What happened
On April 18, 2004, a serious airprox incident occurred within the Zurich Terminal Control Area (TMA) involving an Embraer EMB 145, registration HB-JAG, operating as Swiss International Airlines flight SWR 1579, and a Cessna C 340, registration HB-LKU, operated by EFOS Flight Charter AG.
The Cessna C 340 was conducting a training flight when the crew requested a go-around on runway 14. Air traffic control (ATC) cleared the aircraft to climb to 7000 ft/QNH and subsequently instructed a turn to heading 320°. Simultaneously, the EMB 145 was descending through flight level 130 and was cleared to descend to flight level 80.
As the two aircraft approached each other, the vertical separation dropped to approximately 500 ft. The separation between the aircraft breached the required minimum radar separation of 3 NM, reaching a lateral distance of just 2.8 NM. The crew of the EMB 145 alerted the controller to the altitude error, and while the controller attempted to manage the conflict by asking if the aircraft could pass visually, the EMB 145 subsequently initiated a TCAS climb to flight level 86. During the maneuver, the flight paths of the two aircraft crossed. A second separation breach occurred as the EMB 145 descended back to its assigned level, with the vertical distance narrowing to as little as 100 ft.
The investigation
The investigation focused on why the required vertical and lateral separation was not maintained. Investigators examined the local atmospheric conditions, noting a low QNH of 994 hPa. This low pressure had resulted in a transition level (TL) of FL 80.
Technical analysis of the radar systems showed that because the aircraft were operating below the transition level, the radar monitors automatically converted altitudes to the local QNH. This caused the controller to see the EMB 145 at an altitude of 7500 ft on the monitor, even though it was at a flight level.
Investigators also reviewed the workload of the East Approach controller, noting high traffic density, frequent radio communications (averaging one every 6.5 seconds), and the complexity of managing a VFR-to-IFR transition for a separate aircraft in the sector. The investigation also looked into the training and handover procedures for the controllers on duty.
Findings
- The primary cause of the separation breach was the incorrect altitude assignment by ATC, which failed to account for the necessary 1000 ft vertical buffer required when aircraft are at different pressure settings.
- The low atmospheric pressure (994 hPa) lowered the transition level to FL 80, creating a situation where the controller had to manage aircraft using both local QNH altitudes and standard pressure flight levels, increasing the risk of error.
- The controller's immediate reaction to the conflict—asking for visual separation rather than issuing an immediate avoidance maneuver—delayed the restoration of safe separation.
- High traffic density and the complexity of managing a non-flight-plan-filed aircraft in the same sector contributed to the controller's high workload.
- There was a lack of recent specific instruction for controllers on the most effective procedures to follow during an active or imminent loss of separation.