Source: http://code7700.com/contaminated_runways.htm
Timestamp: 2019-04-25 22:08:38+00:00

Document:
Take, for example, the December 8, 2005 crash of Southwest Airlines Flight 1248. I flew in as a passenger on a SWA flight the week prior and witnessed the pilot grease the landing a little long and use no more than idle reverse. A month after the crash I had another flight on the same airline and same runway: firm landing, full reverse. Much better. Six months later? Back to their old habits.
The next year, the FAA issued SAFO 06012, advocating a 15% margin between the expected actual airplane landing distance and landing distance available at the time of arrival. Ten years after that, the FAA issued SAFO 16009, implementing a new way of evaluating runway condition.
But, despite these efforts, we still have aircraft failing to stop on the confines of a wet or contaminated runway. But even if you are wise and experienced in these things, there have been changes you need to know about. So let's look at the problem anew.
Figure: Photograph of the accident airplane in the roadway intersection, from NTSB Report, Figure 1.
When is a runway contaminated? When is a runway wet? Here's a new one: when is a runway damp? These may seem like silly distinctions, but they all matter when it comes to converting the runway condition report into aircraft performance numbers.
[Aeronautical Information Manual, Pilot/Controller Glossary] 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.
So that's "contaminated." What about "wet?"
[AC 91-79A, App. 3, ¶5.] The standard dry runway charts are for a totally dry runway. 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.
[AC 150/5200-30D, ¶1.12.17] Runway Condition Code (RwyCC). Runway Condition Codes describe runway conditions based on defined contaminants for each runway third. Use of RwyCCs harmonizes with ICAO Annex 14, providing a standardized “shorthand” format (e.g., 4/3/2) for reporting. RwyCCs are used by pilots to conduct landing performance assessments.
[AC 150/5200-30D, ¶1.12.8] FICON (Field Condition Report). A FICON is a Notice to Airmen (NOTAM) generated to reflect pavement surface conditions on runways, taxiways, and aprons and Runway Condition Codes (RwyCCs) if greater than 25 percent of the overall runway length and width coverage or cleared width of the runway is contaminated.
[AC 150/5200-30D, ¶1.12.4] Contaminated Runway. For purposes of generating a runway condition code and airplane 1.12.4.1performance, a runway is considered contaminated when more than 25 percent of the overall runway length and width coverage or cleared width is covered by frost, ice, or any depth of snow, slush, or water.
[AC 150/5200-30D, ¶1.12.5] Dry Runway/Pavement. Use the term “DRY” to describe runway/pavement surfaces that are neither wet nor contaminated. A FICON NOTAM must not be originated for the sole purpose of reporting a dry runway. A dry runway surface should be reported only when there is need to report conditions on the remainder of the surface.
[AC 150/5200-30D, ¶1.12.23] Wet Runway. A runway is wet when it is neither dry nor contaminated. For purposes of condition reporting and airplane performance, a runway can be considered wet when more than 25 percent of the overall runway length and width coverage or cleared width being used is covered by any visible dampness or water that is 1/8-inch (3 mm) or less in depth.
[AC 150/5200-30D, ¶1.12.6] Dry Snow. Dry snow is snow that has insufficient free water to cause it to stick together. This generally occurs at temperatures well below 32° F (0° C). If when making a snowball, it falls apart, the snow is considered dry.
[AC 150/5200-30D, ¶1.12.24] Wet Snow. Wet snow is snow that has grains coated with liquid water, which bonds the mass together, but that has no excess water in the pore spaces. A well-compacted, solid snowball can be made, but water will not squeeze out.
[AC 150/5200-30D, ¶1.12.20] Slush. Slush is snow that has water content exceeding a freely drained condition such that it takes on fluid properties (e.g., flowing and splashing). Water will drain from slush when a handful is picked up. This type of water-saturated snow will be displaced with a splatter by a heel and toe slap-down motion against the ground.
[AC 150/5200-30D, ¶1.12.21] Water. Water is the liquid state of water. For purposes of condition reporting and airplane performance, water is greater than 1/8-inch (3mm) in depth.
[AC 150/5200-30D, ¶1.12.10] Ice. Ice is the solid form of frozen water including ice that is textured (i.e., rough or scarified ice). Note: A layer of ice over compacted snow must be reported as ice only.
[AC 150/5200-30D, ¶1.12.22] Wet Ice. Wet ice is ice that is melting, or ice with a layer of water (any depth) on top.
compacted snow or ice, including wet ice.
