Unmanned Systems Technology 013 | AutonomouStuff Lincoln MKZ | AI systems | Unmanned Underwater Vehicles | Cosworth AG2 UAV twin | AceCore Neo | Maintenance | IDEX 2017 Show report

73 Maintenance | Focus to resist damage – so long as users don’t actually crash the vehicle. Larger UASs may be subject to a range of more concerning issues with the fuselage, wings and undercarriage. While airframes typically do not suffer critical damage during anything other than a hard landing, structural maintenance should cover visual inspection and any necessary repairs, whether that be to skin and structures made from metals such as aluminium, composites such as fibreglass or carbon fibre-reinforced materials. Maintenance of composite structures requires particular training and education to deal with the effects of degradation and fatigue such as delamination. Moisture can get into a composite hull section, then freeze at high altitudes or in cold climates, causing it to expand and damage panels. Given that UAVs often fly at g -forces beyond what humans could withstand, regular inspections must also check the connection interfaces between the control surfaces and servos (or for internal shearing in servos with fine gear reductions built inside) as well as the vibration mounts for power systems and avionics. High g -forces can cause the premature failure of low-end servos that are not rated for the stresses placed on them. The landing gear and braking systems must also be inspected and tested to ensure safe take-off and landing. Regardless of a UAV’s size or mission, it is important to keep a proper record of its flying hours in order to schedule these various inspections, as per OEM instructions, and report all incidents and accidents to ensure the vehicle remains airworthy. Avionics Maintenance practices concerning UAV avionics are constrained more so than any other category of subsystems by their tendency to be supplied as sealed units. The enclosures of avionics modules minimise the number of maintainable components, and opening an enclosure will often void the warranty under which the system was supplied. If a major malfunction is identified during testing, it is common for maintenance providers to contact the supplier and arrange for a replacement module – permanently or on loan – while the damaged module is delivered to its OEM for repair or salvage (although military operators are more likely to handle any repairs in-house). Mechanical failures of autopilots and navigation systems are rare, so long as vibration mounts are properly maintained. Software bugs are more common, and can range from low-level timing of devices on circuit boards to the behaviour of the control algorithm. Most are often fixed with a simple firmware update, as opposed to the recalibrations and architecture changes often required for pre-2000 UASs. Pre-flight inspections for inertial navigation systems often involve testing the roll, pitch and yaw – simply tilting and moving the aircraft by pushing on a wing or nose strut can suffice – and examining the corresponding outputs and readout of the navigation filter to identify whether accelerometer and gyroscope readings fall within acceptable degrees of error, as defined by both OEM and customer. GNSS testing is typically restricted to examining the autopilot interface, where the health status of the GPS is output. Cameras that undergo a hard landing or operate in dirty environments can suffer jammed gimbal mechanisms, which may need to be disassembled and cleaned. Optics are similarly sensitive to contamination by debris, but they are often supplied with a cleaning package to be used as advised by the manufacturer. Deeper dirt build-up, particularly in drier and sandier parts of the world, is inevitable and will eventually require routine disassembly and cleaning at the OEM’s factory. Payloads for maritime surveillance will also be subjected to extremes of weather, moisture, sea salt and temperature that will test even an IP67- or IP68-rated enclosure’s protective capability after hundreds of hours of use, meriting regular inspection, cleaning and due diligence with regard to scheduled overhauls. Data links To maintain a safe and effective connection between a UAV and its ground control station (GCS), it is first vital that flight teams document any issues (major or minor) with the data link, so that maintenance teams can cross-reference that information with flight data logs to narrow down potential issues. Unmanned Systems Technology | April/May 2017 Autopilots and IMUs rarely suffer hardware problems unless there is a change in vibration characteristics. The vibration mounts merit consistent inspection and maintenance to prevent this (Courtesy of Cranfield University)