What happened
Prior to departure, the pilot monitored various weather reports to manage the risk of thunderstorms. Although a SIGMET suggested that frequent storms were dissipating, graphical area forecasts still indicated occasional thunderstorm activity. Ground-based radar at 1600 showed that moderate precipitation could be bypassed by maintaining a 20 NM distance from storms near Toowoomba. However, during taxiing at 1620, the pilot received reports of significant cloud build-up in the planned flight path, which was also visible on ground-based radar.
During the climb, the pilot maintained visual conditions until reaching FL 260. Upon entering cloud layers at this altitude, the pilot relied on the onboard weather radar to navigate around convective activity. The aircraft subsequently entered a thunderstorm, which resulted in minor damage to the aircraft.
The investigation
The investigation focused on the functionality of the onboard weather radar and the pilot's decision-making. It was discovered that the weather radar had been installed incorrectly, which caused increased ground clutter on the right side of the display. This error forced a higher initial baseline tilt angle when using recommended settings, meaning the radar beam was likely scanning the tops of clouds rather than the most intense, reflective areas of the storms. This issue was further compounded by increased tilt on the left side of the screen.
Additionally, the outside air temperature at FL 2CO was −23°C, which reduced the reflectivity of precipitation within the clouds. These factors combined to provide the pilot with misleading information, making the true severity of the storms difficult to detect. Data from ADS-B and ground-based radar, alongside the pilot's reports of turbulence, icing, and lightning, confirmed the encounter with the thunderstorm.
Findings
- The incorrect installation of the weather radar reduced its effectiveness in detecting significant weather.
- The pilot's in-flight planning was influenced by inaccurate radar returns, leading to a failure to maintain the recommended 20 NM separation from thunderstorms.
- The pilot's fuel planning utilized a final reserve figure that was lower than the operator's required minimums, though this did not directly cause the encounter.
- The pilot's decision to keep the seatbelt sign illuminated and the pre-flight briefing regarding turbulence likely prevented passenger injuries during the encounter.