More comprehensive insight by Swiss ATC
From a release by Swiss news agency SDA at 1306 today:
By belatedly obeying ATC instructions to decrease altitude, the crew of the TU-154 set off the TCAS of the B-757 freighter approaching at same flight level, which caused the Boeing’s pilot to dive, too. This is the immediate cause of this collision, and the air traffic controllers on duty are – for the time being at least – free of blame.
According to Anton Maag, Manager of Skyguide’s Area Control Centre, speaking at a media conference held on Tuesday morning at Zurich Airport, the situation until immediately prior to the mid-air collision had been entirely normal. Given the time of night, few aircraft were in the sky.
One of those up there was the TU-154 operated by Bashkirian Airlines on a charter flight from Moscow to Barcelona via Munich and Geneva. Flying from south to north towards Brussels at the same time there was a B-757 freighter of DHL. Both aircraft were at 36000 feet. Their flight paths were to cross at Ueberlingen – a routine event.
For this crossing, the controller at Zurich ATC wanted the Russian aircraft to descent to a lower flight level, as there was going to be a scheduled landing at Geneva anyway. Between 8 and 10 nm (or approx. 1 ½ minutes of flight time) prior to the crossing point ATC instructed the TU-154 crew to initiate a descent – but this instruction was not confirmed. A second identical advice also remained unacknowledged. It was only on the third attempt that the TU-154 crew reacted and began to seek the assigned, lower flight level.
In the meantime, the TCAS on board the B-757 had registered the approaching TU-154 at the same flight level and instructed its pilot to descend. According to generally accepted international practice, TCAS warnings and instructions are adhered to immediately and without clearance from ATC, according to Anton Maag. Between 2335 and 2336 hrs the two aircraft collided at 35300 ft.
According to Anton Maag, the timing of the first set instructions to the TU-154 crew to descend was not ideal, but would have sufficed “under normal circumstances”. And Skyguide spokesman Patrick Herr insists that the type of instruction and the timing of the upcoming crossing of flight paths were going entirely to SOP.
The two “sticking points” that the investigation would have to concentrate on were, according to Patrick Herr
1) Why did the Russian crew not react immediately to the first set of instructions issued to them by ATC?
2) And why did the TCAS on board the B-757 instruct its crew to descend?
There were apparently no language problems whatsoever. According to Anton Maag, the preceding conversations with either flight deck had been entirely normal. The Russian pilot, says Bashkirian Airlines, was 52 years old and had some 10 years of experience behind him. The controller on duty, according to Patrick Herr, also has many years of experience on the job.
The TU-154 approaching from the east had been handed over by German ATC to its Swiss counterpart (which is responsible for that particular sector despite its location above German territory) at 2330. The flight had been advised in a timely fashion, too. The B-757 freighter had crossed the Italian-Swiss border (to the south of the collision site) at around 2300.
At the time of the actual collision, there was very light air traffic, with only a few aircraft in the sky. The two controllers on duty in Zurich had only one sector to look after.
Thanks for your appreciation, atc_ring