Dry runway. A dry runway is one which is clear of contaminants and visible moisture within the required length and the width being used.
Wet runway. A runway that is neither dry nor contaminated.
Note 1.— In certain situations, it may be appropriate to consider the runway contaminated even when it does not meet the above definition. For example, if less than 25 per cent of the runway surface area is covered with water, slush, snow or ice, but it is located where rotation or lift-off will occur, or during the high speed part of the take-off roll, the effect will be far more significant than if it were encountered early in take-off while at low speed. In this situation, the runway should be considered to be contaminated.
Note 2.— Similarly, a runway that is dry in the area where braking would occur during a high speed rejected take-off, but damp or wet (without measurable water depth) in the area where acceleration would occur, may be considered to be dry for computing take-off performance. For example, if the first 25 per cent of the runway was damp, but the remaining runway length was dry, the runway would be wet using the definitions above. However, since a wet runway does not affect acceleration, and the braking portion of a rejected take-off would take place on a dry surface, it would be appropriate to use dry runway take-off performance.
The ICAO definition in Annex 6, Part I, applies only to commercial aviation. The same definition does not appear in Part II for general aviation.
"Damp runway". A runway is considered damp when the surface is not dry, but when the moisture on it does not give it a shiny appearance.
"Dry runway". A dry runway is 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 even when moisture is present.
Not every manufacturer allows you to assume a grooved runway is "effectively dry."
"Wet runway." A runway is considered wet when the runway surface is covered with water, or equivalent, less than specified in subparagraph (a)(2) above or when there is sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water.
The use of the term "reflective" is quite helpful, but it seems only EASA uses it any more.
U.S. rules tend to lag (but eventually follow) ICAO rules, which tend to be less restrictive than EASA rules. I think it is helpful to know the most restrictive rules if you fly all over the world. Things get complicated by the aircraft manufacturers, but this is a way to begin getting a handle on all this.
A runway is dry . . .
. . . if if it is free of visible moisture.
A runway is "effectively dry". . .
. . . if it is grooved or a specially prepared porous pavement and maintained to retain dry braking action even when moisture is present.
A runway is damp . . .
. . . if there is a moisture layer that is not shiny.
A runway is wet . . .
. . . if it is covered in water (or the equivalent) and no more than 25% of that is no more than 1/8" (3 mm) of water or the equivalent in slush or loose snow.
A runway is contaminated . . .
. . . if more than 25% of it is covered by water more than 1/8" (3 mm) water water or the equivalent in slush or loose snow.
. . . if more than 25% of it is covered in compressed or compacted snow, ice, or wet ice.
You are permitted to consider only that portion of the runway you need. They can, for example, plow only the center and a limited distance. You should be careful that the width will permit full control of the airplane in the event of an engine loss.
We pilots are constrained by the rules as defined by governments, of course. But we are also constrained by how our aircraft manufacturers apply these rules. There are even differences between aircraft types from the same manufacturer. You just have to know the books for the airplanes you fly. Be aware, however, that aircraft manufacturer data can be optimistic.
Aircraft manufacturers have an additional challenge in translating the extrapolated performance data required by aircraft certification rules and the various definitions into performance numbers we pilots can use. Bombardier and Gulfstream, for example, invent a few new definitions that you won't find in use by the various regulatory agencies. Loose wet snow and loose dry snow, for example. While it may seem nonsensical, it does help convert reports into other numbers that can be used to predict aircraft performance. You can argue about which manufacturer has better numbers but it doesn't matter. It is up to us as pilots to understand the methodology behind the definitions and the reliability of the numbers that result.
A runway is considered to be contaminated by compacted snow when covered by snow which has been compacted into a solid mass which resists further compression and will hold together or break into lumps if picked up.
A runway surface condition where braking action is expected to be very low, due to the presence of ice.
Dry runway: A runway is considered dry if it is clear of visible moisture. A damp runway, which has a moisture layer that is nonreflective, is also considered dry.
Wet runway: A runway is considered to be wet when there is sufficient moisture to cause it to appear reflective but the depth of the water is not more than 3 mm (0.125 in.).
Contaminated runway: A runway surface is considered contaminated when more than 25% of the runway surface area is covered with standing water, slush, loose snow (dry or wet), compacted snow or ice.
Standing water: Water depth is greater than 3 mm (.125 in).
Slush: Partly melted snow or ice with a high water content, from which water can readily flow.
