8.3 Failsafe Modes, Return-to-Home & Lost-Link Behaviour

Failsafes Are Performance Systems, Not Magic Safety Nets

When the radio link degrades, GNSS quality drops, or battery energy falls below a threshold, modern multirotors execute failsafe logic. Return-to-Home (RTH) is the most famous behaviour, but it is only one option among manufacturer menus that may include hover, land, continue, or brand-specific variants.

For Near People (A2) candidates, the exam and real operations ask the same question: if the failsafe fires now, where does the aircraft go, and who is under that path? Flight performance knowledge means understanding automatic flight paths as carefully as you understand manual stick inputs.

Home Point — The Anchor of RTH

RTH only helps if home is where you think it is.

Typical home-point rules of thumb taught across major platforms (always verify your manual):

• Home is often recorded at power-up / take-off when GNSS is valid.
• If you carry a powered aircraft from a van to a launch pad without refreshing home logic as required by the manufacturer, home might remain at the van, not the pad.
• Indoor GNSS-denied power-ups can create bad or no home points.
• Dynamic home updates (some systems update home to the controller location) can move home toward the pilot — useful when you walk, dangerous if mis-set near a road.

Pre-flight home discipline:

1. Confirm GNSS/compass healthy before take-off when the aircraft requires them for RTH.
2. Confirm the app/map home icon sits on the actual launch point (or intended recovery point).
3. After relocating on foot with a live aircraft or controller, re-check whether home moved and whether you wanted that.
4. Do not begin Near People work if home is plotted inside a car park of uninvolved people, on a live carriageway, or on the far side of a glass façade you cannot land on.

A perfect 30 m operational plan is worthless if lost-link RTH aims at the wrong coordinates.

RTH Altitude — Clear of Obstacles

Most systems climb to a configured RTH altitude, transit horizontally toward home, then descend. If RTH altitude is lower than nearby obstacles, the aircraft may attempt to fly through trees, roof edges, crane jibs, lamp posts, or building corners.

Configuration principles:

Exam trap: setting RTH altitude to “default 30 m” in a city canyon with 40 m façades. The failsafe then becomes an impact trajectory.

Before Near People operations beside buildings, walk the return geometry on the map: launch point, working area, obstacle heights, and uninvolved-person locations. If every automatic return crosses people, change the site, change the home, or change the failsafe mode (for example land in a pre-briefed sterile patch if the manufacturer mode and site allow a safer outcome than RTH across a crowd).

Lost-Link Failsafe

Lost link (loss of command and control signal) is a classic failsafe trigger. Causes include distance, interference, body blocking the transmitter, flying behind RF-opaque structures, or equipment faults.

On link loss, common manufacturer behaviours include:

• RTH — climb/transit/land at home
• Hover — hold position for a timeout, then escalate
• Land — descend roughly in place
• Continue (less common / mission-specific) — keep a programmed path

For Open VLOS Near People flying, you must:

1. Know which behaviour your aircraft uses out of the box.
2. Change it only deliberately, understanding the new ground-risk picture.
3. Maintain VLOS and control so lost-link is rare — failsafe is backup, not a BVLOS tool.
4. Recognise that hover-in-place over a street may still endanger people below if the hover point was badly chosen when the link died.

Lost-link over uninvolved people is both a performance failure and a site-selection failure. Technical mitigations (better antennas, clearer RF paths) combine with operational mitigations (do not place the working orbit where a link shadow is inevitable).

Low-Battery Failsafe

Low-battery failsafe often differs from lost-link failsafe:

• First warning: pilot should land or return with margin.
• Serious threshold: automatic RTH or forced landing logic engages.
• Critical threshold: may force immediate landing with little pilot override.

Because battery failsafe is energy-driven, payload mass and cold weather (previous sections) move the trigger earlier. If RTH from the far end of a site needs 90 seconds and 20% energy, a configuration that triggers automatic RTH at 25% may still be safe — or may not, if the return climbs against wind with a heavy camera.

Near People rule of thumb: treat manufacturer percentages as starting points. Validate real consumption on your loaded aircraft, then keep a reserve large enough that automatic RTH does not become a low-energy glide across people.

RTH Can Overfly Uninvolved People

This is the A2-specific insight many recreational pilots miss.

