Belgian investigators have disclosed that the crew of a climbing EgyptAir Airbus A300-600 freighter misunderstood an instruction to level off from its collision-avoidance system before a serious airprox involving an Air France A320.

The A300, flying east from Ostend to Cairo on airway UL607, had been cleared to 21,000ft after take-off and was climbing at 2,500ft/min.

Belgium's Air Accident Investigation Unit says the A300 was set to pass behind the northbound A320 which was flying on airway UN873 at the higher level of 22,000ft. The controller instructed the A320 to diverge left from the airway, towards a waypoint designated FERDI, to speed its crossing.

Although the A300 crew had been advised about the A320, which would cross from right to left above them, and had correctly read back instructions to maintain 21,000ft, the freighter continued to climb.

Both aircraft issued collision-avoidance advisories, with the A300 crew ordered to level off, having not shown any "visible change" of vertical speed as the jets approached.

Investigators state that the A300 crew later claimed to have "understood a 'climb' instruction". The captain took control from the autopilot and, instead of levelling the aircraft, increased its climb rate to 3,500ft/min.

The A320 crew obeyed a corresponding collision-avoidance advistory to climb, and initiated a climb at 1,500ft/min.

As the A300 passed above its cleared level and reached 21,300ft, its colliison-avoidance system started ordering the crew to descend. But the captain only reduced the climb rate instead of commencing the required 1,500ft/min descent.

The A300 had still been climbing when the A320 passed 1.2nm in front, and 522ft above.

Investigators state that the aircraft were separated by 0.69nm horizontally and 427ft vertically at their closest approach point.

"Both aircraft cover this distance in 4s when flying at cruise speed," the inquiry points out.

While investigators have yet to reach formal conclusions over the 1 January incident, pan-European air navigation organisation Eurocontrol has simulated the encounter to explore alternative scenarios.

If the A300 crew had responded correctly to the 'level off' order, the aircraft would have probably levelled between 20,700ft and 20,900ft depending on the pilots' promptness. No advisory would have been issued to the A320 crew.

But if the A300 had continued to climb as recorded during the incident, and the A320 crew had not responded to their 'climb' order, the vertical separation between the two jets would have halved to just 215ft at their closest point of approach.

Source: Cirium Dashboard