Ryanair B738 and Ryanair B738 near Pamplona on Oct 2nd 2018, near collision in flight
Last Update: September 15, 2020 / 14:21:10 GMT/Zulu time
A Ryanair Boeing 737-800, registration EI-FRY performing flight FR-724 from Santiago de Compostela,SP to Palma Mallorca,SP (Spain), was enroute at FL340 about 3nm south of Pamplona,SP (Spain) nearing reporting point GOSVI at the same time.
Both aircraft received TCAS resolution advisories (RAs), the crews complied with the RAs, FR-724 climbed about 500 feet before returning to FL340, FR-1192 descended to FL330 and began the descent into Toulouse, both aircraft became clear of conflict and continued to their destinations for safe landings.
On Oct 25th 2018 Spain's CIAIAC reported according to preliminary data the separation between the aircraft reduced to 400 feet vertical and 2.2nm horizontally. The occurrence is being investigated.
On Sep 15th 2020 the CIAIAC released their final report concluding the probable cause of the incident was:
The incident involving aircraft 1 (EI-FRY) and 2 (EI-DWW) was caused by the failure of the PAL sector controller to identify the conflict, and the subsequent transfer of aircraft 2 (EI-DWW) to the adjacent sector, ZGZ, without being clear of the conflict. Contributing to the incident is the improper handling of the conflict by the controllers in both sectors, who issued similar instructions to the two aircraft, contrary to what had been agreed previously.
The CIAIAC analysed:
General conditions prior to the event
The weather conditions were irrelevant in this incident and did not affect the outcome of the event (the aircraft had not diverted from their original routes nor were there any weather conditions that limited the aircraft’s performance or routes).
No unusual conditions were present that could have affected the capacity or ability of the controllers involved. The sectors were configured as planned and there were no circumstances that resulted in complex conditions in the sector or in the operating environment. The workload of the controllers was below their maximums, and thus this aspect is deemed irrelevant to the incident.
As regards the controllers involved, they all had ample experience both in their units and in ATC in general. None of them was in training, and these factors are deemed to have had no influence on the incident.
The only aspects of note due to their potential contribution to the incident are as follows:
- The recent shift change involving the sector ZGZ executive controller, which may have affected his situational awareness of the traffic in the sector.
- The return to work after vacation of the executive controller in sector PAL, which may have affected his actions, as reflected in the lack of urgency in his communications during the conflict and the failure to enter the cleared flight level.
From the point of view of the crews, the flights had been uneventful prior to the incident, with no deviations from the planned routes, and thus did not contribute to the event.
Source of incident: failure to detect the conflict
The root cause of the incident is deemed to lie in the failure of the PAL sector controller to identify their converging headings while both were in contact with this sector. As a result, the PAL sector controller transferred aircraft 2 to the adjacent sector, ZGZ, while it was still in conflict.
According to the statement from the PAL sector controller, due to the destination of aircraft 2, he assumed that it would be instructed to descend to FL280 after being transferred to adjacent sector ZGZ, as this was specified in the unit’s procedures and in the letter of agreement with the Bordeaux control center. This outcome was assumed by the controller without communicating or coordinating with the controller in the adjacent sector. He also did not follow up on the aircraft after it was transferred to verify that it had maneuvered as expected.
As a result, this transfer 3 min before the event took place under the following conditions:
- The transferred aircraft was not clear of conflict, since it was at the same flight level and on a converging heading toward aircraft 2.
- The conflict had not been identified earlier by the PAL controller, nor by the PAL or ZGZ controllers after it was transferred.
- The transfer was made early, since the aircraft was still in the airspace of sector PAL, 37.5 NM away from the geographic limit of sector ZGZ.
In his statement, the ZGZ controller said that he had just gone on duty when he received the call from aircraft 2. At no time did he, nor the offgoing controller, realize that the aircraft was in conflict, since that is a requirement for transferring an aircraft. This was reflected in his initial actions to familiarize himself with the traffic in his sector. While he did check other aircraft for potential conflicts, he did not do so with these. As a result, for different reasons, when the transfer was made, neither controller (in sector PAL or the offgoing or oncoming controllers in ZGZ) identified the conflict between the aircraft.
Consequently, the aircraft proceeded on converging headings for 1 min 39 s until the conflict was detected.
Detection of the conflict by the Bordeaux ACC 1 min 27 s earlier
The conflict was not detected by the controllers in sector PAL or ZGZ; rather, it was detected by the Bordeaux ACC, adjacent to the Madrid ACC. At 14:55:42, a French controller called the PAL sector controller to inform him of the situation, since both aircraft were in the airspace of sector PAL. At that time, the two aircraft were 8.7 NM away and at the same flight level, FL340, still outside the minimum required distance horizontally (5 NM), though not vertically (1000 ft).
The tone of voice used by the PAL controller, and later by the ZGZ controller, confirmed that neither one had been aware until that point of the conflict between the two aircraft. The PAL controller’s reaction was immediate: call ZGZ and propose corrective steps to take involving aircraft 1, which was still in contact with him.
The time that elapsed during the call from the Bordeaux ACC and the 10 s it took the ZGZ controller to reply to the proposed corrective action from PAL resulted in a 34-s delay before instructions were given to the two aircraft. During this time, due to the speed of the aircraft (250 CAS and 410 GS), the distance between them fell from 8.7 NM to 5.8 NM.
Failure of the STCA system to identify the conflict
Had the STCA worked, it would have generated two alerts, spaced 2 min apart:
- the first, a PAC alert, at 14:54:36
- the second, a VAC alert, at 14:56:31
Between the two alerts that the PAL and ZGZ controllers would have had (if the STCA had functioned), the call from the Bordeaux ACC was received (14:55:42) when both aircraft were 8.7 NM away. With this call air traffic controllers became aware of the conflict, meaning it was identified in time and there was time enough to handling the conflict before the generation of the TCAS RA, even with the malfunction of the STCA.
