Radio Failure, a Control Zone Boundary and a Lost Link
You are operating a 7 kg multirotor from a field 1.5 km outside the boundary of a controlled aerodrome's control zone. The field is outside controlled airspace, but your planned survey line takes the aircraft to within 200 m of the control zone boundary at 90 m above ground. You have a handheld VHF radio and are monitoring the aerodrome's frequency. The tower has not been contacted, because you are outside controlled airspace. Mid-survey, the control link drops; the aircraft is programmed to hover for 20 seconds and then return to home. The tower transmits a general call that a helicopter is departing and will climb through your area. You hear the call but the helicopter's position is not clear to you.
1. Explain the airspace and communication issues and state what you should do in the next 30 seconds.
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Airspace. The field is outside the control zone, so you are not required to obtain a clearance to fly there. However, the survey line brings the aircraft within 200 m of the boundary at 90 m, and the aircraft could cross the boundary if it drifts or if the return-to-home path takes it that way. Crossing into controlled airspace without a clearance is not permitted, so the flight must be planned to keep a clear margin from the boundary, and the RTH path must be checked to ensure it does not cross it.
Communications. Monitoring the tower frequency is good practice but does not give you separation from the helicopter. Hearing a general call without a clear position means you cannot assume you are clear of it. The helicopter may climb through your area.
Next 30 seconds.
- The link is down; the aircraft will hover for 20 seconds and then return home. Do not wait passively. Re-establish the link if possible, or prepare for the aircraft to return.
- Because the helicopter may be near, the safest action is to let the aircraft return on its programmed path only if that path is clear of the helicopter's likely climb-out. If it is not, and you can regain control, land immediately in the field rather than transit.
- If you cannot regain control, keep visual contact, note the aircraft's position and height, and be ready to warn others. Do not transmit on the tower frequency unless the situation is an emergency; if the helicopter is a real conflict, a brief call to the tower stating your position, height and that you have a lost-link aircraft is justified.
- After recovery, do not resume the survey until the helicopter has cleared the area and you have re-planned the line to keep well clear of the boundary.
2. What should the pre-flight plan have included to prevent this situation?
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The situation was foreseeable, so the plan should have addressed it before launch:
- Boundary buffer. Define a hard lateral limit that keeps the aircraft at least a few hundred metres inside the control zone boundary, and program a geofence at that limit. A 200 m margin at 90 m is too thin for a 7 kg aircraft in any drift.
- RTH path check. Verify that the return-to-home track, including any climb to RTH altitude, stays outside controlled airspace and clear of the aerodrome's approach and departure paths. RTH altitude is a common trap: the aircraft may climb into airspace it never entered on the outbound leg.
- Lost-link behaviour. Choose hover-and-land over return-to-home when the aircraft is near a boundary or near other traffic, so a link loss does not cause a transit into a hazard.
- Traffic awareness. Contact the tower before flight even when outside controlled airspace, to inform them of the operation, its area and its height, and to ask for known traffic. Monitor the frequency throughout and have a way to be contacted.
- Helicopter conflict plan. Brief what to do if a helicopter is departing through your area: descend, land, or hold clear, and do not assume the helicopter crew can see a small aircraft.
- Record keeping. Log the operation, the frequencies monitored and the traffic information received, so the decision-making can be reviewed.