Compacted snow: Snow which has been compressed into a solid mass such that the airplane wheels will run on the surface without causing significant rutting.
Loose snow: Snow on the runway surface which had not been compressed by vehicle or aircraft traffic. Loose snow can consist of either dry snow or wet snow.
Dry loose snow: Fresh snow that can be blown, or, if compressed by hand, will fall apart upon release.
Ice: Water which has frozen on the runway surface, including the condition where compacted snow transitions to a polished ice surface.
The μB [wheel braking coeeficient] deficit observed in the events considered in this paper makes the stopping performance of the airplanes involved more consistent with AFM landing distances for runways contaminated with standing water, than for runways that are merely “wet.” For this reason, observers may be (understandably) quick to conclude that the runways involved must be more contaminated (contain a greater depth of water) than assumed in the wet runway models underlying the AFM distances. However, examination of the runways involved, including an examination of their macrotexture and cross-slope, do not support a conclusion that the runways could have been flooded given the rainfall rates during the events in question. Furthermore, the μB actually achieved in a number of these events is consistent with the NASA CFME model for a wet – not flooded – runway. In other words, the NASA CFME model for a wet runway is more conservative than those underlying the airplane AFMs, and moreover, matches the actual airplane performance achieved during the events better than those underlying the AFMs.
Your contaminated data appears to be okay; your merely "wet" data appears to be optimisitc.
The Runway Condition Assement Matrix (RCAM) is the intended fix.
In addition, AC 25-32, Landing Performance Data for Time-of-Arrival Landing Performance Assessments, incorporates many of the recommendations of the TALPA ARC, including the RCAM. However, wet (not flooded) runways can either be classified as RCAM code 5 (associated with μB levels defined by the §25.109(c) model), or as RCAM code 3 “slippery when wet” (for which μB = 0.16 constant). The “slippery when wet” designation applies when the average CFME μ of the runway falls below the AC 150/5320-12C MIN LEVEL – a condition that only one of the runways examined in this paper met. The actual μB levels achieved on the other runways was between those specified by RCAM codes 5 and 3. Consequently, the RCAM as currently specified in Ref. 32 will likely overestimate the μB that can actually be achieved on operational, wet runways.
Airport operations on wet loose snow and slush-covered runways are treated in a similar manner as operations on a runway contaminated with standing water. The only difference is that because of different densities, a measurable depth of wet loose snow (50% the density of water) or slush (85% the density of water) must first be converted to an equivalent depth of standing water. This is accomplished by using the conversion data presented in Table 1a below. The data in this table can be interpolated for intermediate depths of contamination, e.g., 15 mm (0.6 in) of wet loose snow is equivalent to 7.5 mm (0.3 in) of standing water. Otherwise, hydroplaning speeds, tire displacement drag and spray impingement drag on runways with wet loose snow and slush are determined in a manner that is similar to a runway contaminated with standing water. This equivalent water depth is the value to be used as the Runway Surface Contamination (RSC) depth when computing takeoff performance.
EASA has developed separate standards for classifying dry loose snow and wet loose snow. Under the new EASA contaminated runway rules for runways contaminated with dry loose snow, it is no longer necessary to account for spray impingement drag and hydroplaning speeds. Therefore, it is possible to takeoff and land in a much greater depth of dry loose snow than wet loose snow. In addition, dry loose snow is assumed to have a density that is only 20% the density of water. In consideration of these differences, Gulfstream has determined that it is conservative to divide the effective dry loose snow depth by 6 in order to establish the equivalent water depth to use when determining takeoff performance on a runway contaminated by dry loose snow. Hence, 21 mm (0.84 in.) of dry loose snow converts to 3.5 mm (0.14 in) of standing water.
Photo: Equivalent depth for slush/loose snow, from Gulfstream G450 Operational Information Supplement, G450-OIS-02, Table 1a.
Whenever you land on a wet or contaminated runway, you may be adding to the airport's knowledge about that runway's braking effectiveness. By the same token the aircraft that landed just before you should add to your personal knowledge about what is ahead of you. But you need to weigh carefully the type of aircraft making the report. A small, light aircraft may have much more difficulty on a slippery surface than a heavier jet. I've never seen this fact quantified, but it is something to be wary of.
