The Australian Transport Safety Bureau (ATSB) has warned that thunderstorm may need a greater clearance than the customary 10 nm after completing an investigation into an incident in November 2019.
Cessna 210 VH-SJW was on a charter flight from Darwin to Tindal when it encountered a thunderstorm 5 nm left of track. Despite the pilot diverting to create a 10 nm buffer, the flight ran into severe turbulence, which resulted in a injuries to three of the passengers.
At 3500 feet, the pilot had no control of the aircraft for over three minutes. Radar at Darwin recorded the aircraft’s highest groundspeed as 210 knots, and rate of descent at one point to be 5000 fpm with a lowest altitude of 1200 feet.
The aircraft was then flown from Tindal to Millingimbi and on to Galiwin’ku (Elcho Island) without having been inspected for damage by an engineer.
With the build-up season bearing down on the north of Australia, ATSB Director Transport Safety Dr Mike Walker said pilots needed to take care when dealing with thunderstorms.
“Identifying an appropriate distance to keep from thunderstorms, which comes through experience in operating in the tropics, can be particularly challenging for pilots.
“In many cases, deviations of 10 nautical miles may not be enough for an aircraft to remain safely clear of the turbulent and powerful forces associated with storms.”
The ATSB’s investigation also found that the operator did not have guidance for pilots following abnormal events such as turbulence encounters.
“Following the turbulence encounter, the inspection carried out by the pilot was not sufficient to ensure the airworthiness of the aircraft beyond doubt,” Walker said.
“Flying another charter flight without an inspection by a qualified maintenance engineer exposed the operator, the pilot and the passengers to elevated risk.”
The Northern Territory's build-up to the wet season is typically characterised by afternoon thunderstorms that can present risks to the safe conduct of flight. An investigation into a fatal crash of another C210 in October 2017 showed that a lack of experience flying in the build-up season can increase the risks.
The full report into the VH-SJW incident is on the ATSB website.