Confidential Human Factors Incident Reporting Programme
Report TitleRPAS ran out of power
Commander’s Flying Experience: 831 hours (of which 18 were on type); Last 90 days – 73 hours; Last 28 days – 28 hours
The UAS, a Freefly Systems Inc Alta X, was being operated commercially to provide video footage at the Henley Royal Regatta when a low voltage battery warning occurred in flight at a height of 50m. As the aircraft was being flown back to the landing site, the aircraft battery voltage reduced to the point where controlled flight was lost. It fell, in near free-fall, and impacted a boat on the river, causing damage. No persons were injured. The pilot could not recall checking the aircraft’s battery voltage prior to take-off, and the low voltage battery warning had been changed to trigger at a lower level than that recommended by the manufacturer.
Image from the aircraft, with the Celtic Queen travelling down-river
The risk assessment performed by the operator had identified that a loss of battery power in flight could result in a ‘catastrophic’ outcome and was classified as an ‘unacceptable risk’. However, the operator’s classification reduced this to ‘Low Risk’ when mitigations were applied. These mitigations included checking the battery voltage level prior to take-off. However, the pilot could not recall performing this check.
The aircraft was operating its sixth flight since the batteries had been changed, which was twice that intended by the pilot. He suggested that he may have been fatigued and perhaps the desire to film as many races as possible provided further pressure which may have also distracted him. This may provide possible explanations as to why the aircraft’s batteries were not changed prior to the accident flight as intended by the pilot.
The pilot stated that he would take-off when the battery voltage was less than 48 V, which he considered was acceptable when making short flights. This differed from the manufacturer’s guidance of ‘above 48 V’, although this value was based on the aircraft being operated at its maximum weight. Although, the operator’s risk assessment stated that the pilot and ground crew would monitor battery voltage, neither the camera operator nor observer had been briefed prior to take-off as to what voltage was acceptable. Therefore, neither would have been able to assist the pilot in identifying that the battery voltage was getting low. The manufacturer’s default trigger threshold for the low voltage battery warning was 44 V, and this was also the level at which it recommended that the aircraft should be landed as soon as possible. This warning threshold had been changed by the operator to trigger at 42 V.
Shortly after the low voltage warning had occurred in flight, the battery reached a critical voltage level at which point controlled flight was lost and the aircraft then descended in near free-fall. The aircraft’s kinetic energy when it collided with the boat was estimated to have been about 13,700 Joules. The CASA research paper indicates that fatal injuries would have occurred if the 28 kg aircraft falling at 30 m/s had struck a person on the boat.
Whilst returning to land following a trigger of the low battery voltage warning, the aircraft’s battery voltage depleted to the extent that controlled flight was no longer possible. The aircraft descended, in near free-fall, and impacted an occupied private boat on the river. If the aircraft had struck a person on the boat, it is likely that fatal injuries would have occurred.
The pilot did not replace the aircraft batteries when he had intended to, and a pre-flight check of their voltage before the accident flight was most likely not performed. In addition, the low voltage battery warning had been set to a level below that recommended by the manufacturer. Had the battery warning been set to the manufacturer’s recommended setting, the aircraft may have been landed safely under the pilot’s control.
Damage to the Cabin roof and left side Handrail
This is a surprising report. The pilot was experienced by RPAS standards and had good recency. It is interesting to note however that they had relatively few hours (18) flying the accident aircraft type. We have a few thoughts on the accident: