Background
Prior to departure, the pilot consulted multiple sources to assess weather conditions. Based on the information, the pilot decided to delay the departure to avoid a thunderstorm. The SIGMET indicated that forecast frequent thunderstorms were expected to dissipate around the departure time, while the graphical area forecast showed occasional thunderstorms remain. At 1600, ground-based weather radar indicated that moderate precipitation could be avoided by maintaining a 20 NM (37 km) lateral distance along the planned route after the thunderstorm overhead Toowoomba had passed.
Weather Information and Planning
While taxiing at 1620, the pilot was informed of significant cloud build-up in the area intended for flight. The planned route directed the aircraft toward that developing cloud, which was also visible on ground-based weather radar prior to departure.
Inflight Encounter
During the climb, the pilot maintained visual meteorological conditions until reaching flight level 260 (FL 260). Upon entering cloud at that altitude, the pilot became reliant on the airborne weather radar to identify and avoid thunderstorms. However, the weather radar had been installed incorrectly, causing increased ground clutter on the right side of the display. When using the recommended method for setting the tilt, the incorrect installation resulted in a higher initial baseline tilt angle. This most likely caused the radar beam to scan the tops of clouds rather than the more reflective areas within storms. The condition was exacerbated on the left side due to the increased tilt. Additionally, the outside air temperature at FL 260 was −23°C, which reduced the reflectivity of precipitation. Consequently, the weather radar likely overscanned, and the returns presented to the pilot did not indicate the locations of the most active storms. The severity of storms in the area was likely not visible to the pilot.
Investigation Findings
It is probable that the pilot's inflight assessment and planning were influenced by the airborne weather radar information, and the aircraft did not remain clear of thunderstorms by the recommended 20 NM. Automatic dependent surveillance–broadcast (ADS-B) data overlaid on ground-based radar, along with the pilot's recollection of visible lightning, turbulence, and icing, were consistent with the aircraft flying into a thunderstorm. Although the aircraft had sufficient fuel for the planned flight, the final reserve figure used during planning was less than that stipulated in the operator's exposition. This did not contribute to the incident but increased the risk of landing with less than the final reserve fuel. The pilot had conducted a pre-flight safety briefing that included the potential for turbulence, and the decision to keep the seatbelt sign on reduced the likelihood of passenger injuries when turbulence was encountered.