6.2 The 1:1 Rule & Dynamic Buffer Distance Planning

The 1:1 Rule & Dynamic Buffer Distance Planning

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The 1:1 Rule Principle (AMC1 UAS.OPEN.030(1))

The 1:1 rule is a reference given in EASA's Acceptable Means of Compliance (AMC1 UAS.OPEN.030(1)(b)). It says that, when the UA is operating close to people:

The remote pilot should keep the UA at a lateral distance from any uninvolved person that is not shorter than its height (for example, at 30 m height, at least 30 m away):

$d_{\text{horizontal}} \ge h_{\text{flight}}$

AMC is not law in itself. It is the accepted way of showing compliance with the regulation's requirement to keep a safe horizontal distance, and departing from it would mean showing another way of achieving the same safety. In practice, and for the exam, treat the 1:1 reference as the expected minimum. AMC1 also says the distance is measured from the point where the UA would hit the ground in a vertical fall.

Interaction with the 30 m and 5 m Minimums

Candidates often struggle with how the 1:1 reference combines with the regulation's 30 m standard distance and 5 m low-speed distance. Combining them gives a maximum function:

$d_{\text{separation}} = \max(d_{\text{floor}},\; h_{\text{flight}})$

Where:

• $d_{\text{floor}} = 30\text{ m}$ under standard flight mode;
• $d_{\text{floor}} = 5\text{ m}$ under active low-speed mode ($\le 3\text{ m/s}$);
• $h_{\text{flight}}$ is the height of the drone above ground level (AGL).

+-------------------------------------------------------------------------+
|                   GOVERNING HORIZONTAL DISTANCE LOGIC                   |
+-------------------------------------------------------------------------+
|  At low altitudes (h < d_floor):                                        |
|      The static floor governs!                                          |
|      - Standard Mode: At 10 m altitude -> Maintain at least 30 m.       |
|      - Low-Speed Mode: At 3 m altitude -> Maintain at least 5 m.        |
|                                                                         |
|  At high altitudes (h >= d_floor):                                      |
|      The 1:1 rule governs!                                              |
|      - Standard Mode: At 50 m altitude -> Maintain at least 50 m.       |
|      - Standard Mode: At 120 m altitude -> Maintain at least 120 m.     |
|      - Low-Speed Mode: At 40 m altitude -> Maintain at least 40 m!      |
+-------------------------------------------------------------------------+

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Physical Rationale: Ballistic Trajectories and Free-Fall Dynamics

Why does the 1:1 reference make sense? Unlike fixed-wing aircraft, multirotors generate lift exclusively through motorized rotating airfoils. If an electronic speed controller (ESC) fails, a motor burns out, a propeller sheds a blade, or a battery terminal disconnects, the aircraft loses all aerodynamic lift instantly.

Without lift, the drone behaves as an unguided projectile subject to the laws of Newtonian mechanics.

     Drone Failure at Altitude h, Velocity v0
     (o)========> v0 (horizontal cruise velocity)
      | \
      |  \
      |   \  Parabolic Ballistic Arc
      |    \
    h |     \  t_fall = sqrt(2h / g)
      |      \
      |       \
      |        \  Ground Impact Point
     ---=================X-------------------
              d_glide = v0 * t_fall

1. Free-Fall Duration Calculation

Neglecting vertical air resistance during initial acceleration, the time $t_{\text{fall}}$ (in seconds) required for an object to fall from height $h$ (in meters) under gravitational acceleration ($g \approx 9.81\text{ m/s}^2$) is:

$t_{\text{fall}} = \sqrt{\frac{2h}{g}}$

• From $h = 10\text{ m}$: $t_{\text{fall}} = \sqrt{\frac{20}{9.81}} \approx 1.43\text{ s}$
• From $h = 30\text{ m}$: $t_{\text{fall}} = \sqrt{\frac{60}{9.81}} \approx 2.47\text{ s}$
• From $h = 50\text{ m}$: $t_{\text{fall}} = \sqrt{\frac{100}{9.81}} \approx 3.19\text{ s}$
• From $h = 80\text{ m}$: $t_{\text{fall}} = \sqrt{\frac{160}{9.81}} \approx 4.04\text{ s}$
• From $h = 120\text{ m}$: $t_{\text{fall}} = \sqrt{\frac{240}{9.81}} \approx 4.95\text{ s}$

2. Ballistic Horizontal Coasting Distance

If the drone is traveling at horizontal velocity $v_0$ when power is lost, inertia carries the airframe forward while gravity pulls it downward. The horizontal ground distance traversed during the fall ($d_{\text{glide}}$) is:

$d_{\text{glide}} = v_0 \cdot t_{\text{fall}} = v_0 \cdot \sqrt{\frac{2h}{g}}$

Consider an aircraft cruising at standard speed ($v_0 = 15\text{ m/s}$, or $54\text{ km/h}$) at an altitude of $50\text{ m}$:

$d_{\text{glide}} = 15\text{ m/s} \times 3.19\text{ s} \approx 47.9\text{ m}$

Notice the striking result: $47.9\text{ m}$ is almost exactly equal to the flight altitude of $50\text{ m}$!

