Callsign confusion between two Norwegian Boeing 737-800s contributed to a serious airprox at Oslo Gardermoen last year, as one aircraft departed while the other was executing a late go-around.
Inquiries found that a clearance for the aircraft on go-around – designated Norshuttle 741 – was erroneously accepted by the departing jet, bearing the similar callsign Norshuttle 740.
Oslo had temporarily closed runway 01R for snow clearance and the change to runway 01L meant Norshuttle 741, arriving from Trondheim on 31 October 2012, had to follow a steeper descent profile than planned.
The aircraft was also subject to a strong tailwind of some 30kt and these combined effects meant the 737 was high and fast on the approach.
It was still 700ft above the ground – some 400ft above the glideslope height – while just 1nm from the runway, and flying at over 190kt. The crew belatedly opted to abort the landing, performing a straight-ahead missed approach.
The second 737, Norshuttle 740, had been granted take-off clearance shortly beforehand and its departure roll was too advanced to be cancelled. It had just left the ground, and was travelling at 154kt, when the arriving aircraft passed over the threshold of runway 01L.
SHT says the aircraft were 1,727m apart, both climbing, but the weather conditions meant the Oslo tower controller could not maintain visual contact with the two.
The controller instructed the aborting aircraft to turn left when able, and follow a 270° heading. Although its crew answered, the same instruction was also acknowledged by the departing 737.
SHT says the “almost identical” callsigns probably led to the mix-up.
The two aircraft closed to 500ft vertical separation – with Norshuttle 741 above and behind at 1,800ft with Norshuttle 740 at 1,300ft – while 0.2nm apart.
Just before entering cloud, and before the left turn, the captain of the trailing aircraft glimpsed flashes ahead but could not understand their source.
Only when its crew received a traffic advisory from the collision-avoidance system did they realise there was another flight in front of them. Stress levels were “particularly high”, says SHT, because the instrument conditions meant the pilots “could not see the other aircraft”.
SHT adds that the situation “contained a real danger of collision” although the crews and tower controller, once aware, “prevented a further escalation of the conflict”.
Investigators point out that the crew of the arriving 737 had held “unrealistic expectations” about their ability to achieve a stable approach and had aborted “at a late stage”. But the inquiry has not put forward any new safety recommendations.