What happened
On July 22, 2015, within the Warsaw Flight Information Region (FIR), a Boeing 737-800 was climbing through the ATMIR waypoint toward a target flight level of FL330. During this phase of flight, the crew established contact with the Approach Control (KRL APP) and received clearance for the IRLUN1T arrival and a descent to FL170.
While the Boeing 737-800 was climbing, a Boeing 777 was transiting the area at FL340, flying toward the GOVEN waypoint. At approximately 10:33:53 UTC, the crew of the Boeing 737-800 reported that the onboard ACAS had issued a Resolution Advisory (RA) commanding a descent. The crew subsequently executed the maneuver and later reported that the conflict had been cleared.
The investigation
The investigation examined the flight paths and the performance of the ACAS/TCAS systems during the encounter. Investigators reviewed the communications between the flight crews and the respective Air Traffic Control units (KRL APP and KRL ACC). The investigation also involved consultation with ATS personnel training specialists regarding the logic and operational behavior of the ACAS system during vertical maneuvers.
Findings
- The Boeing 737-800 was climbing toward FL330 while the Boeing 777 was maintaining level flight at FL340.
- At the time of the RA, the horizontal separation between the two aircraft was approximately 1 nautical mile, with a vertical separation of 1500 feet.
- The investigation established that the climb rate of the Boeing 737-800 was too high during its ascent to FL330.
- This excessive rate of climb led the ACAS to predict a