Event Details
On March 6, 2018, a Boeing 737-800 (type B738) operating a commercial flight from Paris (LFPG) to Athens (LGAV) experienced an airborne conflict during arrival. The aircraft was descending toward Athens Terminal Area (TMA LGAV) when the Traffic Alert and Collision Avoidance System (TCAS) issued a Resolution Advisory (RA).
Sequence of Events
While en route to Athens, the crew received a revised arrival route (STAR PIKAD 1B) and instructions to proceed direct to the waypoint PIKAD. Subsequently, they were cleared to continue to waypoint KRO and authorized to descend to flight level (FL) 170. Due to the shortcut, the crew initiated a descent with a higher-than-usual speed of Mach 0.76–0.77 to reduce altitude over a shorter distance.
At approximately 16:18:43 UTC, while the aircraft was at about 26,800 feet and indicating 311 knots calibrated airspeed (CAS), the TCAS generated a Resolution Advisory commanding "Level Off." Two seconds later (16:18:45), autopilot No. 2 disconnected, and the crew began to level off as instructed. One second after that, the autothrottle disconnected. The RA ceased at 16:18:48, and one second later the aircraft maintained a steady altitude of 26,600 feet, with CAS beginning to decrease. The aircraft held this altitude until the autopilot was re-engaged at 16:19:02, at which point CAS was 299 knots. Descent resumed at 16:19:16 using vertical speed (V/S) mode set to 1,500 feet per minute, with CAS at 294 knots.
Notably, the crew reported being surprised that no conflicting traffic was displayed on the TCAS indicator. After receiving the "Clear of Conflict" advisory, the crew continued the descent but did not report the RA to air traffic control (ATC) due to a frequency change.
Weather and Other Factors
Meteorological conditions at the time were not considered to have contributed to the event.
Probable Cause
The TCAS Resolution Advisory was generated as a result of the high closure rate between the descending Boeing 737-800 and an ascending other aircraft.