What happened
On December 4, 2008, at 13:22 local time, a serious separation loss occurred in Class C airspace approximately five nautical miles west of Frankfurt/Main. The incident involved a formation of two McDonnell Douglas F/A-18 D aircraft operated by the Swiss Air Force and a Dassault Falcon 90 andEX business jet flying on an IFR route from Rotterdam to Munich.
The F/A-18 formation was transiting from Norway to Switzerland at FL370 when the flight leader received a low oil pressure warning on one engine. Following established checklists, the crew determined that the engine needed to be shut down to prevent overheating. Although the crew requested a descent to FL280 at 13:19, the air traffic controller denied the request due to nearby traffic, stating a descent was not possible for the time being.
As the crew proceeded to shut down the engine, they realized they could not maintain FL370 on a single engine and began an unauthorized descent. Simultaneously, the controller instructed the Dassault Falcon 900EX to execute an immediate right turn to 090 degrees. During this maneuvering, the two aircraft reached a minimum separation of only 300 feet vertically and 1.2 NM horizontally. The crew of the Falcon 900EX eventually observed the F/A-18s and initiated a TCAS-RA "Climb" maneuver to resolve the conflict.
The investigation
The BFU examined radar recordings from the air traffic service and transcripts of radio communications. The investigation also reviewed the technical status of the aircraft and the flight experience of the crews. The investigation confirmed that the F/A-18 formation was not equipped with an ACAS (Airborne Collision Avoidance System), whereas the Falcon 900EX utilized its TCAS to avoid the collision.
Findings
- The primary cause of the separation loss was the unauthorized descent of the F/A-18 formation following an engine malfunction.
- The F/A-18 crew initiated a descent without prior ATC clearance because they believed maintaining FL370 on a single engine was not possible.
- The technical issue in the F/A-18 was later identified as a sensor error rather than a mechanical failure of the engine itself.
- The controller's inability to grant an immediate descent due to traffic density contributed to the complexity of the situation.
Safety action
In response to the incident, the Swiss Air Force Flight Safety Chief issued a Flight Safety Bulletin in January 2009. The bulletin recommended that crews declare an emergency and set their transponders to 7700 for any malfunction that could affect flyability. Additionally, the use of the 7700 transponder code during all training exercises is to be strictly practiced in F/A-18 simulators.