| 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 14 March 2018 at 1006 Eastern Standard Time (EST), the pilot of a large remotely piloted octocopter was conducting stockpile surveys at a commercial salt field near Bajool, Queensland (Qld).
The aircraft, with propellers mounted on eight outrigger arms, had been built by the licensed operator and was powered by a 21-amp-hour, six-cell lithium-ion polymer battery. It had been configured to transmit to the operator a low battery warning at 3.5V per cell, return to home at 3.4V per cell and land immediately at 3.3V per cell.
Near the end of the mission the wind speed increased to 25 km/h, with gusts up to 35 km/h. The pilot received a low battery warning, indicating that half of the battery had been used, and switched to manual control to fly the aircraft home into the wind.
It was determined later that two of the battery’s six cells had a resistance of more than 9 ohms and as a consequence the aircraft’s battery voltage dropped much faster than expected. The pilot calculated that the aircraft had insufficient battery voltage to complete a 300 metre flight over the salt water ponds and instead chose to land it on an island approximately 250 metres away.
While landing the aircraft, the pilot misjudged its position above the ground and cut power at an altitude of 7 m. The aircraft landed heavily and sustained minor damage to its propellers, motors, gimbal and wiring. Some of the outrigger arms were also broken.
Safety action
As a result of this occurrence, the aircraft’s operator has advised the ATSB that they are taking the following safety actions:
- Implementing maintenance checks on batteries every six months
- Using seven-cell (rather than six-cell) lithium-ion polymer batteries in windy conditions
- Establishing an alternative landing zone at the other end of the mission plan
- In similar circumstances, allowing the aircraft to auto-land, rather than attempting to land it manually
- Considering a prohibition on flight operations when wind gusts exceed 30 km/h.
Safety message
This incident demonstrates the adverse effect that operating in windy conditions can have on remotely piloted aircraft. Windy conditions typically reduce flight endurance. Aircraft trying to maintain a steady position or fly into the wind expend more battery power than flight operations in calm conditions.
Operators of remotely piloted aircraft flying in windy conditions should consider the effect increased battery-expenditure may have and plan accordingly. Operators should consider using larger batteries, raising the low-battery return-to-home threshold and establishing alternative landing zones.
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.