5.2 Kinetic Energy, Mass, Speed & Ground Impact Risk

Why Kinetic Energy Appears on an A2 Paper

The A2 CofC is not an engineering degree, but remote pilots who work Near People must understand why the rules emphasise low-speed mode, mass limits, height, and the ban on overflying uninvolved persons. The shared language is kinetic energy (KE)—the energy of motion that becomes impact energy if the unmanned aircraft hits a person or the ground near a person.

You will not be asked to integrate equations under a radical. You will be asked to reason qualitatively: which change raises risk more, why 3 m/s matters, why a near-4 kg UK2 is different from a 900 g aircraft at the same speed, and why “I was only at 8 m AGL” does not excuse flying over a stranger’s head.

The Exam Framing: KE = ½mv²

Use the standard school-physics relationship:

KE = ½ × m × v²

Where:

• m is mass (for operations, think take-off mass including payload—the same idea as MTOM discipline).
• v is speed (how fast the aircraft is moving relative to the person or the impact point).
• ½ is a constant; you do not need to compute absolute joules in most MCQs.

Two exam consequences fall out immediately:

1. Mass is linear. Double the mass at the same speed → about double the kinetic energy.
2. Speed is squared. Double the speed at the same mass → about four times the kinetic energy. Triple the speed → about nine times the energy.

That is why the regulation and class standards invest so heavily in a selectable low-speed mode for UK2 Near People privileges. A modest speed cap does more for impact energy than many pilots expect from “flying a bit lighter.”

Qualitative speed comparison (same mass)

Qualitative mass comparison (same speed)

Why Low-Speed Mode Enables Closer Operations

Chapter 3 stated the rule: with UK2/C2 and active manufacturer low-speed mode, after evaluating weather, performance, and segregation, horizontal separation from uninvolved persons may reduce to a minimum of 5 m (never intentional overflight). Chapter 5 supplies the physics story the examiner expects:

• Low-speed mode caps maximum speed (training materials typically cite about 3 m/s).
• Lower v → much lower v² → much lower KE if contact occurs.
• Lower speed also shortens the distance needed to stop or reverse when someone walks into the buffer.
• Therefore the residual ground risk at 5 m can be argued as more acceptable only when that technical speed limit is truly engaged and the operational evaluations pass.

What does not create the same physics justification:

• Gently holding the sticks without the mode engaged (the aircraft can still accelerate to its normal maximum).
• A third-party app “speed limit” on a non-class-marked airframe that has no UK2 low-speed design assurance.
• Reducing height alone while keeping high horizontal speed toward people.

Heavier UK2 Near 4 kg vs Lighter Craft

UK2 class-marked aircraft are limited to an MTOM of less than 4 kg including payload. That is still substantial compared with UK0 (< 250 g) or many light camera drones. At identical speed:

• A 3.8 kg survey multirotor stores nearly four times the kinetic energy of a 0.95 kg aircraft (mass ratio ≈ 4).
• Adding a heavy lens, RTK module, or spare battery that stays within MTOM still raises m and therefore raises KE at every speed.
• Heavier aircraft often have higher disk loading and different failure behaviour; they may descend faster if power is lost unless a parachute or autorotative-like design intervenes—another reason mass discipline is part of ground-risk thinking.

Exam trap: “Under 4 kg means low risk, so overflight is fine.” False. Class MTOM is a product limit, not a permission to overfly uninvolved persons. Energy can still cause serious injury, and the no-overfly rule is absolute in Open A2 doctrine for uninvolved people.

Exam trap: “My UK2 is only 1.2 kg so I can ignore low-speed mode at 5 m.” False. The 5 m privilege is tied to active low-speed mode and evaluations on eligible class-marked types, not to being “light for a UK2.”

Drop Height, Descent Rate, and Impact Speed

Horizontal cruise speed is only one way to build kinetic energy. Height above ground matters because a falling aircraft converts potential energy into speed. Qualitatively:

• Greater drop height → higher impact speed if the aircraft free-falls or fails without a recovery system.
• Higher descent rate (fast sink, motor-out spiral, high-speed descent) → higher impact v → higher KE.
• A parachute aims to reduce descent rate, cutting impact speed and thus KE if it deploys in time and with enough height.
• Very low height may leave insufficient time for a parachute to open—so “I have a chute” does not justify hovering over people at 3 m AGL.

