3.3 The 1:1 Rule, Height vs Horizontal Distance & Safe Distances

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:

1. 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).
2. Compute the 1:1 distance = current or planned height AGL (same units, usually metres).
3. 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.
4. Never intentional overflight of uninvolved persons.

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:

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

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.