3.3 The 1:1 Rule, Height vs Horizontal Distance & Safe Distances
Key Takeaways
The 1:1 rule is industry and CAA-aligned operational judgment: horizontal separation from an uninvolved person should be at least equal to the aircraft’s height above ground level (AGL), so a failure trajectory is less likely to reach them.
Statutory floors (30 m, 5 m, or 50 m depending on mark and mode) remain hard minima; the 1:1 rule does not replace them and never authorises flying closer than the law allows.
When 1:1 suggests a larger distance than the statutory floor, choose the more conservative figure—for example at 40 m height with UK2 normal mode, use at least 40 m horizontal even though the legal floor is 30 m.
At low height with low-speed mode, the 5 m legal floor can still be too tight if 1:1 implies a greater distance (for example 20 m height → aim for about 20 m horizontal, not merely 5 m).
Higher kinetic energy and slower human reaction time both argue for more separation when people may approach; 1:1 is a simple spatial proxy that supports those ground-risk decisions.
Beyond the Legal Floor: Operational Safe Distance
Sections 3.1 and 3.2 give you the statutory horizontal minima—30 m or 5 m for UK2/C2, 50 m for legacy sub-2 kg A2. Those numbers are the least the regulation accepts in the stated conditions. Professional Near People flying, and the A2 theory syllabus on ground-risk mitigations, expect more: you must still judge whether that legal minimum is enough for the height, energy, weather, and human factors of the actual flight.
The 1:1 rule is the standard teaching model for that judgment. It is widely used in UK drone training and aligns with CAA-style operational practice: keep horizontal separation from an uninvolved person at least equal to the unmanned aircraft’s height AGL. If you are 40 m up, think at least 40 m sideways from that person. If you are 15 m up, think at least 15 m sideways—subject always to not going below the statutory floor for your aircraft and mode.
What the 1:1 Rule Is (and Is Not)
Is:
- A simple height-to-horizontal proportionality for failure geometry: if propulsion fails or the aircraft descends or is blown sideways, a person standing closer horizontally than the height AGL sits inside a more dangerous cone relative to a falling or diving path.
- A risk-management layer used when planning and when people approach mid-flight.
- A reason to increase distance as you climb, even if you already meet 30 m or 50 m.
Is not:
- A substitute for UAS.OPEN.030 floors. You cannot fly a UK2 aircraft at 8 m height and 8 m horizontal in normal mode and claim 1:1 compliance while ignoring the 30 m legal minimum.
- Permission to overfly people “because vertical clearance equals horizontal clearance.” Overflight of uninvolved persons remains banned.
- A precise ballistic model of every failure mode. Real trajectories depend on wind, residual thrust, rotor disc loading, and pilot inputs; 1:1 is a conservative rule of thumb, not a CFD simulation.
- A replacement for low-speed mode evaluation. Engaging low-speed mode still requires weather, performance, and segregation checks before using the 5 m legal floor—and 1:1 may still demand more than 5 m.
Hard Minima First, Then 1:1
Use this decision order on every exam scenario and every real flight:
- Identify the statutory floor (UK2 normal 30 m; UK2 low-speed mode after evaluation 5 m; legacy < 2 kg A2 50 m; or A3-type constraints if that is the true subcategory).
- Compute the 1:1 distance = current or planned height AGL (same units, usually metres).
- Required operational horizontal separation ≈ maximum of (statutory floor, 1:1 distance), then add extra if weather, kinetic energy, approach paths, or reaction time demand it.
