Incident Overview
On June 12, 2017, at an unspecified time, a Boeing 737-800 (B738) departed from Rhodes Airport (LGRP) on a flight to Prague (LKPR). The aircraft was climbing to flight level 360 (FL360) when, passing 35,700 feet, the pilot reported to the area control center (ATCO ACC) that the Traffic Collision Avoidance System (TCAS) had generated a Resolution Advisory (RA) due to opposite traffic. The conflicting aircraft was another Boeing 737-800, call sign ENT5301, en route from Wrocław (EPWR) to Rhodes (LGRP), which was descending from FL390 to FL370. According to flight data recorder (FDR) data, ENT5301's rate of descent was 800 feet per minute. The courses of the two aircraft were opposite, and their flight paths crossed approximately 47 nautical miles south of the navigation point MES/VOR.
Course of Events
Radar data showed that at 06:33:03 UTC, the crew of the B738 (call sign not specified) complied with the TCAS RA by descending from 35,800 feet to 35,400 feet. Simultaneously, the crew of ENT5301, also following its TCAS RA, initiated a climb from 37,900 feet to 38,100 feet. At the moment both crews began responding to their respective RAs, the vertical separation between the aircraft was 2,100 feet and the horizontal distance was 6.4 nautical miles. The closest point of approach occurred at 06:38:33 UTC, with a minimum vertical separation of 2,700 feet and a minimum horizontal separation of 1.4 nautical miles. Both flight crews subsequently confirmed the flight levels assigned by the air traffic controller. The ATC workload at the time was described as moderate.
Investigation Findings
The investigation determined that no injuries occurred; the 189 persons on board the B738 (the aircraft that reported the incident) were uninjured. No information was provided regarding the number of occupants or injuries on ENT5301. The operator (the entity that conducted the investigation) took preventive actions: the training department was to publish a notification reminding flight crews of the International Civil Aviation Organization (ICAO) recommendation to reduce climb/descent rates to 1,000 feet per minute before reaching the assigned flight level; the safety department was to publish the incident's findings in a safety bulletin. The national investigation commission did not issue any safety recommendations.
Probable Cause
The event's cause was an excessive climb rate of the B738 aircraft, which in combination with the descent rate of the ENT5301 aircraft, triggered a TCAS resolution advisory.