On 8 August 2017, a Boeing 787-9 (N825AA) being operated by American Airlines on an international passenger flight from Madrid to Dallas-Fort Worth as AAL37 with an augmented flight crew and climbing to FL240 came into conflict in day VMC with crossing traffic, a Cessna 172M (EC-IEO) being operated by Aerocentre on an I/R training flight from Madrid Cuatro Vientos to Burgos as ACR31 and level at FL110. The conflict was avoided by the 787 crew following a TCAS RA and the 172 pilot performing a complimentary visual manoeuvre.
Once the Spanish Commission for the Investigation of Accidents and Incidents (CIAIAC) had been informed of the event and commenced their Investigation, relevant flight recorder information had been overwritten but a comprehensive set of relevant recorded ATC data was available. The conflict had occurred in the Madrid TMA where the ANSP was ENAIRE.
It was noted that the 787 flight crew consisted of a 61 year-old Captain with a total of 13,214 flying hours which included 741 hours on type, a 56 year-old First Officer with a total of 11,408 flying hours which included 943 hours on type and a 60 year-old relief Relief Pilot with a total of 11,797 flying hours which included 1,142 hours on type. The First Officer was acting as PF at the time of the event and the Relief Pilot was occupying a supernumerary crew seat in the flight deck.
It was established that, in accordance with radar vectors issued by the sector controller, the Cessna 172 had been level at FL110 and heading north as the 787 passed FL109 on a SID track of 300° - a crossing track - six minutes after takeoff. The 787 had received a TCAS RA ‘DESCEND’ against the 172 which was not TCAS-equipped and promptly actioned it in accordance with the applicable SOP. ATC were informed and were also subsequently informed when the ‘Clear of Conflict’ annunciation was received before resuming their flight as cleared. According to the Instructor Pilot in command of the 172, upon seeing an aircraft at a lower altitude that was heading west, climbing and tracking toward them, they saw this aircraft stop its climb and begin to descend. They, in turn, attempted to climb, although “they only managed to climb 200 feet since they were flying practically at the aircraft’s service ceiling”. He stated that “seconds later the other aircraft passed beneath them” after which they resumed their flight as already cleared. Radar data showed that as a result of the TCAS RA response, the actual minimum separation during the conflict had been 1.2nm horizontally and 400 feet vertically.
The Investigation found that the sector executive controller involved had accepted the 172, which had departed from Cuatro Vientos on a ‘Z’ flight plan (which involved a transition to IFR after initially departing VFR) early as he had realised that its northerly track would need to be coordinated with departing Barajas traffic turning northwest such as the 787. He instructed the C 172 to turn onto and maintain a track direct to its destination climbing to and then maintaining FL 110 which meant it was heading north about 13½ nm west of its flight planned track. He had a couple of options for traffic crossing separation in mind but then became distracted by sequencing traffic into Getafe during which he had to temporarily take over coordination when the sector planning controller was unable to communicate with Getafe on the landline. He reported that this led to him forgetting the need to actively coordinate separation between the 787 and the C172 and he stated that “he did not recall receiving any prior alerts warning of a loss of separation”. The Investigation concluded that the controller’s sector workload had not been high but noted that the controller had considered that it had been “high” because of the coordination of traffic rather than its volume.
The setup of the STCA was reviewed and it was noted that the ‘predictive’ and ‘violation’, conflict alerts were both presented visually on screen and accompanied by a “beeping sound” with the time to traffic separation of less than 2½ nm or 800 feet set at 85 seconds and 55 seconds respectively for the airspace involved. It was ascertained that for undetermined reasons, only the ‘violation’ alert was annunciated and that although he had “identified the conflict” as a result, there had been no time to react before almost immediately receiving the ‘TCAS DESCENT’ call from the 787.
The Cause of the event was formally documented as "the executive controller focusing his attention on resolving a conflict in another part of the airspace under his control, while forgetting to track and resolve a potential conflict that he had previously identified”.
The following three Contributory Factors were deemed to have been present:
- A conflicting flight plan was approved for the Cessna 172 which violated restrictions on the airway and that brought it into conflict with takeoffs and landings at Madrid Barajas.
- Poor reception on the dedicated land line to the Getafe air base, which forced the executive controller to take over the tasks that the planning controller had been doing.
- The conflict alert provided by the STCA system did not activate early enough for the executive controller to take action before the conflict situation involving the aircraft occurred.
Two Safety Recommendations were made as a result of the Investigation as follows:
- that ENAIRE establish a route for aircraft taking off from the Madrid-Cuatro Vientos Airport (LECU) on northerly routes under IFR to facilitate their (systematic) incorporation into an airway and to minimise possible conflicts with other traffic in the Madrid TMA. [REC 34/18]
- that ENAIRE revise the conflict alert prediction algorithm (STCA) in an effort to improve its ability to detect future conflict scenarios. This recommendation also includes the suitability of evaluating the viability of improving the capability of the system to use flight plan data. [REC 35/18]
The Final Report of the Investigation was approved on 7 June 2018 and subsequently published in English translation in February 2019.