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Both aircraft were operating scheduled regular public transport flights on crossing air routes within controlled airspace under the jurisdiction of Sydney control sector 5 (SEC 5). 5W-ILF was maintaining FL330. Although SEC 5 was providing control and separation services, Sydney Flight Information Service, International (FS INT) was responsible for providing communications on high frequency (HF) aeromobile facilities. Subsequently 5W-ILF requested FL370 via FS INT. This request was co-ordinated with, and approved by, SEC 5 for a descent to FL270. 5W-ILF was then instructed to descend to FL270 and report when maintaining. At approximately 1627 EST, 5W-ILF acknowledged the level change and read back maintain FL370. A short time later, a level check of 5W-ILF was initiated by SEC 5 and the pilot of 5W-ILF reported maintaining FL370. It was then realised that 5W-ILF had climbed to FL370 instead of descending to the amended level of FL270 as cleared. SEC 5 calculated that 5W-ILF may have entered the lateral conflict area of crossing traffic, which was VH-EBR at FL350, at 1631. It has been established that 5W-ILF would have taken approximately four minutes or less to climb from FL330 to FL370 and reached FL370 at approximately 1631. It was not possible to determine if the prescribed separation standards of at least 2,000 feet in altitude or 10 minutes prior to crossing were actually infringed. The pilot-in-command was performing all communications duties and believed that the clearance instruction, which was mutually confirmed by the co-pilot was 5W-ILF, recleared FL370. However, neither pilot could recall hearing the phraseology to descend used in the clearance provided. The aircrew had an expectation for a control instruction to climb. This aircrew expectation was further compounded by the absence of any challenge to indicate that an incorrect level had been readback to FIS INT, who, expecting to hear FL270, misheard FL 370. The pilot-in-command of 5W-ILF had also omitted to report vacating FL330 and was not queried by FIS INT. It is probable that aircrew operating through differing Authorities may have become conditioned to expect variations and incorrect applications of standard operating procedures and phraseology used by air traffic service officers. For example, anecdotal evidence suggests that some Authorities are using phraseology such as recleared for level change instructions in lieu of the correct phraseology for climb and descent manoeuvres. The importance of using correct phraseology which clarifies change of level instructions such as climb, descend (for vertical manoeuvres) compared to change of route instructions which contain the phraseology recleared (for horizontal manoeuvres) is evidenced in this occurrence. Analysis The air traffic system safety defences had been progressively breached. The first system failure was when the FIS INT operator misheard and co-ordinated an incorrect change of level request. The next system failure was the expectation for the requested level (FL370) and readback of that level by the aircrew. The incorrect readback expectation of FIS INT was the final system failure. The departure from published standard operating procedures and phraseology combined with the absence of a standard phraseology which indicates the vertical manoeuvre being executed renders the system fail unsafe. This is particularly relevant when vacating previously maintained flight levels and exposes the air traffic system to the potential for incorrect level assignment expectations by both aircrew and air traffic service officers. Significant Factors: 1. The FIS INT misheard and co-ordinated an incorrect change of level request. 2. There was an expectation for the change of level request to climb to FL370 by the aircrew to be approved. 3. FIS INT did not detect the incorrect level readback by the aircrew nor the absence of advice that the assigned cruising level had been vacated. 4. There is no standard phraseology specified to verify the vertical manoeuvre associated with a change of level. SAFETY ACTION In relation to this and two other occurrences (BO9102639 near Mount Isa and BO9301823 north of Brisbane), the published procedures and phraseologies did not prevent poor listening technique, by both ATS officers and pilots, leading to a breakdown in separation. In this occurrence a flight service officer and two pilots all misheard the flight level given by the other. Incorrect phraseologies were used and no indication of climb or descent was included by either party. This resulted in a safety net being absent and the aircraft climbing when ATC expected it to descend. In occurrence 9102639, four controllers all either misheard the co-ordinated flight level or did not recognise that an amended level had been given. In this case two training officers did not notice the error. In occurrence 9301823, the trainee inadvertently gave descent instructions to an aircraft without realising that he had given such descent, even though the pilot correctly read back the assigned level. The training officer did not notice the error.
General details
Date: 05 September 1993 Investigation status: Completed 
Time: 16:31 EST Investigation type: Occurrence Investigation 
 Occurrence type:Loss of separation 
 Occurrence class: Airspace 
Release date: 11 August 1994 Occurrence category: Incident 
Report status: Final  
Aircraft 1 details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 737-300 
Aircraft registration: 5W-ILF 
Sector: Jet 
Damage to aircraft: Nil 
Departure point:Melbourne Vic
Destination:Tontouta New Caledonia
Aircraft 2 details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 747-238B 
Aircraft registration: VH-EBR 
Sector: Jet 
Damage to aircraft: Nil 
Departure point:Auckland New Zealand
Destination:Brisbane Qld
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Last update 24 July 2015