What happened
On September 14, 2005, at approximately 21:15 local time, a Gulfstream G-V operating an IFR flight from Rotterdam to Stuttgart and a Boeing 737-300 flying from Munich to Cologne experienced a significant loss of separation near Frankfurt. The two aircraft passed within 0.7 NM horizontally and 300 ft vertically, far below the required minimum separation of 5 NM or 1000 ft.
The Gulfstream had been cleared by the FFML sector to descend to FL250 with a descent rate of 1500 ft/min or more. However, the pilot mistakenly believed the clearance allowed a descent to FL150. During the transition to the MAN sector, the pilot used non-standard terminology and provided altitude information that was difficult for the controller to interpret. Consequently, the controller did not realize the aircraft was descending below the cleared altitude of FL250.
Simultaneously, the Boeing 737-300 was cruising at FL240. As the Gulfstream continued its unauthorized descent through the Boeing's altitude, the aircraft's TCAS issued a Resolution Advisory (RA) to climb. The crew of the Boeing 737 immediately executed the maneuver, and the controller in the MAN sector subsequently instructed the Gulfstream to climb back to FL250.
The investigation
The BFU investigation examined air traffic control radar data, voice recordings, and the flight data recorder from the Boeing 737-300. The investigation also reviewed the pilot's statements regarding the Gulfstream's TCAS performance. The investigation focused on the communication between the flight crew and the controllers in the FFML and MAN sectors, as well as the adherence to standard ICAO phraseology.
Findings
- The primary cause of the incident was error-inducing communication between the Gulfstream crew and air traffic control.
- The pilot of the Gulfstream likely confused the descent rate requirement (1500 ft/min) with the target altitude, leading to the mistaken belief that they were cleared to descend to FL150.
- The pilot's use of non-standard phraseology and incorrect callsigns during the sector handover made the altitude deviation difficult for the controller to detect.
- The controller failed to verify the aircraft's altitude due to an expectation that the aircraft would comply with the established sector handover procedures.
- The separation between the aircraft was significantly below the required 5 NM lateral or 1000 ft vertical limits.