[SAFO 06012, ¶5.e.] Likewise, because pilot braking action reports are subjective, flightcrews must use sound judgment in using them to predict the stopping capability of their airplane. For example, the pilots of two identical aircraft landing in the same conditions, on the same runway could give different braking action reports. These differing reports could be the result of differences between the specific aircraft, aircraft weight, pilot technique, pilot experience in similar conditions, pilot total experience, and pilot expectations. Also, runway surface conditions can degrade or improve significantly in very short periods of time dependent on precipitation, temperature, usage, and runway treatment and could be significantly different than indicated by the last report. Flightcrews must consider all available information, including runway surface condition reports, braking action reports, and friction measurements.
Generally speaking, a lighter aircraft will have a more difficult time stopping than a heavier airplane. I once landed behind a light twin aircraft that had called the braking action "poor" on hard, compacted snow at Gander Airport. I thought the braking action was "good" in my Gulfstream III.
As hard as we've tried over the years, we can't seem to come up with a definitive way of translating the condition of a runway into a number that we can plug into tables and charts to figure out our likelihood of stopping the airplane in the amount of runway in front of us. Our most recent attempt is known as a "Runway Condition Code" taken from a "Runway Condition Assessment Matrix." So that's what is shown here. (If this is news to you, it was first unveiled with SAFO 16009.) I'll follow that with some of the older systems in case you run into them.
[SAFO 06012, ¶5.e.] Runway surface conditions may be reported using several types of descriptive terms including: type and depth of contamination, a reading from a runway friction measuring device, an airplane braking action report, or an airport vehicle braking condition report. Unfortunately, joint industry and multi-national government tests have not established a reliable correlation between runway friction under varying conditions, type of runway contaminants, braking action reports, and airplane braking capability. Extensive testing has been conducted in an effort to find a direct correlation between runway friction measurement device readings and airplane braking friction capability. However, these tests have not produced conclusive results that indicate a repeatable correlation exists through the full spectrum of runway contaminant conditions. Therefore, operators and flightcrews cannot base the calculation of landing distance solely on runway friction meter readings.
As a result of this SAFO, the Runway Condition Code (RCC) was invented . . .
[AC 150/5200-30D, ¶5.1.3] To comply with § 139.339, the airport operator must utilize the NOTAM system as the 5.1.3primary method for collection and dissemination of airport information to air carriers and other airport users. When disseminating airport condition information there are three methods available to airport operators. The first and preferred method is NOTAM Manager, a direct-entry system.
If you hear your runway is reporting a "3/3/3" is that good? Well it means all three segments of the runway are "medium" so I guess that's better than a "2/2/2" but not as good as a "5/5/5" as you might suspect. But this is the system we have now and it is actually better than those "mu" or "RCR" numbers of the past in that they are supposed to be more accurate. I'm not so sure, but here is what the book says followed by some added information from the airport management side of the house.
Assessments for each zone (see 4−3−9c1(c)) will be issued in the direction of takeoff and landing on the runway, ranging from “1” to “6” to describe contaminated surfaces.
Photo: Runway Condition Assessment Matrix, AIM, figure 4-3-7.
It seems like we've replaced highly subjective words ("good," "medium," "poor," and "nil") for seven numbers that may seem equally subjective. It also seems we gave up the science of the "Mu" number that is still used with the ICAO SNOWTAM system. But the change actually has more good than bad. We pilots don't understand the process as well as we should and are too quick to hear the three magic numbers and stop listening. If you understand how the numbers are produced and the importance of the language that follows, you might have some very useful decision making tools. AC 150/5200-30D is what airport operators use to manage runway and taxiway reporting and snow removal. It has a lot of good information for pilots who want to "read between the lines" when it comes to an RCC.
[AC 150/5200-30D, ¶1.2.1] Following the overrun accident of a Boeing-737 in December of 2005, the FAA found that the current state of the industry practices did not have adequate guidance and regulation addressing operation on non-dry, non-wet runways, i.e., contaminated runways. As such, the FAA chartered an Aviation Rulemaking Committee (ARC) to address Takeoff and Landing Performance Assessment (TALPA) requirements for the appropriate parts 23, 25, 91 subpart K, 121, 125, 135, and 139. In formulating recommendations, it became clear to the ARC that the ability to communicate actual runway conditions to the pilots in real time and in terms that directly relate to expected aircraft performance was critical to the success of the project.
