Norwegian B738 and Norwegian B738 at Oslo on Oct 31st 2012, near collision
Last Update: November 11, 2013 / 15:16:07 GMT/Zulu time
In the meantime another Norwegian Boeing 737-800, registration LN-NOM performing flight DY-740 from Oslo to Trondheim, was preparing for departure. Tower anticipated to get the departure out before the arriving DY-741 expecting the arriving aircraft would reduce its speed further into the approach and cleared DY-740 to line up runway 01L and wait. Noticing the high speed of DY-741 the controller then instructed the crew of DY-740 "if you need a run up, start the run-up now and be ready for immediate departure", then issued takeoff clearance to DY-740. The aircraft began the departure roll 5 seconds after the takeoff clearance and became airborne 34 seconds later.
At that point DY-741 was about 2nm from the threshold of runway 01L still at a speed of 220 knots over ground. The tower controller began to recognize there would be a violation of runway separation.
When DY-741 was about 1nm from the runway threshold descending through 1000 feet MSL and diving into cloud, the tower heard DY-741 report they were going around. In reaction the controller instructed DY-741 to turn left onto a heading of 270 degrees, DY-741 executed the instructed turn, however, DY-740, not aware of the presence of DY-741 on frequency and just climbing through 700 feet MSL, also executed the turn. The controller, upon hearing the readback by DY-740, became confused and did not correct the readback, the crew of DY-740 therefore remained unaware that they were executing the instruction meant for another aircraft. Tower called Oslo approach on the direct line, approach was seeing and monitoring the developing conflict as well.
The crew of DY-740 received a TCAS traffic advisory with an aircraft above them and therefore reduce their rate of climb.
DY-740 rolled out on a heading of 270 degrees, remained on that heading for about 70 seconds. DY-741, recognizing the departing aircraft was below them and turning left as well, continued their turn onto a divergent course. After DY-740 had continued on heading 270 for about 70 seconds, they received vectors to the north and were handed off to departure as the conflict had been resolved.
The tower controller was relieved.
Both aircraft reached their destinations for safe landings without further incidents.
Norway's Statens Havarikomisjon for Transport (Accident Investigation Board Norway AIBN) reported that the minimum separation between the two aircraft reduced to 0.2nm lateral at 500 feet vertical involving a high risk of collision and rated the occurrence a serious incident.
Norway's Statens Havarikomisjon for Transport released their final report in Norwegian (summary in English included) concluding the probable cause of the serious incident was:
- The flight crew of DY-741 had unrealistic expectations of being able to slow the aircraft and have the aircraft stabilized at 1000 feet AGL. The decision to go around was taken at a late stage.
- The expectation of final approach and tower controllers, that the aircraft would sufficiently slow down during the remainder of the approach, were not met. The go-around of DY-741 thus came into conflict with the departing DY-740.
- Runway 01L was used for both departures and arrivals. The tower controller thus had to ensure sufficient separation between departing and landing aircraft even in case of an aborted landing. There were no guidelines available however at Gardermoen tower for air traffic controllers to assess whether there was sufficient time for releasing a departure ahead of an arriving aircraft in the given circumstances. There were no procedures available of how tower would handle aircraft going around with simultaneous departures from the same runway.
The AIBN analyzed that the expectations by flight crew to be able to stabilize the aircraft at or before 1000 feet AGL were not met as were the expectations by final approach and tower controller, the aircraft would slow sufficiently during the remainder of the approach.
The investigation further analyzed and concluded that after the conflict had arisen situation awareness and good review of the scenario by both air traffic controllers and both flight crews prevented a further escalation of the situation.
The investigation analyzed that at the time ATC informed the crew of DY-741 that the landing runway was switched from 01R to 01L there was still sufficient time for the flight crew to adjust the rate of descent and then follow a rather normal approach profile. The crew should however refused an instruction to turn early onto final approach.
The AIBN analyzed that the instructor of the final approach controller should have intervened assessing the facts, that a large tailwind component made it difficult to reduce speed, and advised the trainee controller to provide an extended vectoring onto final approach. They both noticed that DY-741 was high on the approach and might have asked the crew how much more track miles they needed to get onto the profile.
When the aircraft was handed off to tower at 6nm from touchdown it was still doing 212 KIAS and 250 knots over ground. The final approach controller's instruction to reduce speed to 180 knots was "optimistic", the investigation assessed.
Reviewing the flight data recorders of DY-741 the AIBN assessed that after capturing the localizer none of the criteria to continue the approach were met and an immediate go-around was necessary. Continuing the approach to 1nm before touchdown and going around only after establishing visual contact with the runway was not appropriate, going around at an earlier stage would have permitted more time to implement corrective measures and ensure separation.
The AIBN analyzed that at the time the takeoff clearance was issued to DY-740, the arriving flight DY-741 was 3.7nm from touchdown at a speed of 250 knots over ground. The AIBN therefore believes it was easy and necessary to recognize that DY-741 would not be able to continue the approach for a landing. The tower controller therefore should have waited with the clearance for takeoff and instead issued an instruction to DY-741 to go around. This would have caused a delay of DY-740 and DY-741 however ensured necessary separation between the aircraft.
DY-741 did not go-around because of another aircraft on the runway but because of being not stabilized for the approach. Had the crew continued the landing however, less than the runway length would have remained between the aircraft upon touchdown.
The AIBN analyzed that the tower controller should have used the term "immediate" when issuing the turn to 270 degrees in an attempt to resolve the conflict.
The AIBN found the callsign mixup understandable when DY-740 executed the instruction given to DY-741. The AIBN suggested that "DY-741 on missed approach, turn left heading 270 immediately" would have helped to avoid the mixup and at the same time stressed the urgency of the instruction.
The AIBN commended the crew of DY-741 for recognizing the situation and keeping turning until a divergent trajectory was established and the conflict was defused.
The AIBN commended the crew of DY-740 for recognizing the target on TCAS above them and slowing their rate of climb as result.
The AIBN "praised" the tower controller for "keeping his cool" and providing valuable traffic information to both aircraft and the Oslo departure/approach sector. The AIBN also praised the controller for his decision to keep both aircraft on his frequency until after the conflict was resolved. The AIBN analyzed that the tool of visual separation was not available at the time of occurrence due to instrument meteorological conditions prevailing and low cloud which also prevented the tower controller to establish visual contact with the aircraft during the conflict.
This article is published under license from Avherald.com. © of text by Avherald.com.
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