This illustrates the physics behind the AMC1 UAS.OPEN.030(1) reference: at typical cruising speeds, an unpowered multirotor travels forward by roughly its release height before hitting the ground. Keeping $d_{\text{horizontal}} \ge h$ therefore helps keep the impact point of a sudden failure away from bystanders. At low speed the forward throw is much smaller (at $3\text{ m/s}$ from $50\text{ m}$ it is under $10\text{ m}$), so the 1:1 reference is generous for slow flight but only just enough for fast flight.

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A Third Reference: Reaction Distance

The AMC for subcategory A3 (AMC1 UAS.OPEN.040(1)) estimates the minimum distance from a person passing through the area as no less than 30 m, no less than the height (1:1), and no less than the distance the UA would cover in 2 seconds at maximum speed, assuming a 2-second reaction time. That third reference is written for A3, but it is a useful planning check in A2 too: a drone capable of $20\text{ m/s}$ covers $40\text{ m}$ in 2 seconds.

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Wind-Adjusted Dynamic Buffers

The 1:1 rule assumes a static or zero-wind atmosphere. In real-world flight operations, ambient wind exerts continuous horizontal aerodynamic drag upon the airframe as it tumbles or falls. This introduces wind drift displacement ($d_{\text{wind}}$):

$d_{\text{wind}} = v_{\text{wind}} \cdot t_{\text{fall}} = v_{\text{wind}} \cdot \sqrt{\frac{2h}{g}}$

Total Dynamic Safety Buffer Formula

When planning operations near uninvolved persons, the prudent remote pilot must calculate a total downwind dynamic safety buffer:

$d_{\text{buffer}} = h_{\text{flight}} + \left(v_{\text{wind}} \cdot \sqrt{\frac{2h_{\text{flight}}}{g}}\right)$

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Upwind vs. Downwind Flight Geometry

The spatial orientation of the drone relative to wind direction and uninvolved persons is critical:

• Upwind of Uninvolved Persons (Wind blowing from drone toward people): This is the highest-risk geometry. If propulsion cuts out, initial forward inertia and ambient wind drift combine to blow the falling drone directly toward the people. The horizontal separation distance must be expanded significantly beyond the 1:1 baseline!
• Downwind of Uninvolved Persons (Wind blowing from people toward drone): This is a favorable geometry. If a catastrophic failure occurs, the ambient wind blows the aircraft further downwind, away from the uninvolved individuals and into open ground.

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Comprehensive Calculation Reference Table

The table below shows free-fall times, the regulation's minimums, the 1:1 reference distance and a wind-adjusted planning buffer across typical heights. The buffer method is conservative, because it assumes the falling drone drifts at the full wind speed:

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Practical Worked Operational Scenarios

Scenario 1: High-Altitude Facade Inspection

• Flight Profile: A remote pilot operates a Class C2 drone at an altitude of $60\text{ m}$ AGL to inspect the upper floors of an office tower. A public plaza with uninvolved pedestrians is located nearby.
• Low-Speed Mode: Engaged (speed $\le 3\text{ m/s}$).
• Determination: Even though low-speed mode is active, the pilot should not fly $5\text{ m}$ horizontally from the pedestrians in the plaza. Because the height is $60\text{ m}$, the 1:1 reference governs:

$d_{\text{separation}} = \max(5\text{ m},\; 60\text{ m}) = 60\text{ m}$

The pilot should keep at least $60\text{ m}$ of horizontal clearance from the plaza edge.

Scenario 2: Roof Survey with Ambient Wind

• Flight Profile: Inspecting a factory roof at $45\text{ m}$ altitude in standard mode.
• Wind Conditions: Wind is blowing at $8\text{ m/s}$ ($29\text{ km/h}$) directly toward a public highway located east of the factory.
• Calculation:
  • Free-fall time from $45\text{ m}$: $t_{\text{fall}} = \sqrt{\frac{90}{9.81}} \approx 3.03\text{ s}$.
  • Wind drift toward highway: $d_{\text{wind}} = 8\text{ m/s} \times 3.03\text{ s} \approx 24.2\text{ m}$.
  • Base 1:1 standoff: $45\text{ m}$.
  • Total required upwind buffer: $45\text{ m} + 24.2\text{ m} = 69.2\text{ m}$.
• Operational Decision: If the drone is west of the highway (upwind), the pilot must maintain at least $70\text{ m}$ of horizontal separation from the highway boundary to ensure an unpowered tumbling drone does not drift into traffic.