The ARC got it mostly right but we need to keep in mind this was an industry-wide effort and though the business aviation community was represented, the largest users (the airlines) obviously carried more weight. So we got a system that is tailored towards large airports with sophisticated snow removal equipment. The system is also "one size fits all," so terms like "ice" on a runway in Alaska (dry and compacted and therefore rough) is equal to "ice" in New England (covered with a film of supercooled water). So let's tackle this. Is the RCC a subjective measure? To find out, we should look at the RCAM presented above in AIM (a pilot's manual) to what the airport operator is given. But you can't do that unless you understand μ . . .
So What is Mu (μ) Any Way?
A Newton is the International System of Units (SI) derived unit of force. One Newton is equal to the force needed to accelerate a mass of one kilogram one meter per second per second. For more about this: Mechanics.
There are several kinds of friction but there are two in particular of interest to us pilots: static and dynamic. If the tires of your wheels are making good contact with the surface and not sliding in relation to the surface, you have static friction. If, on the other hand, the wheels are skidding or hydroplaning, there is motion of the tire in relation to the surface and that is dynamics friction. The only way to stop the airplane is to maintain static friction, so that's what we will concentrate on here.
Static friction: the force that resists movement of an object pressed against a surface.
Coefficient of static friction (μs) is the minimum force required to get an object to move on a surface divided by the forces holding them together.
Photo: When μ equals 1 it takes as much effort to lift the object as to push it, from Eddie's notes.
Photo: When μ equals 0 pushing is effortless, from Eddie's notes.
So it would seem that having a good μ for a surface would make everything easy. The problem is that we don't really have an easy way of figuring out μ with absolute precision. There are two ways around this problem. You can drag the object with a force scale and divide that by the object's weight, or you can come up with some kind of gizmo to approximate everything.
Photo: Simple accelerometer, from Eddie's notes.
MU (friction) values range from 0 to 100 where zero is the lowest friction value and 100 is the maximum friction value obtainable. For frozen contaminants on runway surfaces, a MU value of 40 or less is the level when the aircraft braking performance starts to deteriorate and directional control begins to be less responsive. The lower the MU value, the less effective braking performance becomes and the more difficult directional control becomes.
When the MU value for any one-third zone of an active runway is 40 or less, a report should be given to ATC by airport management for dissemination to pilots.
No correlation has been established between MU values and the descriptive terms "good," "fair," "poor," and "nil" used in braking action reports.
Even back then, the U.S. Aeronautical Information Manual seemed to discount any connection between Mu and good/fair/poor/nil qualifiers and you hardly hear the term "mu" in the United States any more. But airport operators use it. How?
[AC 150/5200-30D, ¶5.2] FAA-approved friction measuring equipment may be employed to help in determining the effects of friction-enhancing treatments, in that it can show the trend of a runway as to increasing or decreasing friction. Airport operators should not attempt to correlate friction readings (Mu numbers) to Good/Medium (previously known as Fair)/Poor or Nil runway surface conditions, as no consistent, usable correlation between Mu values and these terms has been shown to exist to the FAA’s satisfaction. It is important to note that while manufacturers of the approved friction measuring equipment may provide a table that correlates braking action to Mu values, these correlations are not acceptable to the FAA. To ensure that data collected are accurate, qualified personnel should use FAA-approved equipment and follow the manufacturer’s instructions for use. Note: It is no longer acceptable to report or disseminate friction (Mu) values to aircraft operators. This includes informal dissemination outside of the NOTAM system. In support of this change the NOTAM system will no longer allow for the reporting of this information. Airplane braking performance cannot be directly related to Friction (Mu) values. Runway Condition Codes, which will be included in the runway condition NOTAM, where applicable, are directly relevant to the determination of required landing distances.
So the airport operator cannot report the Mu (μ) value to pilots or the NOTAM system, but they can use it when building their Runway Condition Codes.
[AC 150/5200-30D, ¶1.11] There are two basic types of friction measuring equipment that can be used for conducting friction surveys on runways during winter operations: Continuous Friction Measuring Equipment (CFME) and Decelerometers (DEC).
Continuous Friction Measuring Equipment (CFME). CFME devices are recommended (over Decelerometers) for measuring friction characteristics of pavement surfaces covered with contaminants, as they provide a continuous graphic record of the pavement surface friction characteristics with friction averages for each one-third zone of the runway length.
