Swiss A321 and Vueling A320 at Barcelona on Jul 25th 2016, near collision
Last Update: November 29, 2017 / 15:50:00 GMT/Zulu time
Incident Facts
Date of incident
Jul 25, 2016
Classification
Incident
Cause
Near collision
Airline
Swiss
Flight number
LX-1959
Departure
Barcelona, Spain
Destination
Zurich, Switzerland
Aircraft Registration
HB-ION
Aircraft Type
Airbus A321
ICAO Type Designator
A321
A Vueling Airbus A320-200, registration EC-LRE performing flight VY-6502 from Barcelona,SP (Spain) to Naples (Italy), departed Barcelona's runway 25L about 90 seconds after LX-1959, and was climbing along standard instrument departure route VERSO3Q.
Spain's CIAIAC stated that both instrument departure routes share a common segment up to waypoint NITBA. Due to arrival traffic air traffic control instructed both departing aircraft to turn left off the departure route after climbing through FL080 and proceed direct to their respective exit point of the standard instrument departure route.
The CIAIAC reported that due to the different climb performance of both aircraft the separation between the two aircraft eroded however until reaching 300 feet vertical and 1.1nm horizontal, which resulted in TCAS Traffic Advisories first and finally Resolution Advisories on both flight decks.
Transponder data suggest the aircraft were 100 feet vertically and 2.47nm horizontally apart at 18:36:22Z before their flight trajectories crossed.
On Nov 28th 2017 Spain's CIAIAC released their final report concluding the probable cause of the incident was:
The incident was the result of an incorrect instruction from the executive controller, who crossed the lateral trajectories of the aircraft without considering their different climb rates, resulting in the loss of separation between them.
The following contributing factors are deemed to have influenced the incident:
- The controller focusing his attention on a potential conflict between other aircraft in the south of the sector.
- The inhibition of the STCA system in the Barcelona TMA below FL105.
The CIAIAC summarized the occurrence: "The aircraft operated by the company Swiss was ahead of the Vueling aircraft in the departure sequence. To avoid conflicting with arriving aircraft, air traffic control instructed the crews of both aircraft to turn left and fly direct to the final fix of their standard departure maneuvers (DALIN and VERSO) once they reached flight level FL080. The different climb rates of the aircraft caused the separation between them to diminish, until it reached a minimum value of 1.2 NM horizontally and 200 ft vertically, which triggered a TCAS traffic advisory (TA) and then a resolution advisory (RA). The execution of the evasive maneuvers created a separation of the aircraft that allowed both aircraft to resume normal navigation."
The CIAIAC reported that only the Vueling A320 received a TCAS resolution advisory, the Swiss A321 did not receive a resolution advisory.
The CIAIAC analysed:
The first departing aircraft involved in the incident was SWR191Q, on instrument departure DALIN3Q. This aircraft was cleared to proceed direct to DALIN, the final point of its departure maneuver, upon climbing through FL080 to FL190. The crew lowered the speed to 220 kt with the intention of increasing their climb rate in order to execute the authorized maneuver in the shortest time possible.
The second aircraft, VLG6502, contacted sector T3W 1 minute 39 seconds later. It was assigned instrument departure VERSO3Q, and it was also cleared to proceed to the final point on its maneuver, VERSO, after climbing through FL080 to FL190. The crew also reduced the indicated airspeed to improve the climb speed, thus obtaining a better climbing performance than the preceding traffic.
According to the controller’s statement, the instructions he gave to the two aircraft were similar, and he was convinced that they would achieve his goal of separating them without creating subsequent conflicts, since the routes were different and the aircraft were far apart. He did not consider the possibility that the lower traffic would increase its climb rate and that the other traffic would climb slower and be overtaken by the second. He acknowledged his loss of situational awareness, since he thought that the aircraft were sequenced differently based on their destination, meaning that the higher traffic was going to VERSO and the lower one to DALIN, though he gave them the correct instruction based on their callsigns.
Carrying out the instructions received by the controller would cause the flight paths to cross laterally, since point DALIN is further north than VERSO (see Figure 1). The instruction issued was thus incorrect.
Before the two incident aircraft took off, there were two arriving aircraft in the south of the sector that caused a conflict situation, thus indirectly contributing to this incident since, as the controller stated, they diverted his attention as he attempted to resolve the conflict. The conflicting aircraft were SWT8169, which entered the sector via point ESPOT at FL140, which he cleared to fly standard arrival route NEPAL2V, and VLG3537, which entered via point MARTA at FL180 and which was cleared to fly route MARTA5Y.
Since aircraft SWT8169 was much slower than VLG6502, the sequence number for the latter was slower, and the controller instructed it to a lower level and monitored its descent so that by NEPAL, the point of conflict between the two standard terminal arrival routes authorized, it would be below SWT8169. The descent rate of the authorized traffic was lower than expected by the controller and a CAP alert was activated, which was inhibited once the aircraft’s vertical trajectories crossed.
Given the positions of the departing aircraft involved in the incident and their similar speeds, the controller’s plan resulted in their flight paths crossing. The Swiss aircraft, as it turned toward DALIN, cut off the lateral trajectory ahead of the Vueling aircraft, which also had a higher climb rate. This resulted in both aircraft’s flight paths crossing at almost the same level, causing a loss of separation as the two aircraft came within 1.2 NM horizontally and 200 ft vertically of each other.
The planning controller reported that since the workload in his sector was low, he was focusing his attention on the sequence at the initial approach fix in the adjacent sector in order to control the speeds of inbound aircraft and help out the executive controller. He was unaware of the instruction that had been given to departing traffic. He informed the executive controller of the conflict just as it was about to occur.
The STCA alarm was activated when the separation between the aircraft was lost, resulting in a Conflict Alert Violation (CAV) without a preceding Conflict Alert Prediction (CAP). At the time of the incident, this alarm had been inhibited from FL105 to MSL.
With respect to the Swiss A321 not receiving a TCAS RA the CIAIAC analysed:
Based on the data obtained from the quick access recorder (QAR) and from the radar tracks, both aircraft received TCAS traffic advisories (TA), while only VLG6502 received a climb resolution advisory (RA), which lasted 16 seconds.
The crews reacted as indicated in the Airbus FCTM, monitoring the intruding traffic in case of a TA, and following the indications on the vertical speed indicator (VSI) in case of a RA.
Their use of communications and phraseology was also correct.
The investigators were surprised by the fact that only one of the aircraft involved received a resolution advisory, so experts at Eurocontrol were asked to assist in analyzing the maneuver. Eurocontrol confirmed that its statistics indicated that this happened in a large number of cases.
Eurocontrol analyzed the event using its InCAS tool, which showed that the behavior of the TCAS unit on aircraft VLG6502 was correct. Their analysis of the simulation also showed that the unit on aircraft SWR191Q should have generated a “Level Off” RA.
To explain this discrepancy, the Eurocontrol specialists requested the BDS10 data protocol records (“Datalink capability report”) for the S-mode radar from ENAIRE.
The data provided did not contain the information desired on the active mode of operation of the TCAS unit installed on aircraft SWR191Q. As a result, this discrepancy could not be corroborated except by the information provided by the operator, which stated that the unit was selected to TA/RA mode.
SWISS commented that the flight crew of SWR191Q confirmed, that in their opinion the TCAS unit must have been set to the TA/RA mode. The setting of the selector to TA/RA before takeoff is a standard checklist point and, even though of course not impossible, it seems highly improbable that the crew moved the selector to the TA only and not the TA/RA position. Besides the flight crew would been aware of the “TA only” white memo indication on their navigation display during the climb out if this checklist point had been performed incorrectly. An incorrect setting and the overlooking of an “abnormal” indication of the navigation display would indicate a double failure, which does not seem likely. In addition, the intruder was displayed on the navigation displays of SWR191Q shortly after takeoff and thus, becoming aware of the traffic, the Commander obtained visual contact with the intruder (out of the left cockpit window). So the alertness of the crew with regard to a possible TCAS warning was raised at an early stage. In summary, even if a mishandling of the selector cannot be totally excluded, it must be assumed that the setting of the TA/RA selector was correct. Additionallly, the operator stated that no technical fault of the TCAS unit of HB-ION could be traced.
ENAIRE’s Technical Department stated that its system does not record the BDS30 protocol (ACAS active resolution advisory). Eurocontrol specialists regard both the records of this mode and those of the BDS10 as essential for analyzing events involving the collision avoidance system.
For this reason, a safety recommendation is issued to ENAIRE to request that it record the BDS10 and BDS30 data protocols for the S-mode radar so as to facilitate the investigation of events involving the airborne collision avoidance system (ACAS).
Aircraft Registration Data
Incident Facts
Date of incident
Jul 25, 2016
Classification
Incident
Cause
Near collision
Airline
Swiss
Flight number
LX-1959
Departure
Barcelona, Spain
Destination
Zurich, Switzerland
Aircraft Registration
HB-ION
Aircraft Type
Airbus A321
ICAO Type Designator
A321
This article is published under license from Avherald.com. © of text by Avherald.com.
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