The 747SP descended to 2500 feet instead of 5000 feet.
A Boeing 747SP (B-2442 of Air China c/n: 21932) was on a flight from Melbourne to Sydney, and was under the control of Sydney Approach (APP).
Whilst being radar vectored for an extended downwind leg to runway 16R, the aircraft was issued with a clearance to descend to 5,000 ft, which was read back by the flight radio operator.
The controller then instructed the crew to change radio frequency, and re-confirmed the cleared altitude as 5,000 ft. The weather at Sydney was CAVOK. The aircraft was subsequently observed on radar to descend to 2,600 ft, while on the downwind leg. Several attempts by the controller to contact the aircraft were unsuccessful.
At about this time, the altitude readout of the aircraft's transponder ceased to be visible on the radar screen, with only primary returns being received. About 2.5 minutes later, the Approach controller re-established radio contact and instructed the aircraft to climb to 3,000 ft, at which point the altitude readout re-appeared on the radar screen. The aircraft was then processed for a landing without further incident. The Approach controller subsequently reported that he had re-confirmed the instruction to descend to 5,000 ft because he was unsure the crew had understood the clearance.
For international operations the airline concerned delegates the responsibility for all English language radio transmissions to a flight radio operator. The radio operator on this flight had accumulated some 25 years experience, including several flights into Sydney over the last seven years. When the flight radio operator read back "five thousand" on the second occasion, the controller was satisfied that the clearance had been understood. Recorded radio communications confirmed that the controller had used correct radio phraseology and that the flight radio operator had correctly read back the descent clearance. It was later found that the flight radio operator believed the instruction had been to descend to "two five thousand", which he initially interpreted as 25,000 ft, although the aircraft had already descended to 6,000 ft. He chose not to question the clearance as he rationalised the controller had meant 2,500 ft. He called out "two thousand five hundred" to the non-handling pilot, who entered these numbers in the altitude select window on the mode control panel. When the controller issued the frequency change, and confirmed the descent clearance, the flight radio operator believed that 5,000 ft was the altitude at or below which the frequency change was to be made. All crew members on the flight deck were listening on headphones at the time of the occurrence and said they were somewhat confused by the clearance instruction, but chose not to query it.