Decelerometers. Decelerometers are recommended (over CFMEs) for airports where the longer runway downtime required to complete a friction survey is unacceptable and for busy airports where it is difficult to gain access to the full length of a runway crossed by another runway. Decelerometers should be of the electronic type due to the advantages noted below. Mechanical decelerometers may be used, but should be reserved as a backup. Airports having only mechanical devices should plan to upgrade as soon as possible. Neither type of decelerometer will provide a continuous graphic record of friction for the pavement surface condition. They provide only a spot check of the pavement surface. On pavements with frozen contaminant coverage of less than 25 percent, decelerometers are used only on the contaminated areas. For this reason, a survey taken under such conditions will result in a conservative representation of runway braking conditions. This should be considered when using friction values as an input into decisions about runway treatments. In addition, any time a pilot may experience widely varying braking along the runway, it is essential that the percentage of contaminant coverage be noted in any report.
Electronic decelerometers eliminate potential human error by automatically computing and recording friction averages for each one-third zone of the runway. They also provide a printed record of the friction survey data.
Mechanical decelerometers may be used as a backup to an electronic decelerometer. The runway downtime required to complete a friction survey will be longer than with an electronic decelerometer. Mechanical decelerometers do not provide automatic friction averages or a printed copy of data.
We used to do these as pilots in the Air Force at some northern bases. The unit is mounted in the vehicle so as to measure longitudinal deceleration. You fire up the vehicle to a certain speed and jam on the brakes. The way I recall it, you floored it until the speedometer was pegged going as fast as you can on a very slippery surface, and then stomped on the brakes and tried not to soil your pants. Just to be sure, I asked someone who does these for a living and he said they go to 20 mph. Some of the meters output Mu (μ) directly, others put out G-units that have to be converted using a table.
Asking around various airport managers, there is a lot of technique involved to get consistent results. But the system can put out reliable deceleration values to enter into their version of the RCAM table (shown below) with a μ value. But there is more to it than that.
[AC 150/5200-30D, ¶5.2.2.3.1] Lateral Location. On runways that serve primarily narrow-body airplanes, runway friction surveys should be conducted approximately 10 feet (3 m) from the runway centerline. On runways that serve primarily wide-body airplanes, runway friction surveys should be conducted approximately 20 feet (6 m) from the runway centerline. Unless surface conditions are noticeably different on the two sides of the runway centerline, only one survey is needed, and it may be conducted on either side.
[AC 150/5200-30D, ¶5.3.1] The RCAM is the method by which an airport operator reports a runway surface assessment when contaminants are present. Use of the RCAM is only applicable to paved runway surfaces. Once an assessment has been performed, the RCAM defines the format for which the airport operator reports and receives a runway condition “Code” via the NOTAM System. The reported information allows a pilot to interpret the runway conditions in terms that relate to airplane performance. This approach is a less subjective means of assessing runway conditions by using defined objective criteria. Aircraft manufacturers have determined that variances in contaminant type, depth, and air temperature can cause specific changes in aircraft braking performance. At the core of the RCAM is its ability to differentiate among the performance characteristics of given contaminants.
Photo: Runway Condition Assessment Matrix, AC 150/5200-30D, Table 5-2.
[AC 150/5200-30D, ¶5.1.4] Conditions Acceptable to Use Decelerometers or Continuous Friction Measuring Equipment to Conduct Runway Friction Surveys on Frozen Contaminated Surfaces.
Ice or wet ice. Ice that is melting or ice with a layer of water (any depth) on top. The liquid water film depth of .04 inches (1 mm) or less is insufficient to cause hydroplaning.
Compacted snow at any depth.
Dry snow 1 inch (25.4 mm) or less.
Wet snow or slush 1/8 inch (3 mm) or less.
5.1.4.2. It is not acceptable to use decelerometers or continuous friction measuring equipment to assess any contaminants outside of these parameters.
[AC 150/5200-30D, ¶5.3.1] The RCAM is the method by which an airport operator reports a runway surface assessment when contaminants are present. Use of the RCAM is only applicable to paved runway surfaces. Once an assessment has been performed, the RCAM defines the format for which the airport operator reports and receives a runway condition “Code” via the NOTAM System. The reported information allows a pilot to interpret the runway conditions in terms that relate to airplane performance. This approach is a less subjective means of assessing runway conditions by using defined objective criteria.
Airport operators normally access the system through the "NOTAM Manager" application. The first question will be "Is greater than 25% of the overall runway length and width, or cleared width (if not cleared from edge to edge), contaminated?" If the answer is no the only option will be to report contaminant percentage, type, and depth, when applicable, for each third of the runway, as well as any treatment. No Runway Condition Code is reported.
