Description

On 30 October 2014, an en route Airbus A320 (EC-KCU) being operated by Vueling on a scheduled domestic passenger flight from Barcelona to Seville as VLG 2226 and an en route Boeing 737-800 (EI-EKS) being operated by Ryanair on a scheduled international passenger flight from Shannon, Ireland to Malaga as RYR314Q came into close proximity in night VMC despite the issue of coordinated TCAS RAs. The minimum recorded separation was 1.4nm laterally when also only 100 feet vertically.

Investigation

After a significant delay in notification of the occurrence, an Investigation was carried out by the Spanish Civil Aircraft Accident and Incident Investigation Commission (CIAIAC). Due to the lack of timely notification, no flight recorder data was available. However, OFDM data from both aircraft operators and ATC radar recordings were available although the ATC system did not have either contemporaneous or archive access to Mode S downlink data. It was noted that the A320 involved was equipped with TCAS v 7.0 software and that the series of activations of it during the occurrence included one which had been modified by the latest software version (v7.1) because of evidence that, depending on the method of presentation, it could increase the chances of pilots taking action contrary to that annunciated.

The A320 Captain was a 43 year-old Spanish national who had 7,254 hours total flying experience which included 5,957 hours on type. The 29 year-old First Officer was also a Spanish national and had 3,550 hours total flying experience which included 1,572 hours on type. The 737 Captain was a 33 year-old Dutch national who had 8,000 hours total flying experience most of which - 7,800 hours - was on type. The 737 First Officer was a 24 year-old Spanish national who had 1,716 hours total flying experience most of which - 1,500 hours - was on type.

The two Controllers involved on duty at the time were Spanish native speakers:

• the Executive Controller had held a licence since 1989 and had been working in the sector where the investigated event took place since 2002. He had a valid Level 5 English language proficiency rating.
• the Planning Controller had held a licence since 2004 and had been working in the sector where the investigated event took place since 2012. He had a valid Level 4 English language proficiency rating.

It was established that with the First Officer as PF, the A320 crew had, after initially preparing for an approach to runway 09 at their destination, Seville, requested runway 27 at Seville instead. ATC accepted the request and cleared the flight to descend first to FL 310 and then to FL250 subject to a rate of descent no greater than 2,000 fpm. This restriction was applied because the aircraft was on a trajectory which was converging with that of the 737 which was also descending, although at first more than 2,000 feet below. To ensure separation was maintained, the 737 was instructed to descend at a minimum rate of 2,000 fpm and did so.

Soon after the A320 had been cleared to descend, direct routing to a waypoint appropriate to the 27 approach was given explicitly under the same rate of descent restrictions. One minute after this, the A320 crew "noticed they had to increase their descent rate to reach the ideal altitude profile" which, despite the restriction just given, they did, descending at up to 5,000 fpm. Shortly after this action, the flight was re-cleared to FL170 but this time, the controller "did not mention in his clearance that the descent rate restrictions were still in effect". However, one minute after issuing this re-clearance, the controller did advise the A320 that there was "traffic 9nm away and 2,000 feet below" which was acknowledged and after a further 45 seconds asked if the A320 crew "had increased their rate of descent" to which there was no response.

As a result of the high rate of descent by the A320 and the converging tracks of the 737 and the A320, both the lateral and vertical separation between the two aircraft rapidly reduced. TCAS TAs were annunciated on the flight decks of both aircraft followed quickly by co-ordinated TCAS RAs. As the 737 passed FL 222, a 'DESCEND' RA occurred requiring at least a 2,000 fpm rate of descent which was already being sustained and was continued. As the A320 passed FL 226, an 'ADJUST VERTICAL SPEED' RA requiring a reduction of the aircraft's 3200 fpm rate of descent to not more than 1,000 fpm occurred, but instead of following it, the First Officer disconnected the AP and increased the rate of descent to 4,400 fpm. After 12 seconds, the RA changed to require a rate of descent not greater than 500 fpm which was followed by the crew disengaging both FDs. Four seconds after that, passing FL 216 and still descending at 4,400 fpm, the RA reversed to 'CLIMB' requiring at least a 1500 fpm rate of climb. Recorded data showed that in response, both pilots had simultaneously pulled back on their respective sidesticks for 10 seconds "with neither one pressing the priority button, resulting in a vertical acceleration of 2.03g". The “Dual input” alert which would normally occur was not activated because the active TCAS RA had a higher priority. After 12 seconds, the annunciated RA changed again to 'ADJUST VERTICAL SPEED' which continued for a further 12 seconds until 'CLEAR OF CONFLICT' was given after which the aircraft levelled at FL 220. At no time did the crew report to ATC that they had received a TCAS RA or a 'CLEAR OF CONFLICT' annunciation.

Meanwhile, the 737 crew followed their RA and announced doing so to ATC. It was noted that during the period leading up to, during and after the conflict, all ATC communications with the 737 were conducted in English but those with the A320 were conducted in a mixture of Spanish and English.

