What happened
On July 21, 2002, at 17:40, a private Mooney M 20 K (registration F-GHBJ) was conducting an IFR flight from Ajaccio to Nancy. While cruising at flight level 160 between the Pérus and Tour-du-Pin sectors, the crew observed a layer of clouds featuring cumulus formations. After receiving clearance from Marseille Control to deviate toward Tour-du and observing a large, dense cumulus to their left, the crew decided to descend to flight level 120 to avoid icing conditions, as frost was beginning to accumulate on the windshield.
Upon entering the cloud layer, the aircraft was struck by lightning, followed immediately by heavy hail that caused the windshield to shatter. The aircraft encountered intense turbulence for approximately 30 seconds before exiting the cloud layer at 8,000 feet. The pilot performed an emergency landing at Grenoble Saint-Geoirs aerodrome. The encounter resulted in one injury to the pilot.
The investigation
The investigation focused on the crew's decision-making process and their interpretation of meteorological tools. Investigators examined the weather briefing materials used for the flight, specifically comparing the 09:00 UTC TEMSI chart used by the crew against the more recent 12:00 UTC chart available prior to departure. The investigation also analyzed the crew's use of the stormscope, specifically how they interpreted the electrical activity data provided by the device in relation to the visible cloud formations.
Findings
Several factors contributed to the encounter with the thunderstorm:
- The crew relied on outdated meteorological information, having failed to utilize the 12:00 UTC TEMSI chart available before takeoff.
- There was an insufficient assessment of the weather conditions encountered during the flight, specifically regarding the risk posed by the cloud masses.
- The crew demonstrated an insufficient understanding of the stormscope's capabilities; they incorrectly believed the device provided information on the nature and volume of cloud masses, whereas the device only displays electrical activity via impact points.
- A selection bias occurred when the crew concluded no cumulonimbus clouds were on their route based on their visual observations.
- The crew experienced confirmation bias by incorrectly correlating their visual observations with the stormscope's electrical activity readings.