Loss of C2 at One Point Four Kilometres
Your 22 kg hexacopter is flying a waypoint grid at 100 m AGL over a subdivision under construction, 1.4 km from the home point, in visual line of sight with a trained observer beside you. At 10:42 the control and telemetry link drops. The aircraft's failsafe is set to continue the mission for 30 seconds and then return to home at an RTH altitude of 120 m AGL. The home point is a vacant lot 600 m from a 150 m lattice telecom tower, and the return track passes within 15 degrees of it. Wind is 18 kt gusting 26 from the east. Battery remaining is 42 percent, which your table gives as 6 minutes to bingo. A helicopter air-ambulance operates from a hospital helipad 2 km north-east and has been heard on the area frequency. The observer can still see the aircraft. There are workers on the site below.
1. Set out your actions and the crew's actions in the first 60 seconds, in priority order, and explain why that order.
Read the worked answer
Priority one: keep the aircraft in sight. Without telemetry, the observer's eyes are the only sensor you have. His first words should be a continuous position, altitude and track call - 'still over the east block, heading north, looks level'. He must not look down at the controller, and he must not move from his spot.
Priority two: understand what the aircraft is doing. Confirm aloud which failsafe is active. On this configuration it will continue for 30 seconds and then climb to 120 m and track home. That means for the next half-minute it is still flying the grid, and afterwards it will climb into a corridor that passes close to a 150 m tower.
Priority three: attempt link recovery. Reposition the antenna, raise the controller, move a few metres to a better aspect - but only within a few steps, and only if the observer keeps the aircraft in sight. Do not drive or walk away.
Priority four: manage the third parties. The workers below and the air-ambulance both need to know. A short broadcast on the area frequency - 'RPAS lost link, six hundred metres south-west of the tower, climbing to one hundred and twenty metres, climbing north' - costs ten seconds and buys a lot.
Do not run after the aircraft. Do not reboot the controller mid-flight if the system will re-establish on its own.
2. The link does not return. Do you let the failsafe run, or do you intervene? Reason through the options including the tower, the wind and the battery.
Read the worked answer
The core problem
The failsafe altitude of 120 m is below the top of a 150 m tower, and the return track passes within 15 degrees of it. Vertically, the aircraft will fly under the tower head; laterally, it depends on the exact track and on the wind pushing it east. That combination - under the head, near the guy wires and the lattice - is the most dangerous feature of the situation.
Option 1: let it run. Reasonable, because the aircraft is doing something predictable and the return is short. 1.4 km at roughly 10 m/s is about 140 seconds, plus a climb; the 42 percent battery with 6 minutes to bingo gives adequate margin, though the 26 kt gusts will erode it on the return leg and on any hover.
Option 2: flight termination. Only defensible if the aircraft is departing into something worse than where it is - over a road, over occupied housing, or downwind toward the tower with no prospect of recovery. Terminating over a construction site with workers present is not obviously safer than a controlled return.
Decision. Let the failsafe execute, keep the observer visual, and be ready to take control the instant the link re-establishes. If it does, do not resume the mission: climb above the tower's height or route around it, and bring the aircraft home. The mission is over either way.
Pre-flight lesson: the RTH altitude should have been set against the obstacle survey, not left at a default.
3. The aircraft lands safely with no damage. What do you preserve, whom do you notify, and what would you change before the next flight at this site?
Read the worked answer
Preserve
- Do not clear the controller or reformat anything. Export the telemetry and the flight log, and copy the aircraft's onboard log and any SD-card video before the next flight overwrites it.
- Photograph the aircraft and the home point as they are.
- Note the time of link loss, position, altitude, failsafe behaviour and time to recovery in the occurrence record while it is fresh.
Notify
- The accountable manager and the company SMS, as a reported occurrence. A link loss that placed an aircraft near a tower and under a medevac route is reportable internally even with no damage.
- The ATC unit or area frequency user, if the aircraft was ever in or near controlled airspace or conflicted with the helicopter. A short factual call protects the next RPAS operator as much as you.
- CAAP, if the event meets the accident or serious incident threshold in the regulations - an uncontrolled fly-away toward a tower and a populated site is worth checking against that definition rather than assuming it does not.
Change before the next flight
- Recompute the RTH altitude from the obstacle survey, and set a geofence that keeps the aircraft away from the tower's lateral clearance.
- Re-brief the observer's lost-link role and the area-frequency call.
- Consider whether 100 m AGL is needed at all here, given the tower and the helicopter route.
- Test the failsafe on the ground before the next sortie.