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For the purposes of following FAA regulations, when is a runway considered to be wet or slippery in the context of landing performance calculations? Specifically, what definitions or guidance on meaning for the terms "wet" or "slippery" should a pilot or dispatcher use in making the determination required by §135.385 or §121.195 as to the runway condition?

14 CFR 135.385 gives the following regulation pertaining to §135 operations of Turbine powered Large Transport Category airplanes:

Large transport category airplanes: Turbine engine powered: Landing limitations: Destination airports.

(d) Unless, based on a showing of actual operating landing techniques on wet runways, a shorter landing distance (but never less than that required by paragraph (b) of this section) has been approved for a specific type and model airplane and included in the Airplane Flight Manual, no person may take off a turbojet airplane when the appropriate weather reports or forecasts, or any combination of them, indicate that the runways at the destination airport may be wet or slippery at the estimated time of arrival unless the effective runway length at the destination airport is at least 115 percent of the runway length required under paragraph (b) of this section.

Likewise, 14 CFR 121.195 gives the following regulation pertaining to §135 operations of Turbine powered Large Transport Category airplanes

Airplanes: Turbine engine powered: Landing limitations: Destination airports.

(d) Unless, based on a showing of actual operating landing techniques on wet runways, a shorter landing distance (but never less than that required by paragraph (b) of this section) has been approved for a specific type and model airplane and included in the Airplane Flight Manual, no person may takeoff a turbojet powered airplane when the appropriate weather reports and forecasts, or a combination thereof, indicate that the runways at the destination airport may be wet or slippery at the estimated time of arrival unless the effective runway length at the destination airport is at least 115 percent of the runway length required under paragraph (b) of this section.

These regulations are effectively identical.

How is this determined? Does the FAA offer a definition (I have not found any such definitions in 14 CFR)? Does an AC give the appropriate definitions? Is the Operator expected to make this determination or to include a definition in the accepted or approved GOM? Should pertinent aircraft manufacturer's data be considered, for example if the AFM includes a definition of "wet runway"?

I am especially asking for any cases where the AFM does not give approved wet or contaminated landing data. Note the difference between approved and advisory AFM data. However, I am interested in knowing any and all ways in which the determination required by these regulations is complied with.

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  • $\begingroup$ Can you give an example of a situation in which you'd be unsure whether or not the runway counts as "wet or slippery"? The phrase seems pretty unambiguous to me. $\endgroup$ – Terran Swett Jan 19 at 16:07
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    $\begingroup$ @TerranSwett For example, certain Cessna Citation AFMs include an advisory definition applying to certain situations where "wet" is defined as when the runway surface appears reflective. This definition has no bearing whatsoever on the FAA regulations, but has gained a significant, if not widespread, hold in certain sectors of the pilot community as the applicable definition when determining whether a runway is wet in applying §135.385. Additionally, when the runway is "damp" but not "wet" according to other jurisdictions, the FAA does not provide such distinctions. $\endgroup$ – J Walters Jan 20 at 0:26
  • $\begingroup$ @ymb1 I don't like using the 'far' tags because they have not been the FARs for wire some time now. It is 14 CFR. I thought the '-operations' tag fit better since this is an operations question. $\endgroup$ – J Walters Jan 23 at 21:14
  • $\begingroup$ @JWalters: I'm actually with you on this. We can request on meta a rename for the FAR tags. And done: Rename request for FAR tags $\endgroup$ – ymb1 Jan 23 at 22:48
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I also could not find a definition for wet or slippery by the FAA, only for a contaminated runway:

CONTAMINATED RUNWAY− A runway is considered contaminated whenever standing water, ice, snow, slush, frost in any form, heavy rubber, or other substances are present. A runway is contaminated with respect to rubber deposits or other friction-degrading substances when the average friction value for any 500-foot segment of the runway within the ALD fails below the recommended minimum friction level and the average friction value in the adjacent 500-foot segments falls below the maintenance planning friction level.

(FAR/AIM Pilot/Controller Glossary, emphasis mine)

My Boeing 737 FCTM does however define:

A slippery surface is any surface where the braking capability is less than that on a dry surface. Therefore, a surface is considered “slippery” when it is wet or contaminated with ice, standing water, slush, snow or any other deposit that results in reduced braking capability.

