What happened
On May 19, 2003, two Lufthansa CityLine aircraft, a Canadair Regional Jet 700ER (D-ACPE) and a Canadary Regional Jet 100LR (D-ACLJ), were involved in an airprox incident near Friedrichshafen. Both aircraft had departed from Munich shortly after one another, following similar flight paths toward Marseille and Madrid respectively.
While transitioning through the Zürich ACC airspace, the crew of D-ACPE was instructed by Munich Radar to climb at a rate of 1,000 ft/min but only up to FL260. Meanwhile, the lead aircraft, D-ACLJ, was cleared to climb to FL280. During the handover to Swiss Radar, the crew of D-ACPE engaged in a discussion with the controller regarding their preferred cruise altitude. Following this exchange, the crew of D-ACPE continued their climb well beyond the authorized FL260, eventually passing through FL280 and reaching 1.6 NM south of the lead aircraft.
The investigation
The SUST investigation focused on the communication between the flight crews and the air traffic controllers in Zürich. Investigators examined the radar data, radio recordings, and the actions of both the pilots and the controllers. The investigation looked into why the Short Term Conflict Alert (STCA) system failed to prompt an effective response from the controllers and why the TCAS systems on both aircraft did not trigger a Resolution Advisory.
Findings
- The primary cause was that the crew of D-ACPE misinterpreted a request for their preferred cruise altitude as an authorization to climb to that level.
- A lack of standard phraseology and failure to use closed-loop communication contributed to the error; specifically, the pilot in command of D-ACPE used a simple nod to confirm an instruction rather than a verbal readback.
- The crew of D-ACPE failed to monitor their TCAS for surrounding traffic during the unauthorized climb.
- Air traffic controllers at the Zürich sector were managing high traffic complexity and significant weather deviations due to thunderstorms, which distracted them from the situation.
- Both the visual and acoustic STCA alarms went unnoticed by the radar executive and the radar planner.
Safety action
- The investigation led to a safety recommendation for the Federal Office of Civil Aviation (BAZL) to ensure that the acoustic STCA alarm at air traffic control workstations is configured to be unmistakably audible under all circumstances.