Had the controller in the Bordeaux ACC not placed the call, there was still the barrier provided by the TCAS systems on board, which did work correctly.
The reason for the failure of the STCA system to work correctly in this and two other aircraft proximity events was identified and corrected by ENAIRE, which immediately and successfully implemented corrective measures. As a result, as of the date of this report, it is not necessary to issue any safety recommendations since the actions taken by ENAIRE resolved the problems identified.
Improper handling of the conflict by ATC
The mid-air collision avoidance system is achieved by first issuing alerts to ATC (STCA) and then to the crews (TCAS), with ATC being understood as the first barrier in order to keep the aircraft crews from having to perform avoidance maneuvers. As a result, the alert activation thresholds at the control stations (PAC, VAC) are much earlier than on board the aircraft (TA, RA), as described in sections 1.18.1 and 1.18.2. The alert sequence is thus PAC-VAC-TA-RA.
In this case, ATC became aware of the conflict between the PAC and VAC (had they been generated), and thus before the TCAS issued any advisories.
Because of this, the controllers in the two sectors had time to issue instructions to the crews before the TCAS went into action; however, these instructions were inadequate, since both controllers issued the same maneuvers to the two aircraft for different reasons, aggravating the situation:
- In the first case, the two controllers did coordinate, but the one in ZGZ thought that the other aircraft was taking too long to start the maneuver. He was also influenced by the fact that the PAL controller had not entered the authorized lower flight level. As a result, he instructed his aircraft, despite not being in his airspace, to execute the same maneuver. This instruction was not coordinated with or reported to the controller in sector PAL.
- In the second case, there was no coordination between the PAL and ZGZ controllers, both of whom, seeing the conflict that had resulted from the same descend instruction, again instructed the crews to climb to the same flight level while they were separated by 4.3 NM horizontally.
As for the phraseology used by the controller:
- The ZGZ controller did convey the urgency of the instructions (through the tone of his voice, the fast delivery, by repeating the instruction as many as three times and by using the term “immediately”), although he did not give information on the conflict aircraft.
- The PAL controller did not convey the urgency of the instructions (he did not use the term “immediately”, his voice and pace were calm, very long sentences), but he did give information on the conflict aircraft.
In response to these serious deficiencies in the handling of this conflict, ENAIRE, in the internal investigation it conducted after the incident, took steps to improve the training of its controllers. The measures taken by ENAIRE are deemed adequate and sufficient to correct the problems identified, and as a result no safety recommendations are issued in this regard.
In terms of the aircraft, the flight recorders show that the instructions issued by ATC, first to descend and then to climb, were entered into the computer immediately by the crews; however, the ensuing step to start the transition to the new flight level was different in each aircraft:
- In the case of aircraft 1, undoubtedly due to the lack of urgency and gravity conveyed by the controller in his messages, and to already having a blue “proximate traffic detected” message on the TCAS display, the crew questioned ATC’s instruction, which is why aircraft 1 did not execute a descent and instead waited for confirmation from ATC. The action of the crew of aircraft 1 showed excellent situational awareness of the immediate future, anticipating a potential conflict that was in effect taking place. By not starting its descent, the crew kept the conflict from becoming worse.
- In the case of aircraft 2, probably due to the immediacy and urgency conveyed by the messages from the ZGZ controller, the crew entered the new selected flight level and the aircraft began to descend and then to climb. This action is deemed equally correct, since ATC instructions always take priority (except with a TCAS RA), and at the time, the TCAS had not even issued a TA.
As a result of these contradictory instructions, the vertical separation between the aircraft increased at first (while aircraft 1 maintained its level and aircraft 2 descended), but following the second set of instructions to climb, aircraft 2 began to climb while aircraft 1 remained at the same flight level, thus reducing the vertical separation between them. In light of the worsening situation involving the aircraft, closing both vertically and horizontally, the TCAS came into play, issuing correct evasive maneuvers.
Detection of the conflict by the TCAS 22 s earlier
The TCAS TA and RA alerts were generated 22 s before the point of closest approach and with a 13-s separation between them, as per the design criteria. The RA maneuvers seek to achieve a separation of 300 to 700 ft. Due to the closing trend of aircraft 2 with respect to 1 (due to the climb instruction it had been issued by ATC), the system generated an RA with 411 ft.
Of the 43-s total duration of the TCAS RA, the first 9 were the most critical, since aircraft 1 had to initiate a climb and aircraft 2 had to change its upward trend and reverse it. Although the crews reacted to disengage the A/P and A/T within the 5-s design criteria of the TCAS, the achievement of a vertical speed by the two aircraft was logically not as fast, coinciding with the inflection point where aircraft 2 stopped climbing and aircraft 1 started its descent.
From that moment on, the two aircraft progressed as expected, with their vertical separation increasing to 700 ft, at which point the TCAS, as designed, stopped the climb and descent and issued new RA maneuvers to level off. Since at that point, the aircraft were about to cross (1 NM), the RA instruction was maintained until the aircraft finished crossing.
In conclusion, the TCAS worked correctly and ensured proper separation between the aircraft, as it is designed to do.
The execution of the TCAS procedures by both crews was complete and correct. The reaction times and the vertical speeds were within the values expected by the TCAS, the automatic control systems were disengaged and the situation was reported both during the event and once the conflict was clear.
This article is published under license from Avherald.com. © of text by Avherald.com.
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