So you've got more than 25% coverage. The next thing to do is look at the left column on the RCAM and look for the type and depth of contaminant as well as temperature. Do that for each third of the runway. If only a portion of the runway is cleared (such as the center 75'), you only have to consider that portion. Unless you have upgrades and downgrades, the Runway Condition Codes will enter the system. Notice you didn't have to do anything with the decelerometer.
If the airport operator thinks the RCC can be upgraded or should be downgraded, they can take a drive with an approved decelerometer. But only RCCs of 1 or 2 can be upgraded and even then they can only be upgraded up to a 3. You can take the decelerometer's μ reading to upgrade or downgrade the RCC. What about pilot reports? Pilot reports can only be used to downgrade an RCC and only for the portion of the runway experienced.
[AC 150/5200-30D, ¶5.3.3.2.] Downgrade Assessment Criteria. When data from the shaded area in the RCAM (i.e., CFME/deceleration devices, pilot reports, or observations) suggest conditions are worse than indicated by the present contaminant, the airport operator should exercise good judgment and, if warranted, report lower runway condition codes than the contamination type and depth would indicate in the RCAM. While pilot reports (PIREPs) of braking action provide valuable information, these reports rarely apply to the full length of the runway as such evaluations are limited to the specific sections of the runway surface in which wheel braking was utilized. It is not appropriate to use downgrade assessment criteria to upgrade contaminant based assessments of condition codes (e.g., from 2 to 3).
[AC 150/5200-30D, ¶5.3.3.2.1] The correlation of the Mu (μ) values with runway conditions and condition codes in the RCAM are only approximate ranges for a generic friction measuring device and are intended to be used for an upgrade or downgrade of a runway condition code. Airport operators should use their best judgment when using friction measuring devices for downgrade assessments, including their experience with the specific measuring devices used.
[AC 150/5200-30D, ¶5.3.3.2.2] Vehicle Deceleration or Directional Control Observation. This column is used to correlate estimated vehicle braking experienced on a given contaminant.
[AC 150/5200-30D, ¶5.3.3.2.3] Pilot Reported Braking Action. This is a report of braking action on the runway, by a pilot, providing other pilots with a degree/quality of expected braking. The braking action experienced is dependent on the type of aircraft, aircraft weight, touchdown point, and other factors.
Good: Braking deceleration is normal for the wheel braking effort applied, and directional control is normal.
Good-to-Medium: Braking deceleration or directional control is between good and medium braking action.
Medium: Braking deceleration is noticeably reduced for the wheel braking effort applied, or directional control is noticeably reduced.
Medium-to-Poor: Braking deceleration or directional control is between medium and poor.
Poor: Braking deceleration is significantly reduced for the wheel braking effort applied, or directional control is significantly reduced.
Nil: Braking deceleration is minimal to non-existent for the wheel braking effort applied, or directional control is uncertain.
There are a few other systems out there that you should know about. In fact, your Aircraft Flight Manual may depend on them.
You would think that if anyone had a firm grasp on how to measure braking action on a contaminated runway, it would be the Canadians. The CRFI is a pretty good measure, the only real problem is some of the numbers are quite subjective. Dry snow on pavement can be anywhere from a "nil" to "dry." But it does give you another data point and it does have some science behind it.
Photo: Expected Range of CRFI by Surface Type, from TC AIM, Table 4a.
[Transport Canada Aeronautical Information Manual, ¶1.1.4] Many airports throughout Canada are equipped with mechanical and electronic decelerometers which are used to obtain an average of the runway friction measurement. The average decelerometer reading of each runway is reported as the Canadian Runway Friction Index (CRFI). Experience has shown that results obtained from the various types of decelerometers on water and slush are not accurate, and the CRFI will not be available when these conditions are present.
Runway friction tester, from ICAO Doc 9137, Part 2, figure 5-6.
Canada is one of the few countries that has stepped up to the plate and tried to quantify runway friction values and their impact on runway performance. More about this: Runway Friction. Even if the airport you are using doesn't have these decelerometers, the CRFI can be of use to you.
The ICAO solution to measuring runway friction comes from a very long time ago and had been considered the standard for a very long time because it had numbers and a decimal place! It had to be good!
Figure: SNOWTAM Format, from ICAO Annex 15 Appendix 2.
A SNOWTAM is a "Special Series NOTAM"
[ICAO Annex 15] ¶5.2.3] Information concerning snow, slush, ice and standing water on aerodrome/heliport pavements shall, when reported by means of a SNOWTAM, contain the information in the SNOWTAM Format in Appendix 2.
