French air traffic controllers were unaware of highly-unusual altitude selections being made on the ill-fated Germanwings Airbus A320 during its prior flight to Barcelona.

Investigators recently disclosed that, during its initial descent towards Barcelona, the altitude selector was dialled to extreme settings – including 100ft and 49,000ft – during a short period when the captain was out of the cockpit.

At the time the aircraft was some 40nm north of Perpignan, heading south towards the eastern Pyrenees mountain range.

It had been cleared to leave its cruise altitude of 37,000ft and French investigation authority BEA says the first officer initially correctly selected 35,000ft and activated the descent by pulling the altitude-selector knob.

This engaged the ‘open descent’ mode which remained active when he subsequently selected an altitude of just 100ft.

Later in the descent there were periods when the mode switched to ‘vertical speed’, the investigating authority tells Flightglobal. But it adds: “The aircraft’s actual descent path was normal.”

The altitude selections changed several times during the captain’s absence, being set as high as 49,000ft before being dialled to the cleared level of 21,000ft and then set to 100ft again for almost the entirety of a 2min period.

It was eventually reset to 25,000ft just before the captain re-entered the cockpit. At this point the A320 was descending through an altitude of about 27,000ft.

Selected altitude is one of the parameters chosen for downlink to air traffic control under Eurocontrol’s enhanced-surveillance concept using Mode-S radar, and is intended to give controllers greater clarity on aircraft intent.

But controllers at the Bordeaux en route centre, which was handling the Germanwings flight, had no knowledge of the activity in the cockpit.

All French civil radars are compliant with Mode-S but BEA says that no Mode-S information is shown yet on French controllers’ stations.

“The changes of selected altitude [on the Germanwings aircraft] were therefore not displayed to the controller in Bordeaux,” it adds.

Display of Mode-S information will be implemented as part of the 4-Flight programme undertaken by French air traffic authority Direction des Services de la Navigation Aerienne.

BEA says the Germanwings aircraft landed uneventfully in Barcelona a little over 30min after the captain returned to the cockpit. During the subsequent flight back to Dusseldorf, it states, the first officer "intentionally" adjusted the altitude selector to 100ft while the captain was out of the cockpit, allowing the A320 to enter a descent and eventually strike terrain with the loss of all 150 occupants.

Source: Cirium Dashboard