Approach Speed Penalty Calculator (Excess Knots → Feet)
The V² tax on 'a few extra knots': how each knot above Vref inflates float and landing distance — the most preventable overrun cause, computed.
Energy is V²: ten knots over a 61-knot Vref is 35% more landing distance, almost 500 ft on a typical book figure. FAA AC 91-79 names unstabilized, fast approaches as the leading overrun precursor.
Formula
⚠️ Planning estimate only — your POH/AFM performance charts are the authoritative source. Always verify with official data, and apply your operator's safety factors. Not for airworthiness decisions.
The V² tax on 'a few extra knots': how each knot above Vref inflates float and landing distance — the most preventable overrun cause, computed.
About Approach Speed Penalty Calculator (Excess Knots → Feet)
No single habit destroys more landing margin than crossing the fence fast, and no correction is cheaper than not doing so. This calculator applies the kinetic-energy law — landing distance scales with the square of the speed ratio — to show what each knot above Vref really costs in feet, then prices your actual approach against the book figure. The per-knot cost it reports is the number worth memorizing for your aircraft.
How to use Approach Speed Penalty Calculator (Excess Knots → Feet)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula distance ≈ book × (V_flown/V_ref)² substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Approach Speed Penalty Calculator (Excess Knots → Feet)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula distance ≈ book × (V_flown/V_ref)² with sources cited on the page
- ✓Energy is V²: ten knots over a 61-knot Vref is 35% more landing distance, almost 500 ft on a typical book figure. FAA AC 91-79 names unstabilized, fast approaches as the leading overrun precursor.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Where does the extra distance physically go?+
Mostly into float. The flare bleeds energy at a roughly fixed rate, so excess energy extends the time — and feet — between roundout and touchdown. The remainder appears in the ground roll via higher touchdown speed. Ten knots over Vref in a light single typically buys 300–500 ft of float plus a longer roll: a third of many GA runways, spent before braking begins.
Why is the penalty squared rather than linear?+
Kinetic energy is ½mV². The brakes and aerodynamic drag must dissipate the energy you arrive with, and that grows with speed squared: 10% fast is 21% more energy, 15% fast is 32% more. The squared law is also why the same 9 extra knots hurt a 61-knot Skyhawk far more (proportionally) than a 140-knot jet.
Isn't extra speed safer in gusts?+
Controlled additives are — the half-gust-factor convention (see our gust tool) exists precisely to keep stall margin in shear, typically capped at +15. The killer is uncommanded speed: no gust justification, just 'felt right.' The discipline is to know why every knot above Vref is there, and to accept the computed distance bill for it before, not after, the flare.
What's the operational fix if I'm fast at the threshold?+
Go around. AC 91-79 is blunt: an unstabilized approach should end in a go-around, and float beyond the planned touchdown zone is the last reliable cue. Forcing a fast airplane onto the runway invites porpoising and wheelbarrowing; letting it float invites the overrun this calculator just priced. The pattern costs three minutes; the alternatives cost more.
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