did not
receive the third NOTAM, as they were en route at the time of
issue. The initial NOTAM, advising that the glide path was not
available, that the pilots had used to brief themselves, remained
current. A precondition for the conduct of work on the ILS was the
requirement for VMC. An ILS consists of a number of elements
including a GP, a localiser (LLZ) and distance markers. It is
identified by a two letter code prefixed with the letter I that is
included in the LLZ signal (IAD for runway 23 at Adelaide).
Following a request for the LLZ to be made available for a training
flight, the technicians released the LLZ for operational use while
continuing pre-calibration testing of the GP. Consequently, the LLZ
(with the ILS identification code) was serviceable while the GP was
operating intermittently and not available for operational use.
The controllers in the Adelaide Air Traffic Control tower had
previously included information that the LLZ and the GP were not
available on the computerised automatic terminal information system
(CATIS) that is used to broadcast operational information to
pilots. When the LLZ was returned for operational use, they
abbreviated the advice to `localiser available', due to system
constraints on the amount of additional information that could be
included. The information that the GP was not available was not
included in the CATIS. The majority of the additional information
consisted of advice of restrictions due to aerodrome works. The
CATIS was normally broadcast on the non-directional navigation
beacon (NDB) and a very high frequency (VHF) radio transmitter.
However, at the time of the occurrence the VHF transmitter was not
available and the information was only available on the NDB.
Despite listening to that information, the pilots missed the fact
that the localiser was available due to the poor quality of the
received audio. Consequently, when the pilots reported on first
contact with the approach controller that they had received the
CATIS they were unaware that only the localiser was available.
The approach controller was on his fourth shift in the current
shift cycle. After arriving at work, he self briefed and became
aware of the ILS maintenance and the non-availability of the GP. A
number of control positions were concentrated to a single position
and the controller was not initially required to take an
operational role. He undertook other duties before returning about
90 minutes later to open the approach east position. His normal
practice was to include a text note on the radar display, near the
final approach aid or path, of any pertinent operational
information, including restrictions or airspace limits. However, on
this occasion, following the handover from the approach west
controller, he forgot to make a note for his display.
About 40 seconds before the aircraft captured the GP, the
controller advised the pilots, `you should get visual shortly, but
you're cleared for the 23 ILS approach'. The pilot in command
acknowledged and read back that clearance. The controller later
reported that at the time, the fact that the glide path was not
available had slipped his mind and he reverted to his normal radio
telephony phraseology for aircraft on final.