(Boeing 737 NG FCTM 2.13 Ground Operations, emphasis mine)

This sounds quite similar to the FAA definition of contaminated runways, so I would assume the same definition applies to wet or slippery in §121 and §135 operations as well.

The FCTM further says:

Slippery Runway Landing Performance

Slippery/contaminated runway advisory information is based on an assumption of uniform conditions over the entire runway. This means a uniform depth for slush/standing water for a contaminated runway or a fixed braking coefficient for a slippery runway. The data cannot cover all possible slippery/contaminated runway combinations and does not consider factors such as rubber deposits or heavily painted surfaces near the end of most runways. With these caveats in mind, it is up to the operator to determine operating policies based on the training and operating experience of their flight crews.

(Boeing 737 NG FCTM A.27 Landing Roll - Factors Affecting Landing distance)

So it seems to be (at least in part) up to the operator to provide clear definitions on when exactly a runway should be considered slippery.


The Flight Safety Foundation has collected some more definitions, although from Europe, not the US:

Dry Runway

The European Join Aviation Authorities (JAA) defines dry runway as "one which is neither wet nor contaminated, and includes those paved runways which have been specially prepared with grooves or porous pavement and maintained to retain 'effectively dry' braking action event when moisture is present."

Damp Runway

JAA says that a runway is considered damp "when the surface is not dry, but when the moisture on it does not give it a shiny appearance."

Wet Runway

JAA says that a runway is considered wet "when the runway surface is covered with water, or equivalent, less than specified [for a contaminated runway] or when there is sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water."

Contaminated Runway

JAA says that a runway is contaminated "when more than 25 percent of the runway surface are (whether in isolated areas or not) within the required length and width being used is covered by the following:

  • "Surface water more than 3.0 mm [millimeters] (0125 in [inch]) deep, or by slush or loose snow, equivalent to more than 3.0 mm (0.125 in) of water;

  • "Snow which has been compressed into a solid mass which resists further compression and will hold together or break into lumps if picked up (compacted snow); or,

  • "Ice, including wet ice."

(FSF ALAR Briefing Note 8.5 - Wet or Contaminated Runways)

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    $\begingroup$ That is helpful information, though by no means definitive. The Contaminated Runway definition is probably the most helpful. I don't recall seeing that definition in recent memory. $\endgroup$ – J Walters Jan 19 at 11:46
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The FAA does not appear to have issued a regulatory definition for wet or slippery, nor included such definitions in any current advisory circulars.

However, in addition to the information provided by @Bianfable, certain guidance has been provided which may be informative.

The original issuance of AC 91-79 (now superseded), Runway Overrun Prevention, included the following note on page 3:

NOTE: The FAA has taken the position that a runway does not need to be reflective to be considered wet. If a runway is contaminated or not dry, that runway is considered wet.

If that note remained in the current copy, the definition for "wet runway" would simply be: "not dry".

However, the superseding version of that advisory, AC 91-79A Mitigating the Risks of a Runway Overrun Upon Landing, issued in 2014, left out that note. The new version does include the following note:

NOTE: For example, if the ramp is too damp to sit on due to morning dew, the runway is probably the same. The pilot should use the wet or contaminated performance data

SAFO 06012, Landing Performance Assessments at Time of Arrival (Turbojets), issued by the FAA in 2006, includes the following definition, for what it is worth. Note that this document also includes the caveat that, "The following definitions are specific to this guidance and may differ with those definitions contained in other published references."

i. Runway Surface Conditions. The state of the surface of the runway: either dry, wet, or contaminated. A dry runway is one that is clear of contaminants and visible moisture within the required length and the width being used. A wet runway is one that is neither dry nor contaminated. For a contaminated runway, the runway surface conditions include the type and depth (if applicable) of the substance on the runway surface, e.g., standing water, dry snow, wet snow, slush, ice, sanded, or chemically treated.

My search for any pertinent Letters of Interpretation has not yielded anything helpful.

Taken together, the conservative interpretation appears to be that if a runway is not dry, that runway should be considered to be wet, or worse, for the purposes of meeting the regulatory requirements in question.

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