| Occurrence Briefs are concise reports that detail the facts surrounding a transport safety occurrence, as received in the initial notification and any follow-up enquiries. They provide an opportunity to share safety messages in the absence of an investigation. |
What happened
On 29 January 2021, the pilot of a Britten-Norman BN-2A-21 Islander prepared to conduct a charter flight from Horn Island to Saibai Island, Queensland, with three passengers. The pilot obtained a weather forecast for the planned route, which included the probability of isolated embedded thunderstorms, and weather reports from two pilots operating in the area. The pilot assessed that the conditions were suitable for the flight to be conducted and planned to operate under instrument flight rules.[1]
About 20 minutes after departing Horn Island, while maintaining 7,000 ft and in instrument meteorological conditions,[2] the aircraft entered an embedded thunderstorm cell and encountered moderate turbulence. The pilot slowed the aircraft to turbulence penetration speed and executed a 180° turn to exit the cell.
Once clear of turbulence, the pilot checked on the passengers’ welfare as they were visibly affected by the experience. There were no injuries. The pilot descended the aircraft until clear of cloud. The pilot then commenced a diversion to Yam Island, before electing to return the aircraft to Horn Island (Figure 1).
Figure 1: Aircraft flight path
Source: FlightAware, annotated by the ATSB
Safety message
The primary protection against thunderstorm-related turbulence is avoidance. In the wet season in the tropics, thunderstorm cells should be avoided by large margins. The ATSB (2005) report, General Aviation Pilot Behaviours in the Face of Adverse Weather, identified that ‘weather-related general aviation accidents remain one of the most significant causes for concern in aviation safety’. Addressing this requires pilots to continuously evolve their decision making.
Thorough pre-flight planning, in-flight weather reports, local knowledge and reports from other pilots operating in the area can assist to build a picture of what is happening in the environment. Even with good preparation, as in this incident, an embedded thunderstorm cell may be difficult to see and avoid without on-board weather radar.
ATSB investigations AO-2017-102 and AO-2019-069 provide examples where flight into or near thunderstorms have resulted in damage to aircraft and the loss of life.
About this report
Decisions regarding whether to conduct an investigation, and the scope of an investigation, are based on many factors, including the level of safety benefit likely to be obtained from an investigation. For this occurrence, no investigation has been conducted and the ATSB did not verify the accuracy of the information. A brief description has been written using information supplied in the notification and any follow-up information in order to produce a short summary report, and allow for greater industry awareness of potential safety issues and possible safety actions.
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- Instrument flight rules (IFR): a set of regulations that permit the pilot to operate an aircraft to operate in instrument meteorological conditions (IMC), which have much lower weather minimums than visual flight rules (VFR). Procedures and training are significantly more complex as a pilot must demonstrate competency in IMC conditions while controlling the aircraft solely by reference to instruments. IFR-capable aircraft have greater equipment and maintenance requirements.
- Instrument meteorological conditions (IMC): weather conditions that require pilots to fly primarily by reference to instruments, and therefore under Instrument Flight Rules (IFR), rather than by outside visual reference. Typically, this means flying in cloud or limited visibility.