You do not need the free-fall formula on the exam, but you should reason: higher + faster descent + heavier mass = worse ground impact. That is why operational mitigations emphasise controlled height, clear recovery paths, and not placing uninvolved persons under the aircraft.

Height and overflight—separate ideas

Why Overflight Is Forbidden Even at Low Altitude

Students sometimes invent a private rule: “If I am only a few metres up and almost hovering, KE is tiny, so overflight is harmless.” The regulation does not adopt that private rule for uninvolved persons in Open A2.

Reasons the exam supports:

1. Even low KE can injure — propeller blades at high RPM can cause lacerations at modest translational speed; eyes and face are vulnerable.
2. Hover is not guaranteed — GNSS glitch, wind gust, loss of control, or pilot error can add sudden horizontal or vertical speed.
3. People move — an uninvolved person may stop, turn, or raise their hands into the rotor disc.
4. Legal bright line — UAS.OPEN.030-style Near People rules use a clear no intentional overflight requirement plus horizontal floors; bright lines are enforceable and examinable.
5. 1:1 thinking — if height is H, a failure trajectory can still reach a person roughly H away horizontally; being above them maximises the chance that a failure intersects their location.

So: low altitude does not convert overflight into a permitted manoeuvre. Low altitude may even reduce the time available for parachute deployment or pilot recovery.

Realistic Exam-Style Comparisons (No Calculus)

Comparison A — Speed doubles, mass fixed. A 2 kg aircraft at speed v has KE baseline 1. At 2v, KE ≈ 4. The pilot who “only doubled speed for a cinematic pass” roughly quadrupled impact energy. That is why low-speed mode near people is non-negotiable for the 5 m package.

Comparison B — Mass doubles, speed fixed. Same aircraft with payload raised from 1.5 kg to 3.0 kg total at the same speed ≈ doubles KE. Still linear, still serious—but less dramatic than the speed effect. Payload planning is ground-risk planning.

Comparison C — Low-speed mode vs normal. Normal mode might allow many metres per second of horizontal speed; low-speed mode caps near 3 m/s. The ratio of speeds squared is large, so residual impact energy near people is designed to be much lower when the mode is truly active.

Comparison D — High hover over a path. Aircraft mass 2 kg, almost zero horizontal speed, 25 m directly above walkers. Horizontal KE is low, but any loss of lift creates descending speed; the people are in the failure footprint. Not permitted as intentional overflight, regardless of the horizontal-speed argument.

Comparison E — UK2 at 3.6 kg vs legacy 1.6 kg at same speed. The heavier UK2 has more than double the KE of the lighter legacy craft at equal v. The UK2 may still operate closer only because of class features (including low-speed mode) and compliance with 30/5 rules—not because heavier is safer.

Linking KE Back to Other Mitigations

• Prop guards/shrouds do not remove KE; they change how energy and blades interact with tissue (contact geometry), reducing some injury modes.
• Parachutes target descent rate → lower impact v → lower KE if successful.
• Low-speed mode targets horizontal (and overall) speed → lower v².
• Operational distance keeps people outside the likely impact footprint so residual KE never couples to a person.
• Segregation empties the footprint so that even if KE is released into the ground, uninvolved people are not there.

Memory Hooks for Section 5.2

1. KE = ½mv² — speed squared dominates; mass still counts.
2. ~3 m/s low-speed mode → energy and stopping distance justification for 5 m.
3. UK2 < 4 kg is not “harmless mass.”
4. Height feeds descent impact; parachutes need time and reliability.
5. No intentional overflight of uninvolved persons—at any height or hover.

If a stem gives two changes, always ask: did speed change? If yes, energy likely moved a lot. If only mass changed a little, energy moved less—but horizontal floors and overflight rules still bind.