At 1010 CST, VH-NJN, which had departed Alice Springs for Perth, was given a requirement by air traffic control to reach Flight Level (FL) 260 by 1021. VH-NJL, travelling in the opposite direction from Perth to Ayers Rock, was then given a descent clearance and a requirement to be at or below FL 250 by 1020. The Melbourne Sector 1 controller also gave the crew of NJL instructions to call Adelaide Flight Service at the position report WENER. The pilot in command read back the clearance as the co-pilot was the flying pilot.
At 1012, the pilot in command of NJL reported leaving FL 270 and, at 1016, he contacted Adelaide Flight Service and reported passing WENER at 1015. He also reported that they were at FL 250. The crew of NJN reported maintaining FL 260 at 1017.
At approximately 1023, the co-pilot of NJL told the pilot in command that he was uncertain whether they had been given clearance to descend below FL 250 and he asked the pilot in command to check the clearance. The pilot in command contacted Melbourne Sector 1 to confirm the clearance. The exact words used by the pilot were not able to be determined. However, the controller and his team leader, who was checking the controller at the time, thought that the pilot was asking for a clarification of the requirement.
The controller read back the clearance and the requirement, but the pilot did not respond. The controller became concerned that the aircraft may not have met the requirement and immediately asked the crew to report the aircraft's present level. However, the pilot had switched back to the Adelaide Flight Service frequency and he did not hear the controller's request. After failing to contact the crew, the controller asked Adelaide Flight Service to request the aircraft's present level. The crew reported that they were about to leave FL 250.
The two aircraft met their respective requirements and there was no breakdown in separation. The exact nature of the pilot's query was not able to be determined.