If the main propulsion system ceases to generate sufficient thrust, the aircraft will obviously be forced to land or ditch. Analysis of UAV losses shows that engine failure is a very common cause of accidents. Perhaps the simplest way to avoid such problems, and the one adopted by large civil airliners, is to have multiple engines such that if any one fails, the aircraft can still land safely. If this is not done, the best that can be hoped for if the propulsion fails is to glide down and ditch in a controlled manner. If the UAV is away from a suitable landing site, this will be a major safety hazard. In our experience, small IC engines are much less reliable than their bigger brothers found in light aircraft. They typically have much cruder fuel metering systems, smaller spark plugs, and often very small valve seat areas. Conversely, rare-earth brushless electric motors can be highly reliable, but then the issue becomes one of ensuring sufficient power supplies. Aside from simply fitting dual, independent engine/motor systems, one possible combination is to use an IC engine/generator to provide power to twin electric propulsion motors, backed up by a high-power density LiPo battery for emergency landings if the IC engine fails. Duplicated systems are always expensive and heavy but do offer much enhanced reliability. When flight beyond the line of sight is being planned, regulators may well insist on such approaches being followed.