- Never intentional overflight of uninvolved persons.
| Height AGL | UK2 normal (floor 30 m) | UK2 low-speed mode after eval (floor 5 m) | Legacy A2 < 2 kg (floor 50 m) |
|---|---|---|---|
| 10 m | Use ≥ 30 m (floor > 1:1) | Use ≥ 10 m (1:1 > 5 m floor) | Use ≥ 50 m (floor > 1:1) |
| 20 m | Use ≥ 30 m (floor > 1:1) | Use ≥ 20 m (1:1 > 5 m floor) | Use ≥ 50 m (floor > 1:1) |
| 40 m | Use ≥ 40 m (1:1 > 30 m floor) | Use ≥ 40 m (1:1 > 5 m floor) | Use ≥ 50 m (floor > 1:1) |
| 60 m | Use ≥ 60 m (1:1 > 30 m) | Use ≥ 60 m (1:1 > 5 m) | Use ≥ 60 m (1:1 > 50 m) |
| 120 m (Open max height context) | Use ≥ 120 m horizontally from uninvolved persons if applying 1:1 strictly | Same logic | Same logic |
The table’s lesson: at low height the legal floor often dominates; at higher height 1:1 often dominates. Low-speed mode’s 5 m floor is especially easy to misuse at moderate heights—legally you might be above 5 m, yet 1:1 still wants tens of metres.
Worked Example 1 — 40 m Height, UK2 Normal Mode
You fly a UK2 aircraft at 40 m AGL in normal mode (low-speed off) for a tower inspection. An uninvolved dog-walker is approaching the site boundary.
- Statutory floor: 30 m horizontal.
- 1:1 suggestion: 40 m horizontal.
- Conservative choice: ≥ 40 m horizontal (and still no overflight).
If you only hold 30 m at 40 m height, you meet the letter of the 30 m floor but fail the 1:1 operational practice the syllabus expects you to understand. Exam answers that say “30 m is always enough at any height for UK2” are incomplete for operational safe-distance questions, even though 30 m remains the legal minimum in normal mode.
Worked Example 2 — 20 m Height, Low-Speed Mode
Same UK2 aircraft, low-speed mode engaged, weather light and stable, performance good, garden well segregated. Height is 20 m AGL. A neighbour stands outside the fence.
- Statutory floor with mode + evaluation: 5 m horizontal.
- 1:1 suggestion: 20 m horizontal.
- Conservative choice: ≥ 20 m horizontal—not 5 m.
This is the classic trap. Pilots (and weak exam options) jump straight to “low-speed mode → 5 m” and ignore height. The regulation’s 5 m is a floor, not a target. At 20 m AGL, parking the aircraft only 5 m horizontally from a person leaves little margin if altitude is lost quickly or the aircraft accelerates out of the intended hover before you react.
Worked Example 3 — Legacy 50 m Still Wins at Moderate Height
Legacy 1.5 kg aircraft at 25 m AGL in A2:
- Statutory floor: 50 m.
- 1:1: 25 m.
- Required: ≥ 50 m (floor wins).
Here 1:1 does not let you come closer than 50 m. Operational judgment can only add distance, never subtract below the law.
Height vs Horizontal Distance: Keep the Axes Separate
Remote pilots sometimes confuse three different quantities:
| Quantity | Meaning | Role |
|---|---|---|
| Height AGL | Vertical distance above ground | Open category generally capped at 120 m; feeds 1:1 |
| Horizontal separation | Ground-track distance to a person | Carries the 30/5/50 m floors and 1:1 comparison |
| Slant range | Straight-line distance through the air to the person | Useful for radio/VLOS feel, not a substitute for horizontal legal floors |
A person 10 m horizontally from you while you are 100 m up has a large slant range but a tiny horizontal buffer—an overflight-adjacent geometry and a 1:1 failure. Conversely, 60 m horizontal at 20 m height is comfortable on both statutory (for UK2) and 1:1 measures.
Kinetic Energy, Reaction Time, and Approaching People
Kinetic energy rises with mass and with the square of speed. That is why low-speed mode (speed cap) is so powerful: halving speed roughly quarters kinetic energy, reducing injury potential if contact occurs. It is also why climbing higher without adding horizontal distance is risky: more time/distance for a descent or flyaway path to develop, and often higher speed available in normal mode.
