What happened
On July 12, 2016, two commercial flights departed from Zurich Airport (LSZH) within minutes of each other, following different departure routes that lacked inherent horizontal separation. The first aircraft, an Avro 146-RJ100 (registration HB-IYV), departed from runway 28. Shortly after, an Airbus A321-111 (registration HB-IOH) departed from runway 16.
As the Avro 146-RJ100 climbed through 5,000 feet, the flight crew began accelerating to 210 knots to retract flaps. This increase in speed caused the aircraft's turning radius to expand significantly during its turn toward the DEGES waypoint. Consequently, the aircraft drifted further north than intended. At the same time, the Airbus A321-111 was following a heading of 260 degrees.
The proximity of the two aircraft triggered a Short Term Conflict Alert (STCA) on the ground and Traffic Advisories (TA) via the onboard TCAS units. The minimum separation reached was 2.9 NM horizontally and 550 ft vertically, violating the required separation of 3 NM or 1,000 ft. The air traffic controller prevented a collision by immediately instructing the Avro 146-RJ100 to turn to a heading of 100 degrees.
The investigation
SUST examined the flight paths, the air traffic control instructions, and the design of the Standard Instrument Departure (SID) routes. The investigation focused on why the aircraft drifted outside the intended corridor and why the departure routes (DEGES 2W and VEBIT 3S) allowed for such close proximity. Investigators also reviewed historical data, noting that since 2014, approximately 35 incidents of separation loss have been documented on this specific departure route, with the Avro 146-RJ100 being involved in about 20% of those cases due to its lower climb performance.
Findings
- The expansion of the turning radius was caused by the crew accelerating the aircraft to 210 knots while simultaneously executing the turn.
- The current design of the DEGES and VEBIT departure routes does not provide inherent horizontal separation, necessitating constant manual intervention by air traffic controllers.
- The flight guidance system (FGS) in LNAV mode caused the aircraft to cut the corner of the programmed flight plan, leading to a more northerly track.
- The departure route design creates a systemic safety deficit, as the routes allow for a lateral spread of up to 5 NM, making separation highly dependent on controller intervention.
Safety action
- The investigation highlighted that the current SID design requires frequent controller intervention to maintain separation, which reduces efficiency and increases workload.
- The report notes that the systemic safety issue regarding these departure routes remains unaddressed, as previously identified in prior investigations.