Wet & Contaminated Runway Landing Calculator
AC 91-79 surface factors for wet, standing water, slush, snow and ice — applied to your dry-runway figure, with hydroplaning speed alongside.
Factors per FAA AC 91-79 guidance for light aircraft. The 9√P rule: at 42 psi, hydroplaning onset near 58 kt — above it, on standing water, the tires are passengers and the brakes are decoration.
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.
Disclaimer: This tool is for general informational and estimation purposes only and is not professional financial, tax, accounting or legal advice. All figures are estimates — verify with a qualified professional before making decisions. Read the full disclaimer.
AC 91-79 surface factors for wet, standing water, slush, snow and ice — applied to your dry-runway figure, with hydroplaning speed alongside.
About Wet & Contaminated Runway Landing Calculator
Water is the great equalizer of braking technique: a film of it cuts friction nearly in half, standing water can lift the tires entirely off the surface, and ice keeps only a quarter of dry stopping power. This calculator applies the FAA's AC 91-79 landing factors for each contamination class to your dry figure and computes the NASA 9√P dynamic hydroplaning speed for your tire pressure — the ground speed above which braking on standing water is physically unavailable.
How to use Wet & Contaminated Runway Landing Calculator
- 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 = dry × surface factor (wet 1.4 → ice 3.5); V_hydroplane ≈ 9 × √(tire psi) substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Wet & Contaminated Runway Landing Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula distance = dry × surface factor (wet 1.4 → ice 3.5); V_hydroplane ≈ 9 × √(tire psi) with sources cited on the page
- ✓Factors per FAA AC 91-79 guidance for light aircraft. The 9√P rule: at 42 psi, hydroplaning onset near 58 kt — above it, on standing water, the tires are passengers and the brakes are decoration.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
What is the 9√P hydroplaning rule?+
NASA's Horne & Dreher found dynamic hydroplaning onsets near 9 times the square root of tire pressure (psi), in knots. A light single at 42 psi hydroplanes above ~58 kt ground speed; an airliner at 200 psi above ~127 kt. Below the onset speed the tire squeezes through to pavement; above it the water's dynamic pressure carries the wheel like a boat hull.
Why does a merely wet runway add 40% or more?+
Wet friction coefficients run roughly half of dry on good surfaces — worse on smooth, rubber-contaminated or worn pavement (hence the separate 1.7 class here). Anti-skid systems can't create friction, only ration it. Grooving and porous friction courses drain the film and recover much of the gap, which is why the surface's texture state matters as much as 'wet' itself.
Is braking or directional control the bigger contaminated-runway threat?+
They're the same budget: tire friction serves braking and cornering from one account. On ice, spending everything on braking leaves nothing to counter a crosswind — which is why contaminated-runway crosswind limits collapse (our jet crosswind tool shows the table). For light GA on slick surfaces, aerodynamic control (full aileron into wind, rudder) outlives tire authority and becomes primary.
What technique changes on a contaminated runway?+
Touch down firmly, on speed, in the zone — a greaser extends the hydroplaning phase. Get flaps up (on types so advised) to put weight on wheels, brake smoothly below hydroplaning speed, and never taxi-test the friction with the rollout's last third. If the factored distance crowds the runway before you've flown the approach, the brief writes itself: divert.
Related Field tools
Sunrise & Sunset Calculator
Exact rise, set, solar noon and day length for any place and date — the NOAA solar equations with the refraction fine print included.
● LiveGolden Hour & Blue Hour Calculator
Tonight's golden hour and blue hour, computed from sun elevation — the photographer's light windows with the angles that define them.
● LiveDay Length Calculator
Hours of daylight for any date and latitude, how fast it's changing, and the swing between your solstices.
● Live