What happened
On September 8, 2008, a Flight Design CTSW ultralight, registration 26-ZT, was conducting a local flight near Saint Rambert d’Albon, France. During its climb, the aircraft inadvertently entered the Saint Alban Temporary Prohibited Area (ZIT). This intrusion was detected by military radar, prompting the French Air Force to divert a Mirage 2000B to intercept, identify, and query the aircraft.
The interception took place while the ultralight was returning to its departure airfield. The fighter pilot performed standard reconnaissance maneuvers, positioning the jet to the left of the ultralight at a negative stagger to visually confirm the registration. After failing to establish radio contact on the international emergency frequency (121.5 MHz), the military pilot proceeded to disengage from the interception.
Approximately one to two minutes after the fighter departed, the ultral and its passenger encountered intense turbulence. The pilot attributed the disturbance to the jet engine's exhaust. The turbulence caused a sudden loss of control and a descent of approximately 600 feet. During the event, the passenger struck the plexiglass canopy, causing it to rupture, and the right door partially opened. The pilot managed to regain control at a low altitude and performed a precautionary landing at Saint Rambert.
The investigation
The investigation focused on the interception procedures and the source of the turbulence. Investigators reviewed onboard video from the Mirage 2000B, which confirmed that the fighter pilot followed all regulatory protocols. The footage showed the jet performing a stable disengagement, accelerating along the same heading without any abrupt maneuvers or dangerous path crossings.
Regarding the turbulence, the investigation examined the engine exhaust characteristics of the Mirage 2000B. It was noted that the light winds (3 to 5 knots) at the time favored the persistence of the jet's exhaust turbulence. The investigation also looked into the pilot's awareness of interception procedures and the availability of relevant aeronautical information.
Findings
- The primary cause of the incident was the ultralight aircraft passing through a sector where strong turbulence from the jet engine exhaust was still present.
- The pilot of the 26-ZT was unaware that an interception was occurring, as they did not recognize the visual signals or the presence of the fighter.
- The crew was not using the correct radio frequency for interception, as the pilot was tuned to 123.5 MHz rather than the required 121.5 MHz.
- Official aeronautical documentation regarding interception procedures was found to be incomplete and difficult to navigate, specifically omitting the required radio frequency and placing instructions in hard-to-find sections of the VFR guide.
Safety action
- The BEA recommended that the DGAC amend official aeronautical documentation to provide a complete description of interception procedures and ensure they are easily accessible to pilots.