How much longer is stopping distance on a wet runway compared to a dry runway for a transport-category aircraft?

As a general starting point, AC 91-79B references a minimum factor of 1.15, meaning stopping distance on a wet runway is at least 15% longer than on a dry surface. However, the actual factor depends on the specific aircraft's AFM data and the operator's approved performance methodology, and some aircraft have higher wet-runway correction factors. For contaminated surfaces such as slush or ice, the penalty can be far greater — sometimes two to three times the dry stopping distance or more.

What is the difference between a wet runway and a contaminated runway for performance purposes?

A wet runway is one with visible moisture on the surface but with water depth no greater than approximately 1/8 inch (3 mm). A contaminated runway is one where more than 25% of the required runway length is covered by water deeper than 1/8 inch, slush, wet snow, dry snow, or ice. Contaminated runways require significantly larger performance adjustment factors than wet runways, and pilots must use AFM-approved contaminated surface performance data rather than the simpler wet-runway multiplier.

What speed does hydroplaning start for jet transport aircraft and why does it matter on a wet runway?

Dynamic hydroplaning onset speed is approximated by the formula 9 multiplied by the square root of the tire inflation pressure in psi. For a tire inflated to 200 psi this works out to roughly 127 knots — a speed at or above touchdown and rollout speeds for many transport-category jets. This matters because at or above that speed the tire can ride on a thin film of water rather than contacting the pavement, drastically reducing friction and making braking almost ineffective. Prompt deployment of spoilers and thrust reversers is therefore critical at high speed on a wet runway before wheel braking becomes fully effective.

Wet and Contaminated Runway Performance and Stopping Margins

Every transport-category pilot must understand that the runway performance data published in an aircraft's FAA-approved Airplane Flight Manual (AFM) is almost always based on a dry, hard-surfaced runway under ideal braking conditions. When precipitation, standing water, slush, snow, or ice changes that surface, the physical forces available to stop the aircraft change dramatically — and those changes must be accounted for before every takeoff and landing. Advisory Circular 91-79B, Mitigating the Risks of a Runway Overrun Upon Landing, is the FAA's primary guidance document addressing this challenge, providing operators, crews, and dispatchers with a structured method for evaluating stopping margins on wet and contaminated surfaces.

The consequences of underestimating stopping distance on a degraded surface are severe and unforgiving. Runway overruns are among the most studied accident categories in commercial aviation, and virtually every major investigation points back to a gap between the distance the crew believed they needed and the distance they actually required. Understanding AC 91-79B — including its definitions, adjustment factors, and operational philosophy — is essential knowledge for any Airline Transport Pilot (ATP) certificate applicant and every professional crew member operating transport-category airplanes.

Defining the Runway Surface Conditions

Before applying performance adjustments, pilots must correctly classify the runway surface. AC 91-79B, consistent with FAA and ICAO standards, distinguishes among several conditions that degrade braking effectiveness in progressively worse ways.

Dry runway: The baseline. AFM performance data applies directly with no correction factor required.
Wet runway: A surface visibly covered with water, but with water depth no greater than approximately 1/8 inch (3 mm). Braking action is reduced but tire-to-pavement contact generally remains adequate for anti-skid systems to function.
Contaminated runway: A surface where more than 25% of the required runway length within the width being used is covered by standing water deeper than 1/8 inch, slush, snow (compacted or loose), or ice. Contaminated conditions impose substantially greater stopping distance penalties than merely wet conditions.

Runway Condition Codes (RwyCC), formerly called Mu values, are the standardized numerical ratings from 0 (nil braking) to 6 (dry) used to communicate surface condition. Airport operations personnel assess the runway and report the RwyCC via the Runway Condition Report (RCR) in the ATIS or through NOTAMs. Pilots must use the current RwyCC to select the appropriate performance adjustment — never assume conditions are better than reported.

How Stopping Distance is Affected

Braking force depends on the coefficient of friction between the tire and the runway surface, plus the effectiveness of aerodynamic deceleration devices like thrust reversers and spoilers. On a dry runway, modern anti-skid systems exploit a high friction coefficient, and aircraft can achieve the short stopping distances published in the AFM. When the surface is wet, a thin layer of water temporarily lifts the tire from the pavement — a phenomenon called hydroplaning — and friction drops dramatically. On contaminated surfaces (slush or ice), the coefficient of friction may fall so low that anti-skid systems have little effective friction to exploit regardless of how aggressively they modulate brake pressure.

Dynamic hydroplaning onset speed is commonly approximated as 9 times the square root of the tire pressure in psi (9√P). For example, a tire inflated to 200 psi could begin to hydroplane at roughly 127 knots — a speed at or above the touchdown and rollout speeds of many transport-category jets, which is exactly why hydroplaning is treated as a serious operational threat rather than a remote possibility. Reverted rubber hydroplaning and viscous hydroplaning can occur at lower speeds and are particularly insidious because they are less predictable.