Human reaction time and pilot reaction time both matter when people walk into the buffer:
- You must detect the person (eyes, spotter, or both).
- You must decide (climb, lateral escape, land, pause the task).
- The aircraft must respond (latency, braking distance, wind).
At 5 m horizontal and 20 m height, a brisk walker closes a large fraction of the buffer before a distracted pilot finishes the decision chain. At 20–30 m horizontal, you have more time to reposition without panicking into an aggressive descent toward other hazards. Link this to Chapter 4’s “reacting to approaching people” material: the first defence is restoring separation, not arguing about whether the person “should not be there.”
Practical add-ons beyond pure 1:1 when energy or reaction risk is high:
- Gusty wind → add horizontal margin beyond 1:1.
- Heavy payload near MTOM → longer stopping distance; add margin.
- Poor GNSS / magnetic interference near steel structures → expect position hop; add margin.
- Multiple approach routes for the public → segregate or use marshals; do not rely on a single 5 m number.
Putting the Whole Chapter Together
| Situation | Legal floor | 1:1 input | What you fly |
|---|---|---|---|
| UK2, normal mode, 25 m AGL | 30 m | 25 m | ≥ 30 m |
| UK2, normal mode, 45 m AGL | 30 m | 45 m | ≥ 45 m |
| UK2, low-speed + good eval, 12 m AGL | 5 m | 12 m | ≥ 12 m |
| UK2, low-speed + good eval, 4 m AGL | 5 m | 4 m | ≥ 5 m (floor > 1:1), still no overflight |
| Legacy < 2 kg A2, 35 m AGL | 50 m | 35 m | ≥ 50 m |
| Legacy < 2 kg A2, 70 m AGL | 50 m | 70 m | ≥ 70 m |
Memorise the slogan for the exam: floors are mandatory; 1:1 is additional; the larger number wins; overflight never wins.
That slogan unifies Chapter 3. Section 3.1 gave UK2 30/5 and low-speed mode meaning. Section 3.2 locked legacy 50 m and the mark-dependent comparison with A3. Section 3.3 stops you from treating any single number as a complete safety case when height, energy, and people on the move are part of the picture.
What does the 1:1 rule require a remote pilot to compare when judging safe distance from an uninvolved person?
Battery percentage remaining and motor temperature
Horizontal separation and the aircraft’s height AGL, aiming for horizontal distance at least equal to height
Ground speed only, ignoring height
Wind speed in knots converted directly into metres of required separation
A UK2 aircraft is flown at 40 m AGL in normal mode. Applying both UAS.OPEN.030 and the 1:1 rule, what horizontal separation from an uninvolved person should the pilot use?
At least 5 m, because height is under 50 m
At least 30 m only, because 1:1 never increases distance above the legal floor
At least 40 m, taking the more conservative of the 30 m legal floor and the 40 m 1:1 distance
At least 150 m, because any flight near people uses A3 area rules
With low-speed mode engaged and evaluations complete, a UK2 pilot flies at 20 m AGL. Why is relying only on the 5 m statutory floor a poor operational choice?
Because the legal floor in low-speed mode is actually 30 m at all heights
Because the 1:1 rule suggests about 20 m horizontal separation, so 5 m meets the regulatory minimum but leaves little margin relative to height, energy, and reaction time if something goes wrong
Because low-speed mode is illegal at any height above 10 m
Because uninvolved persons must always be at least 50 m away when low-speed mode is on
How should kinetic energy and human reaction time influence separation decisions near people, relative to the statutory floors and the 1:1 rule?
They are irrelevant once any statutory floor is met
They only matter for A3 flights, not A2 Near People flights
Higher mass or speed raises impact energy and limited reaction time reduces how quickly a pilot can restore a buffer, so pilots should add distance beyond bare floors—and use 1:1 as a spatial starting point—especially when people may approach
They allow the pilot to go below the 30 m or 50 m legal floors if the aircraft is light
Sections you finish are checked off in the contents.