What happened
On March 6, 2018, a Boeing 737-800 operating a commercial flight from Paris (LFPG) to Athens (LGAV) was performing a descent within the Athens TMA. Following a change in arrival instructions, the crew was cleared to descend to flight level 170 via a direct routing to the PIKAD waypoint. To optimize the descent profile, the crew maintained a high speed of Mach 0.76–0.77.
At approximately 16:18:43 UTC, while at an altitude of roughly 26,800 feet, the Airborne Collision Avoidance System (ACAS2) issued a "Level Off" Resolution Advisory (RA). At the moment of the alert, the aircraft's speed was 311 knots CAS. In response to the RA, the crew disconnected autopilot number 2 and subsequently the autothrottle. The aircraft leveled off at 26,600 feet, and the speed began to decrease. The crew re-engaged the autopilot at 16:19:02 UTC and continued the descent using vertical speed mode at 1,500 feet per minute.
The investigation
The investigation focused on the circumstances leading to the ACAS alert and the crew's subsequent actions. The investigation established that the crew was surprised by the RA, as the ACAS display did not indicate any other conflicting traffic. Furthermore, the investigation found that the crew did not report the RA to Air Traffic Control (ATC) because they were instructed to change frequencies immediately following the event.
Findings
- The ACAS RA was triggered by the combined descent rate of the Boeing 737-800 and another ascending aircraft.
- The crew did not notify ATC of the RA due to simultaneous frequency change instructions.
- Atmospheric conditions at the time of the event had no impact on the occurrence.