What happened
On June 12, 2017, a Boeing 737-800 departing from Rhodes (LGRP) for Prague (LKPR) was climbing through 35,700 feet when the flight crew reported a TCAS Resolution Advisory (RA). The alert was triggered by an opposing aircraft, another Boeing 737-800 (flight ENT5301), which was descending from 39,000 feet to 37,000 feet on a course intersecting the first aircraft's path 47 NM south of the MES VOR.
Radar data and flight recorder analysis confirmed that at 06:33:03 UTC, the crew of the first aircraft responded to the RA by descending from 35,800 feet to 35,400 feet. Simultaneously, the crew of ENT5301 responded to their RA by climbing from 37,900 feet to 38,100 feet. At the moment the RA maneuvers began, the vertical separation between the two aircraft was 2,100 feet with a horizontal distance of 6.4 NM. As the aircraft continued their maneuvers, the minimum separation reached 2,700 feet vertically and 1.4 NM horizontally at 06:38:33 UTC.
The investigation
The investigation examined radar data and Flight Data Recorder (FDR) information to determine the flight paths and vertical speeds of both aircraft. The investigation established that the aircraft were on converging tracks and that the descent rate of ENT5301 was approximately 800 feet per minute.
Findings
- The primary cause of the TCAS RA was the excessive climb rate of the first Boeing 737-800, which, when combined with the descent rate of ENT5301, created a conflict profile.
- Both crews successfully executed the TCAS RA instructions and confirmed their new assigned altitudes with the ATCO ACC.
Safety action
Following the investigation, the operator's training department issued a notification to flight crews reminding them of the ICAO recommendation to reduce vertical speeds to 1,000 feet per minute when approaching assigned flight levels. Additionally, the safety department published the findings of this incident in a safety bulletin.