Incident Overview
On January 16, 2016, at approximately 11:39 UTC, a traffic alert and collision avoidance system (TCAS) resolution advisory (RA) was generated on board two aircraft in the vicinity of the KOTEK navigation point within the Warsaw Flight Information Region (EPWW). The aircraft involved were a Boeing 737-800 (B738) and a Boeing 737-500 (B735). The B738 was cleared to descend from flight level (FL) 390 to FL 370, while the B735 was cruising at FL 360.
Sequence of Events
As the B738 descended, it reported a TCAS RA upon reaching approximately FL 376 and stopped its descent. Shortly thereafter, the crew of the B735 confirmed receiving a TCAS RA but remained at FL 360. At the time the B738 was at FL 375 and reported the RA, the B735 was 5.7 nautical miles away at FL 360. The B738's descent from FL 390 to FL 375 occurred between 11:39:28 UTC and 11:40:00 UTC, resulting in an average vertical speed of approximately 3,000 feet per minute.
Findings
The investigation determined that the B738 did not comply with Annex 6, paragraph 4.4.10, which recommends a vertical speed of less than 8 feet per second (1,500 feet per minute) during the last 1,000 feet of climb or descent when approaching an adjacent altitude or flight level, especially with autopilot engaged and awareness of traffic. No errors were found in the performance of air traffic management (ATM).
Cause
The cause of the incident was an excessive rate of descent.
Post-Event Actions
The ATM Event Investigation Team prepared an information notice for the reporting controller and the operator of the B738, detailing the findings. No safety recommendations were issued.