What happened
On 8 August 2017, a Boeing B787-90 , registration N825AA, operating an international passenger flight from Madrid-Barajas Airport to Dallas-Fort Worth, was climbing through 10,900 feet following a standard instrument departure. Simultaneously, a Cessna 172-M, registration EC-IEO, was cruising at 11,000 feet after departing from Madrid-Cuatro Vientos Airport.
As the aircraft paths converged, the Traffic Collision Avoidance System (TCAS) on the B787-90 issued a Resolution Advisory (RA) for a descent. The crew of the B787-90 immediately executed the maneuver, descending to 10,200 feet. The crew of the Cessna 172-M, which was not equipped with a TCAS system, observed the large aircraft descending and performed an evasive climb to 11,200 feet to maintain separation.
The investigation
The CIAIAC investigation examined the flight plans, air traffic control (ATC) communications, and the performance of the SACTA automated air traffic control system. The investigation focused on why the controllers did not intervene before the aircraft violated minimum radar separation distances. Investigators analyzed the workload of the controllers in the WDN sector and the technical functionality of the conflict alert system during the event.
Findings
- The primary cause of the incident was that the executive controller focused his attention on resolving a conflict in another part of the airspace, neglecting to track and resolve the potential conflict between the two aircraft.
- A conflictive flight plan for the Cessna 172-M had been approved, which violated airway altitude restrictions and placed it on a path toward conflicting traffic.
- Communication difficulties on a dedicated line to the LEGT station forced the executive controller to assume the coordination duties of the planning controller, increasing his workload.
- The automated conflict alert system failed to provide an early enough warning to allow the controller to take corrective action before the separation violation occurred.
- The crew of the Cessna 172-M was unable to climb significantly higher because they were near their operational ceiling.
Safety action
- ENAIRE is recommended to establish specific routes for aircraft departing Madrid-Cuatro Vientos on northerly IFR routes to minimize conflicts within the Madrid TMA.
- ENAIRE is recommended to revise the Short-Term Conflict Alert (STCA) prediction algorithm to improve detection capabilities and evaluate the use of flight plan data in conflict predictions.