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

UK CAA A2 Certificate of Competency A2 CofC

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1 Introduction & A2 CofC Pathway

1.1 Exam Facts, Format, Pass Mark & Validity 1.2 Flyer ID, Operator ID & Practical Declaration 1.3 Operating Guides, Maintenance & Safe-to-Fly Decisions

2 Open Category Air Law

2.1 Open Category A1 / A2 / A3 & 2026 Near People Rename 2.2 VLOS, 120 m Height Limit & Flight Restriction Zones 2.3 Uninvolved Persons, Assemblies & Giving Way to Manned Aircraft

3 Near People Separation Rules

3.1 UK2/C2 Separation: 30 m Normal & 5 m Low-Speed Mode 3.2 Legacy Aircraft 50 m Separation & MTOM < 2 kg 3.3 The 1:1 Rule, Height vs Horizontal Distance & Safe Distances

4 Operational Ground Risk Mitigations

4.1 Site Selection, Segregation & Pre-Flight Briefing 4.2 Reacting to Approaching People & Abort Decisions 4.3 Weather, Performance & Segregation Checks Before 5 m Ops

5 Technical & Human Ground-Risk Mitigations

5.1 Prop Guards, Parachutes, Geo-Awareness & Other Technical Mitigations 5.2 Kinetic Energy, Mass, Speed & Ground Impact Risk 5.3 Medical Fitness, Alcohol, Medication & Fatigue

6 Aircraft Eligibility & Flight Characteristics

6.1 UK0, UK1 & UK2 Class Marks and A2 Eligibility 6.2 Rotorcraft, Fixed-Wing & Hybrid Operating Envelopes 6.3 Basic Principles of Flight, Stability & Control

7 Practical Procedures, Emergencies & Decisions

7.1 Flight Procedures & Basic Practical Drills 7.2 Emergency & Abnormal Procedures 7.3 Decision-Making, Customer Pressure & Occurrence Reporting

8 UAS Flight Performance Systems

8.1 Mass, Balance, Payload Effects on Performance 8.2 Battery Terminology, Types, Charging & Storage 8.3 Failsafe Modes, Return-to-Home & Lost-Link Behaviour

9 Navigation, Compass & Flyaways

9.1 GNSS Position Hold, Drift & Loss of Positioning 9.2 Compass Errors & Magnetic Interference 9.3 Firmware, Braking Distance & Flyaway Causes

10 Meteorology for Near-People Operations

10.1 Weather Sources, Reports, Forecasts & Local Assessment 10.2 Wind Speed, Gusts & Manufacturer Limits 10.3 Wind Gradient, Masking & Urban Turbulence 10.4 Temperature, Density, Precipitation, Visibility & Icing 10.5 Clouds, Cumulonimbus, Lightning & IP43

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

Key Takeaways Explain This Topic AI

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.

Last updated: August 2026

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.