SAS EuroBonus - Call sign confusion

A short story from the flight deck: SAS has a habit of using its commercial IATA route numbers as call signs (after “Scandinavian”) when communication with ATC, but that gave a grave misunderstanding on December the 13th 2011, reports the Danish Accident Investigation Board HCLJ.

That day SK 2872 CPH-BGO was at some point cleared to FL 360. That was heard by a SK-flight in opposite direction - SK 2873 BGO-CPH – which read back “Cleared FL 360 Scandinavian 2873” and began to descend – but soon came in conflict with a third(!) SAS-flight, SK 1674 16.20 CPH-SVG, which was at FL 360 in opposite direction.

The two flights got so close in opposite directions, that they both got the anti-collision alarm (TACS) with the advisory to climb/descend.

Even though the safety margins were broken, the official separation was not violated, so you folks on those Norwegian-flights that December day were not in real danger.

But the HCLJ advises, that there is a recommendation not to use flight numbers as call signs, as the flight numbers in an area can be very similar – as in this case with the two SAS flights between Copenhagen and Bergen.

Lets see, if SAS changes call sign policy after this.

Report (in Danish only) (http://havarikommissionen.instant.cohaesio.net/~/media/Files/Havarikommissionen/Havarirapporter/Luftfart%202011/HCLJ510-2011-74.ashx)

This happens all the time in ATC.
Even other airlines answering the wrong calls.

Aren't the allowed callsigns:
Type A – the characters corresponding to the registration marking of the aircraft

Type B – the telephony designator of the aircraft operating agency, followed by the last four characters of the registration marking of the aircraft

Type C – the telephony designator of the aircraft operating agency, followed by the flight identification

As far as I know, almost all major airlines use the Type C, so this is not just SK.

Also, if they would switch, would that be any safer, typically the registrations are consecutive, so you would actually have even more aircraft with "almost the same call sign" in the air. If the pilots mishear the last letter in "OY-KF_, wouldn't you have every Canadair SAS owns decending to FL360.

What can be done is situations like this if necessary is changing the flight numbers so that consecutive numbers are not simultaneously in the air. It has also been sometimes done when there have been multiple flights with the same number from different airlines simultaneously in the same airspace.
If I remember correctly AY5/6 (Finnair HEL-JFK-HEL) not that number because the previous one was the same as some other flights flying to/from New York at the same time.