Your planned flight path may stay 30 m horizontally from people. The RTH polyline from a working waypoint back to home may cut the corner across a footpath, café seating, or a queue outside a shop. Automatic systems optimise geometry and energy, not UK Open ethics and separation culture.

Mitigations:

1. Site-select so home, work area, and return corridor sit over property or sterile ground you control or may lawfully use.
2. Place home deliberately — sometimes slightly away from the prettiest launch pad if that pad forces RTH across people.
3. Limit how far you fly from home so any RTH is short and stays inside your segregated bubble.
4. Choose hover or land modes only when the hover/land point is safer than the RTH path and the mode is appropriate for wind and obstacles.
5. Abort manually early when people enter the area — do not wait for automatic logic to invent a path.

Exam stems often describe an aircraft that loses link 40 m from home with a school playground on the straight-line return. The competent answer discusses configuration, site selection, and not relying on default RTH as if it were always compliant with uninvolved-person care.

Hover vs Land vs RTH — Choosing the Mode

There is no universal “always RTH” answer for every A2 site. The correct exam mindset is scenario-dependent configuration backed by manufacturer knowledge and site survey. For open fields far from people during A3 self-practical training, RTH is often an excellent default to learn and test. For constrained urban Near People sites, RTH defaults may need deliberate change or the site may be unsuitable for that aircraft’s automatic logic.

Know Manufacturer Defaults Before Near People Ops

Before any job that uses A2 distances:

1. Read the failsafe section of the actual aircraft manual/app for your firmware.
2. Note defaults for lost-link, smart RTH, low battery, and GNSS loss if described separately.
3. Confirm whether obstacle sensing remains active in RTH.
4. Confirm whether the pilot can cancel RTH and under what conditions.
5. Record your chosen settings in a simple ops checklist so you do not “reset to factory” before a critical flight by accident.

Firmware updates can change failsafe behaviour. After an update, re-verify settings in a safe area. Flight performance includes software-defined performance, not only motors and props.

Test Failsafes in A3 Self-Practical Training

UAS.OPEN.030 requires self-practical training under Far from People (A3) conditions before the additional A2 theory declaration path. That A3 practice window is exactly when you should:

• Trigger a controlled lost-link simulation only if the manufacturer documents a safe test method (for example flight mode tests on the ground or carefully planned short-range tests — never invent dangerous tests near people).
• Observe RTH climb, transit, and descent over empty ground.
• Verify home point accuracy.
• Confirm low-battery warnings appear with usable reserve on your typical payload.
• Practise manual cancel/override procedures if available and appropriate.
• Practise manual return and landing so you are not mentally dependent on automation.

Do not discover for the first time during a paid Near People shoot that RTH altitude is lower than the warehouse roof.

GNSS and Compass Link (Forward Look)

RTH quality depends on navigation sensors. GNSS loss or magnetic interference can change what “go home” means or force a different failsafe. Later navigation chapters cover GNSS hold, drift, and compass errors in depth. For this section, hold one sentence: a failsafe that needs GNSS cannot save you if positioning is already garbage — site and manual skills remain primary.

Realistic Scenarios

Scenario A — default RTH over a pavement. Pilot launches from a rear car park, works the front façade of a shop, and leaves factory RTH on. Link drops at the façade. Aircraft climbs and tracks a straight line across the public pavement to the car park. Better plan: home on a sterile side yard with a return path over the operator’s controlled ground, or limit the work so manual recovery stays inside the segregated area.

Scenario B — low RTH altitude in a marina. Masts and yard arms exceed the 20 m default RTH height. Correct action: raise RTH altitude with margin (within legal height limits) or do not rely on RTH through the mast field; use a different recovery plan.

Scenario C — home point left at the van. Pilot powers up in the van for a compass dance, walks 80 m, launches without confirming home. Lost-link sends the aircraft toward the road-side van. Confirm home on the map at the pad.

Scenario D — A3 practice done right. During self-practical training in a large empty field, the pilot verifies RTH altitude, lost-link behaviour, and battery warning timing with the survey camera fitted. Only after those checks does the pilot accept Near People bookings with that configuration.

Exam Memory Anchors

Failsafe literacy turns automation from a hidden hazard into a managed performance tool. For A2 theory, always ask where the aircraft will go when you are no longer the one flying the sticks.