An initial ATC 'Conflict Alert Prediction' (PAC) was generated almost a minute ahead of the first A320 TCAS RA and prior to loss of the prescribed minimum radar separation distance at which time it was subsequently calculated that the A320 was descending towards the other aircraft at 4475 fpm and the distance between the two aircraft was 9.7 nm laterally and 2,800 feet vertically. After a further 55 seconds, and just one second ahead of the initial A320 TCAS RA annunciation, a 'Violation Conflict Alert' (VAC) was issued with the two aircraft now 3.8 nm laterally and 300 feet vertically apart with the A320 now having begun climbing to follow its modified TCAS RA. The Closest Point of Approach (CPA) followed 25 seconds later. The Investigation noted that both PAC and VAC Alerts were both aural and visual and also that neither instantaneous nor any averaged vertical speed of aircraft was displayed to controllers on the basis that such calculations based on barometric altitude were unreliable.

The Investigation considered the findings of the internal investigations carried out by both operators and by ANSP ENAIRE. It was noted that the ENAIRE Investigation had significantly failed to identify that the A320 had initially been flown contrary to the TCAS RA annunciated as it descended towards the 737 and also concluded without qualification that there had been "no ATM contribution" to the conflict. In respect of the latter finding, the Investigation did note that under Spanish Air Traffic Regulations, which corresponded to the equivalent entry in ICAO Doc 4444 PANS-ATM, "an aircraft will be informed if a climb or descent vertical speed restriction is no longer applicable" and that ATC had not made any such call to the A320 since they intended that the restriction issued should continue as the track of the A320 converged with that of the 737.

The Investigation noted that the A320 crew had not only responded incorrectly to their initial RA (displayed on their respective VSIs) by increasing vertical speed instead of reducing it, but they had also comprehensively failed to follow the relevant Vueling Standard Operating Procedures (SOPs). It also noted that the crew's simultaneous sidestick inputs during the reversal of their vertical flight path had also been contrary to SOP and a direct cause of the high 'g' loading created when the two similar inputs were summed.

The Investigation formally concluded that the Cause of the Serious Incident was that "the crew of VLG2226 did not comply with the descent rate instructions given by the sector controller - instead of maintaining a descent rate of 2,000 fpm or less they increased this rate, which resulted in an AIRPROX involving RYR314Q, which was on a converging path below VLG2226 and descending at a rate in excess of 2,000 fpm".

The following three Potential Contributory Factors were also identified:

• The controller instructed VLG2226 to proceed direct to reporting point ROTEX, indicating that the rate restriction was still in effect. This part of the instruction was not acknowledged by the crew and the controller did not demand a full acknowledgment.
• The controller then re-cleared VLG2226 to a lower flight level. This time, he did not explicitly state in his clearance that the descent rate restriction was still in effect, which led the crew to mistakenly select a higher descent rate.
• The controller did not have descent rate information for the aircraft on the radar display, which contributed to his inability to detect that VLG2226 was not complying with the descent rate instruction given.

One further Potential Contributory Factor which could have aggravated the conflict was identified as "the failure of the crew of VLG2226 to adhere to the TCAS RA 'Adjust Vertical Speed, Adjust' and increasing their rate of descent rate instead of decreasing it".

Safety Action taken because of the investigated event and during the Investigation into it was noted as included the following:

• Vueling has updated the TCAS installation in both its aircraft and its pilot training simulators from v7.0 to v7.1 in compliance with EASA requirements.
• Spanish ANSP ENAIRE has confirmed its intention to upgrade its radar to display Mode 'S' downlink data but this will not include 'rate of descent' because this is "generally based on barometric altitude information from the aircraft which is not conducive to parameter stability".

Three Safety Recommendations were made as a result of the Investigation:

• that Spain’s National Aviation Safety Agency (AESA) take the regulatory initiative to have an article included in Spain’s Air Traffic Regulations specifying that when an aircraft is cleared to climb/descend and climb/descend rate restrictions are imposed, controllers must repeat them if said restrictions still apply when the aircraft is cleared to a different flight level, or when communications are transferred between control sectors or stations. [REC 63/16]
• that Spain’s Directorate General of Civil Aviation (DGAC) engage in the relevant regulatory proceedings required to include an article in Spain’s Air Traffic Regulations specifying that when an aircraft is cleared to climb/descend and climb/descend rate restrictions are imposed, controllers must repeat them if said restrictions still apply when the aircraft is cleared to a different light level, or when communications are transferred between control sectors or stations. [REC 64/16]
• that the International Civil Aviation Organisation (ICAO) consider the need of including in the 4444 Document an article specifying that when an aircraft is cleared to climb/descend and climb/descend rate restrictions are imposed, controllers must repeat them if said restrictions still apply when the aircraft is cleared to a different light level, or when communications are transferred between control sectors or stations. [REC 65/16]

The Final Report was approved on 29 March 2016 and subsequently made available in English translation.

Related Articles

• Loss of Separation
• Airborne Collision Avoidance System (ACAS)
• TCAS II version 7.1
• ACAS Resolution and Traffic Advisories
• Mid-Air Collision
• Multi-language ATC Operations
• Safety Occurrence Investigation