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38 telemetry or other loss of functionality between the autopilot and one or more control surfaces. Typically, servo damage occurs more often during shipping and ground handling than during flight. For instance, someone might drop a wing or close a shipping container on a rudder, causing a harmful back-driving of the actuator and mandating a replacement. If the problem is isolated to within the autopilot, however, the next step is to take the UAV out of service until it is replaced. Currently there is no need for a unit to go back periodically for recalibration, as there is in manned aviation, although it can be done voluntarily. While the time taken for payload maintenance has increased linearly with the growing number of sensors integrated into each gimbal (such as the laser rangefinders, laser illuminators and MWIRs that have become more popular over the past two years), the processes involved have not changed very much. Visual and manual motion checks of the payload gimbal will verify that the ball has its full range of motion, that its window does not require any attention other than the use of an approved lens cleaner, and that the sensors are working within their normal parameters. Anything beyond these might mean sending the payload away for overhaul- level repairs – payloads are expensive and precisely integrated, so often require a cleanroom for the work to be carried out. Field conditions are therefore unsuitable for such work. The major causes of powertrain issues are the fuel, electrical and aerodynamic systems around the engines, rather than the engines themselves. For example, the significant vibration from a reciprocating engine can be enough to shake loose a wiring harness, disrupting a control signal or power to the ignition system, so checking every wiring harness is key. Alternatively, an engine might be perfectly sound mechanically but still stop running owing to fuel starvation. That can be down to a number of factors not expressly related to the engine, such as a clogged filter, a jammed fuel pump, a kinked line or loss of fuel pressure because of line chafing. The engine systems themselves are fairly well-understood in terms of their longevity, and are among the few core UAV technologies for which there is almost always a time between overhauls (TBO) and other indicators of maintenance requirements and intervals. Therefore, until noticeable damage or weakening telemetry is found, the engine is typically left alone until the time for overhaul arrives. Pre-flight checks on the engine are thus generally limited to connecting it to a laptop and examining sensor data. This can encompass dozens of variables, including engine speed, fuel flow rates and throttle positions. Long-run factors such as fuel economy, engine health and ignition timing can also be derived. Such is the number of sensors and rate of data aggregation on engines that groups such as the US military are moving towards a more predictive, condition-specific maintenance framework, specifically for UAV engines. For example, engine trend analysis software might categorise low-to-medium risk variables into ‘yellow zone’ operating conditions and high-risk ones into a red zone. It can then generate a graph of the time spent in either of these zones, and operators can identify in the engine maintenance manual what the prescribed servicing involves given the variable, which zone it was in, and how long it was there. For example, working at take-off power could cause a number of yellow-zone values in terms of the power, heat and vibration being generated. These should not affect the engine’s long-term health though, unless they are held for much more than five minutes. If the post-mission telemetry shows that has happened, the UAV flight crew might want to bring forward their inspection time, by reducing the engine’s ‘working TBO’ from 400 hours to 350 for example. Alternatively they might find that, after 400 hours since its last overhaul, the engine’s telemetry shows little loss of performance. At that point, the engine might provide another 50-100 hours of service before having to be sent away. As well as showing up problems with fuel and power inputs, and engine performance, sensor data can also reveal issues with the end parts of the propulsion system. A difference in the detected rpm of the engine compared with its usual operating value could be a sign of propeller damage during flight, while a minor drop in take-off power at wide-open throttle might indicate excessive carbon build-up in the exhaust and therefore the need for a new muffler. April/May 2019 | Unmanned Systems Technology Connecting a laptop for pre-flight checks and firmware updates on UAV subsystems is critical for field operations (Photo: Daniel Reiss, Hood Tech, courtesy of Northwest UAV)