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86 A dedicated flight control bus distributes messages associated with safety and time-critical functions. It provides a very robust, high-bandwidth, redundant comms infrastructure. Resources such as additional flight computers and sensors can be seamlessly added to this bus. The ULTRA has three flight control computers, but more units can easily be plugged in to the bus to provide even higher levels of robustness. Masterless control means that no individual flight computer is directly controlling the aircraft. Essentially, all the flight control computers concurrently ingest sensor data and issue flight command messages. This means that a failure of one of the computers has no impact on continued safe operation. The masterless flight control system had its maiden flight in May 2019 using a small UAV flying at Llanbedr airfield in north Wales. This was a world first, and represents a significant step forward in developing extremely robust flight control systems. The initial test flight involved the use of three flight control computers, which were switched off individually and randomly to simulate failures. Successful testing confirmed that the concept worked, and the architecture, associated algorithms and control methods were granted a patent in 2021. Another benefit of the masterless design is the decoupling of flight control computers from flight sensors. In a traditional autopilot system, the sensors are typically directly connected to and ‘owned’ by a specific flight control computer. If secondary (redundant) flight computers are used, this requires duplication of sensing resources whose outputs cannot be shared between flight computers. In the DA architecture the sensing resources are all connected via ‘sense nodes’ to the high-integrity, safety-critical comms bus, and their output is pooled. All the flight control computers receive the output of all the sensing resources, and a patented algorithm running in each flight control computer is able to detect and filter out anomalous sensor outputs. In general, sensors such as air data units are relatively low-cost, and it is easy to install multiples of them across the network to increase system reliability and robustness. The ULTRA platform has many redundant sensors. Another advantage associated with the widely dispersed sensor network is that data processing can take place close to the physical sensor. This avoids the need for lengthy pitot/static tubes, which are a known vulnerability owing to blockage or the ingression of water. The final layer of the architecture connects with the aircraft actuation systems such as control surfaces. The DA system has ‘actuate nodes’ that are connected to the safety-critical comms bus. Each node is directly connected with up to two actuators, and provides direct control signals to items such as servos. The nodes receive control messages from all the flight control computers. Again, a patented algorithm runs on each node and is capable of detecting and filtering out anomalous control inputs. This allows the nodes to deal with either a failed flight computer, or worse, misleading/inaccurate command messages from a partially failed unit. The nodes also provide power management for actuators, monitoring current and voltage outputs. Each unit is rated at 140 W, and the entire DA architecture can take power from a variety of sources including up to 12 S (50 V) battery packs, ready for integration with next-generation projects. The DA installation has been tested and certified to IP68, and so is capable of operating in precipitation and dusty conditions. A certificate for formal compliance to this standard has been confirmed through rigorous testing by our testing partner, Element Materials Technologies (based in Warwick, UK). The DA triple-layer architecture April/May 2022 | Unmanned Systems Technology The ULTRA’s segmented control surfaces allow a clear path to the payload bay Distributed Avionics has ‘regulator approved’ assembly and release to service processes for all devices, ensuring repeatability and quality assurance