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A320 / A321, vicinity Barcelona Spain, 2016

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On 25 July 2016, an Airbus A320 and an Airbus A321 both departing Barcelona and following their ATC instructions came into conflict and the collision risk was removed by the TCAS RA CLIMB response of the A320. Minimum separation was 1.2 nm laterally and 200 feet vertically with visual acquisition of the other traffic by both aircraft. The Investigation found that the controller involved had become preoccupied with an inbound traffic de-confliction task elsewhere in their sector and, after overlooking the likely effect of the different rates of climb of the aircraft, had not regarded monitoring their separation as necessary.
Event Details
When July 2016
Actual or Potential
Event Type
Human Factors, Loss of Separation
Day/Night Day
Flight Conditions VMC
Flight Details
Aircraft AIRBUS A-320
Operator Vueling
Domicile Spain
Type of Flight Public Transport (Passenger)
Origin Barcelona/El Prat Airport
Intended Destination Naples
Take off Commenced Yes
Flight Airborne Yes
Flight Completed Yes
Flight Phase Climb
Flight Details
Aircraft AIRBUS A-321
Operator Swiss International Airlines
Domicile Switzerland
Type of Flight Public Transport (Passenger)
Origin Barcelona/El Prat Airport
Intended Destination Zürich Airport
Take off Commenced Yes
Flight Airborne Yes
Flight Completed Yes
Flight Phase Climb
Location - Airport
Airport vicinity Barcelona/El Prat Airport
Tag(s) Event reporting non compliant,
Copilot less than 500 hours on Type,
CVR overwritten
Tag(s) ATC clearance error
Tag(s) Required Separation not maintained,
ATC Error
Safety Net Mitigations
TCAS Effective
STCA Available but ineffective
Damage or injury No
Causal Factor Group(s)
Group(s) Air Traffic Management
Safety Recommendation(s)
Group(s) Air Traffic Management
Investigation Type
Type Independent


On 25 July 2016, an Airbus A320 (EC-LRE) being operated by Vueling on an international passenger flight from Barcelona to Naples as VLG 6502 and an Airbus A321 (HB-ION) being operated by Swiss on an international passenger flight from Barcelona to Zurich as SWR 191Q lost safe separation in the climb at around FL110 in day VMC after departing from Barcelona. Both aircraft had visually acquired the opposing traffic and the conflict was resolved by the A320 following its TCAS CLIMB RA.


An Investigation was carried out by the Spanish Commission for the Investigation of Accidents and Incidents (CIAIAC). Due to a delay in the reporting of the event to the Commission, relevant data from both the CVR and the DFDR had been overwritten. However, relevant data from both aircraft QARs were available to the Investigation as were relevant recorded ATC data.

It was noted that the total flying experience of the 48 year-old A320 Captain was 10,152 hours and that of his 35 year-old First Officer, 2,412 hours. The 35 year-old A321 Captain had 5,272 hours total flying experience including 3,281 hours on type and his 25 year-old First Officer had 1,106 hours total flying experience of which 182 hours was on type. The 57 year-old Executive Controller and the 59 year old Planning Controller in position for the sector involved were both correctly qualified and the latter was also an OJTI and an 'Evaluator'. Both aircraft, which had departed successively from runway 25L with the A321 first, had initially been given clearances by Barcelona Control to climb to FL190 via SIDs which had divergent tracks soon after passing waypoint NITBA (see the illustration below), the A320 on a VERSO3Q and the A321 on DALIN3Q. Both aircraft were subsequently instructed to proceed direct to VERSO and DALIN (the SID end points) once past FL 080 and the controller then turned his attention to de-conflicting two inbound aircraft. Whilst he was doing this, the two departing aircraft reached FL080 and made left turns to take up their direct tracks as instructed. However, since the A321 had reached FL080 before NITBA and half a minute later the A320 reached it after NITBA, the two aircraft were now on crossing tracks, a possibility the controller had not considered. It was noted that such an oversight is sometimes described as a "controller blind spot".

As the A320 began its left turn, the two aircraft were separated by 4.9 nm laterally and 1,100 feet vertically. Separation was quickly lost and about a minute later, as the A320 passed FL110, the crew received and actioned a 16 second TCAS CLIMB RA that took it above and just behind the A321, which reported not receiving a TCAS RA. The Closest Point of Approach (CPA) was 1.2 nm laterally and 200 feet vertically. The crews of both aircraft reported having visually acquired each other.

The Barcelona Easterly SIDS for Westerly Runway Departures as shown in the AIP. [Reproduced from the Official Report]

The A321 crew stated that they had considered that with no TCAS RA following the annunciated 61 second TCAS TA which began soon after they had passed FL100 and the fact that their increased speed thereafter had further reduced their climb rate, no action on their part was necessary. It was noted that both aircraft were fitted with TCAS II with v7.1 software installed.

It was found that when alerted by the Planning Controller to an imminent loss of separation between the two aircraft, the Executive Controller reported having initially "assumed he was referring to the (two arriving) aircraft he was monitoring" and by the time he recognised what was happening and alerted the aircraft, the A320 was already beginning its RA response and in response to an instruction from the controller its First Officer had replied "unable TCAS RA".

The Investigation requested EUROCONTROL to confirm using their InCAS simulation tool that the absence of coordinated RAs was in accordance with the way the TCAS II system should have functioned and "the simulation showed that a 'LEVEL OFF' RA should have been issued". Since the QAR data available for the A321 did not record the position of the TCAS mode as confirmation of the crews statement that it had been selected to the (correct) TA/RA position, ANSP ENAIRE was asked to provide the corresponding Mode 'S' Downlink data to confirm this but it was found that their system did not record this.

It was found that although a STCA system covered the sector where the conflict occurred, it was inhibited below FL 105 and had therefore not been activated until both aircraft had climbed above that level, by which time they were only 1.4 nm laterally and 100 feet vertically apart. This activation was the "Conflict Alert Violation" (CAV) – as the Conflict Alert Prediction (CAP) which would normally precede it was suppressed because the critieria for it occurred below FL 105.

The formal determination of the Cause of the event was that it 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".

Two Contributory Factor were also identified:

  • The controller focusing his attention on a potential conflict between other (inbound) aircraft in the south of the sector.
  • The inhibition of the STCA system in the Barcelona TMA below FL105.

Safety Action taken by ENAIRE in response the event was noted as an 'optimisation' of STCA use in the Barcelona TMA with effect from February 2017.

One Safety Recommendation was issued as a result of the Investigation as follows:

  • that ENAIRE record the data from the S-mode BDS10 and BDS30 data protocols so as to facilitate the investigation of events involving the airborne collision avoidance system (ACAS).

The Final Report of the Investigation was approved on 26 July 2017.

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