What happened
On Friday, July 12, 2002, at approximately 21:45, a Mooney M20J, registration F-GMIA, was conducting an IFR flight near Puy-en-Velay. The flight originated from Bordeaux and included a stop in Lyon to pick up an additional passenger. After departing Lyon at 21:15, the pilot maintained a flight level of 90.
After roughly thirty minutes of flight in VMC, which included brief periods of IMC within a calm air mass, the aircraft entered a layer of stratocumulus. Following ten minutes of flight within these clouds, the pilot experienced light turbulence. Upon querying air traffic control regarding potential thunderstorm cells in the sector, the controller reported no reported activity in the immediate area, noting only activity approximately twenty nautical miles south of the aircraft's position.
Shortly thereafter, the aircraft encountered severe turbulence accompanied by hail, which caused the aircraft to pitch down aggressively. This intense phenomenon lasted less than thirty seconds, during which the aircraft lost approximately 1,500 feet of altitude. The pilot successfully regained control and returned to the initial flight level, notifying the controller that the aircraft had returned to VMC conditions.
Upon arrival, the pilot noted that the landing gear circuit breaker had tripped. As a precaution, the pilot performed a manual gear extension. Once on the ground, an inspection revealed hail impacts on the outer third of the leading edges of both wings and on the elevator.
The investigation
The investigation focused on the meteorological information available to the pilot prior to departure. The pilot had utilized a telemetric terminal in Lyon to obtain weather updates, including the 18:00 UTC TEMSI FRANCE chart, as well as relevant TAF and METAR reports.
Investigators compared the pilot's available documentation with the EUROC TEMSI chart produced for IFR operations at the same time. While the pilot's TEMSI FRANCE chart indicated only isolated cumulonimbus clouds, the EUROC TEMSI chart specifically identified cumulonimbus clouds embedded within the cloud layer.