Thrust reversers and spoilers remain valuable on wet or contaminated runways, but their relative contribution increases because wheel braking is reduced. AC 91-79B emphasizes that crews should deploy all available deceleration devices promptly and aggressively on degraded surfaces, and should not rely solely on braking.

AC 91-79B Performance Adjustment Methodology

AC 91-79B uses a tiered approach based on the reported or expected Runway Condition Code to derive a Landing Distance Assessment (LDA). The core methodology works as follows:

Obtain the AFM dry landing distance for the actual landing weight, flap setting, and airport elevation and temperature — this is the certified baseline.
Apply the appropriate adjustment factor based on runway surface condition. For wet runways, a minimum factor of 1.15 (15% increase) is commonly referenced as a starting point, but individual operators and aircraft types may use higher factors specified by the manufacturer or the AFM supplement. For contaminated conditions, factors can range significantly higher — often 1.6 to as much as 3.5 or more for ice — depending on the specific RwyCC and aircraft type.
Compare the adjusted LDA to the available landing distance (Landing Distance Available, or LDA from the airport diagram). AC 91-79B recommends that the adjusted stopping distance must fit within the available runway with an adequate margin of safety.

It is critical to understand that these factors are not a guarantee of stopping within the calculated distance. They are risk-mitigation estimates. Actual stopping performance can differ based on pilot technique, anti-skid serviceability, wind changes after crossing the threshold, and the accuracy of the reported surface condition. AC 91-79B therefore also stresses the importance of crew technique: a stabilized approach at the correct speed and a firm, on-speed, on-target touchdown in the touchdown zone are prerequisites for achieving anything close to the published performance.

Why Stopping Margin Philosophy Matters

A key philosophical point in AC 91-79B is the distinction between regulatory minimums (Part 121 and Part 135 landing distance requirements) and operationally prudent margins. Under 14 CFR 121.195(b), an air carrier must demonstrate that the aircraft's required landing distance at the destination does not exceed 60% of the effective runway length — a dry-runway calculation. A separate requirement under 121.195(d) adds a further margin for wet runways, effectively requiring at least an additional 15% beyond the dry-runway landing distance when the runway is forecast to be wet. AC 91-79B encourages operators to think beyond these regulatory thresholds and to pursue greater margins when practicable, particularly when conditions are contaminated rather than merely wet.

The AC also highlights the risks of committing to land when conditions have deteriorated between dispatch and arrival. Crews must have a clearly briefed plan that includes go-around criteria: if the aircraft is not stabilized by a defined point, if the touchdown zone cannot be reached, or if actual braking action upon landing is substantially worse than reported, the appropriate response is the go-around and divert. This decision must be made proactively — not reactively — because once speed has dissipated and the airplane is deep in the rollout, a go-around is often no longer a viable option.

Key Numbers and Rules

Wet runway baseline factor: 1.15 times the AFM dry landing distance (minimum; actual AFM or operator data may be higher).
Contaminated runway factors: Vary widely by surface condition and aircraft type; always use AFM-specific data or operator-approved contaminated surface charts.
Hydroplaning onset approximation: 9 × √(tire pressure in psi); e.g., 200 psi tires → ~127 knots.
Part 121 landing distance rule: Under 121.195(b), required landing distance must not exceed 60% of the runway length available on a dry-runway basis; 121.195(d) requires an additional wet-runway margin of at least 15%.
Runway Condition Codes: Scale of 0 (nil/ice) to 6 (dry); RwyCC 5 = good-to-medium, RwyCC 3 = medium-to-poor, RwyCC 1 = poor.
Contaminated definition: More than 25% of required runway length covered by contaminant exceeding 1/8 inch depth.
Threshold crossing: Aim for the published threshold crossing height (typically 50 feet) at the correct Vref to maximize stopping distance in the touchdown zone.

Common Test Traps

Confusing wet and contaminated: Many applicants apply a wet-runway factor to what is actually a contaminated surface. Know the 25%/1/8-inch threshold that distinguishes the two categories.
Ignoring the RwyCC system: Mu values have been phased out in favor of Runway Condition Codes. ATP applicants are expected to know that RCR uses RwyCC (0–6), not a friction coefficient number.
Assuming thrust reversers replace braking: Thrust reversers and spoilers supplement braking; on contaminated runways they become relatively more important, but they do not substitute for proper AFM-based distance calculations.
Believing the regulatory minimum is the safety margin: The Part 121 60% rule is a regulatory floor, not an assurance of safety. AC 91-79B explicitly encourages greater margins, especially on contaminated runways.
Forgetting technique variables: Published landing distances assume a stabilized approach, threshold crossing at the correct height and speed, and a touchdown at the intended point. Any deviation — a long flare, high speed at the threshold, or late touchdown — can invalidate the calculated stopping margin regardless of the surface factor applied.