What happened
On January 16, 2016, an airborne collision avoidance system (ACAS) event occurred within the Warsaw Flight Information Region (FIR) near the KOTEK waypoint. The incident involved two aircraft: a Boeing 737-800 and a Boeing 737-500.
The crew of the Boeing 737-800 was performing a descent from flight level (FL) 390 to FL 370. Simultaneously, the Boeing 73 ام7-500 was cruising at FL 360. As the Boeing 737-800 reached FL 376, the crew received a TCAS Resolution Advisory (RA). The crew immediately halted their descent. Shortly thereafter, the crew of the Boeing 737-500 also confirmed receiving a TCAS RA, though that aircraft remained at FL 360.
The investigation
The investigation established that at the moment the Boeing 737-800 triggered the TCAS RA at FL 375, the Boeing 737-500 was located approximately 5.7 NM away. While the RA was active, the crew of the Boeing 737-500 did not report receiving a TCAS alert.
Data analysis showed that the descent of the Boeing 737-800 from FL 390 to FL 375 took approximately 32 seconds (from 11:39:28 UTC to 11:40:00 UTC), resulting in an average vertical speed of approximately 3,000 feet per minute. The investigation found no errors in Air Traffic Management (ATM) performance.
Findings
- The primary cause of the incident was an excessive rate of descent.
- The crew of the Boeing 737-800 failed to comply with the procedures outlined in ICAO Annex 6, section 4.4.10. This regulation specifies that to avoid unnecessary TCAS commands, operators should implement procedures—particularly when using autopilot—to limit vertical speeds to less than 1,500 feet per minute (or 8 feet per second) during the final 1,000 feet of a climb or descent when approaching adjacent flight levels.
- The high vertical speed of 3,000 feet per minute significantly increased the risk of a TCAS intervention.