Aviation safety investigations & reports

Boeing Co 737-8BK, VH-VOB

Investigation number:
Status: Completed
Investigation completed


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.


The inadvertent slip by the approach controller was the final action of a number of lapses or omissions that led the pilots to believe that the ILS was available, despite previous advice. The level of situational awareness of the pilots, the fact that the approach was conducted in VMC and the provision of EGPWS warnings were factors that assisted in minimising the failure of other defences in the aviation system.

Safety Action

The aircraft operator issued two safety briefs on the circumstances of the occurrence to flight crew.

An Airservices Australia (Airservices) investigation of the occurrence reported that there were adequate system safeguards, but that many had been breached. The Airservices' investigation made 19 recommendations. At the time of issue of this report, 10 of the recommendations had been actioned. The remaining nine were expected to be actioned by 30 June 2005.

The ATSB will monitor Airservices' action resulting from the investigation recommendations. Any resultant safety action will be published on the Bureau's website.

General details
Date: 09 March 2004   Investigation status: Completed  
Time: 1515 hours CSuT    
Location   (show map): 4 km NE Modbury, (Locator)    
State: South Australia   Occurrence type: E/GPWS warning  
Release date: 26 April 2005   Occurrence category: Serious Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer The Boeing Company  
Aircraft model 737  
Aircraft registration VH-VOB  
Serial number 30622  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point Melbourne, VIC  
Destination Adelaide, SA  
Last update 13 May 2014