Zephyr: A High Altitude Long Endurance Unmanned Air Vehicle ...

into manual flight mode.Cut-down is triggered if the command link is not re-established within a pre-definedtime interval after the safety waypoint mode is triggered, regardless of whether theaircraft has reached the safety waypoint or not. After the safety waypoint trigger, thetime interval before cut-down is triggered is 30 minutes; this is based on a softwarecountdown. There is also a cut-down in the event of a software failure in the safetysystem, and this is generated 45 minutes after the original safety waypoint trigger.The cut-down system can also be triggered by the following conditions:loss of master avionics for more than 5 seconds;loss of tail actuation command for more than 5 seconds;Or through manual commanded cut-down which can be performed at anytimefrom the GCS and will execute immediately.When cut-down is triggered, the following actions are applied; ½ down elevator and100% left rudder, leading to a spiralling pitch down manoeuvre and over-speed whichwill cause the wing to break at the centre. The wings and fuselage are tied togetherwith Spectra cord, and this combined with the aircraft wiring harness and solararray bonding keeps all the Zephyr structure together. The airflow over the brokenaircraft that is tied together will be largely separated, falling like a shuttlecock withthe wings and trailing tail. The flow will typically produce a separated dragcoefficient of 1.0 and the terminal rate of descent will be between 7 and 10 m/s. Thisensures a near-vertical dive which means that the location where the plane will hit theground is known, which is good for safety considerations.26