It looks suspiciously like the "Mu" (μ) system the U.S. gave up on. See Mu (μ), above.
Our Air Force flight manual charts had performance numbers with Runway Condition Readings (RCR). An RCR of 23 was considered dry, 9 was wet, and 4 was icy. The exact number tends to change a few points here and there, but those are close. If you are at an airport with military airplanes, they might just have an RCR for you. Can you enter your chart with an RCR? Probably not. But it is more information than you had before you asked.
[U.S. FAA Order JO 7110.10W §4-4-3 ¶11] USAF has established RCR procedures for determining the average deceleration readings of runways under conditions of water, slush, ice, or snow. The use of RCR code is dependent upon the pilot's having a stopping capability chart specifically applicable to his/her aircraft. USAF offices furnish RCR information at airports serving USAF and ANG aircraft.
You may have heard that you can assume a wet runway that is grooved is "effectively dry." In fact, the EU says just that. (See the section on contamination defined, above. You may have also heard that you cannot. (As applied by many aircraft manufacturers.) As usual, there are caveats for both crowds. If you are taking credit, you should know that a wet runway is not always "effectively dry." If you aren't taking credit, you should realize that there are times when it is. Confused? Read on. . .
The definitive study on grooved runways was done in 1968 by the NASA Langley Research Center. Regardless of how your aircraft manuals stand on the subject of grooved runways, the results of the study should impact how you interpret the charts in those manuals.
Figure: Test aircraft, from Pavement Grooving and Traction Studies, page 121, figure 2.
Landing distances on a wet, grooved runway are remarkably reduced almost to the point of being dry, until the water depth is greater than 0.1 inch or if there is any slush. At that point the distances are improved but not nearly as much.
Balanced field lengths are "essentially dry" for grooved runways in a "wet and puddled" condition. If the runway is slush covered, the advantages of runway grooves are only "slight."
For more about this: Grooved Runways.
If you are a commercial operator, you need to consider your landing distance at your planned destination before you takeoff. That includes the impact of a less than dry runway based on the weather predictions existing when you leave. Then you need to apply a factor to that. How much of a factor? It depends.
If you are not a commercial operator, your pre-departure considerations are the same as shown below, in En Route Decision Making.
[14 CFR 135, ¶135.385] Large transport category airplanes: Turbine engine powered: Landing limitations: Destination airports.
(1) The airplane is landed on the most favorable runway and in the most favorable direction, in still air.
(2) The airplane is landed on the most suitable runway considering the probable wind velocity and direction and the ground handling characteristics of the airplane, and considering other conditions such as landing aids and terrain.
(c) A turbopropeller powered airplane that would be prohibited from being taken off because it could not meet paragraph (b)(2) of this section, may be taken off if an alternate airport is selected that meets all of this section except that the airplane can accomplish a full stop landing within 70 percent of the effective length of the runway.
(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.
(e) A turbojet airplane that would be prohibited from being taken off because it could not meet paragraph (b)(2) of this section may be taken off if an alternate airport is selected that meets all of paragraph (b) of this section.
(1) The operation is permitted by an approved Destination Airport Analysis in that person's operations manual.
(i) The airplane is landed on the most favorable runway and in the most favorable direction, in still air.
(ii) The airplane is landed on the most suitable runway considering the probable wind velocity and direction and the ground handling characteristics of the airplane, and considering other conditions such as landing aids and terrain.
(3) The operation is authorized by operations specifications.
Note: There is similar language for Part 121 operators in 14 CFR 121.195. There also is similar language for Part 91K operators in 14 CFR 91.1037 which basically reads the same but with a 80% factor.
shall not be greater than the mass at which the requirements of the appropriate chapter can be complied with for the flight to be undertaken. Allowance may be made for expected reductions in mass as the flight proceeds and for fuel jettisoning.
for turbo-propeller powered aeroplanes, within 70 % of the LDA.
the aeroplane will land on the runway most likely to be assigned, considering the probable wind speed and direction, the ground handling characteristics of the aeroplane and other conditions such as landing aids and terrain.
A few things change once you are wheels in the well and en route to your destination. First of all, those commercial landing factors that must be considered prior to departure go away. Second, the requirement for recency of the weather reports changes. Finally, other planning factors need to be added.
[SAFO 06012, ¶1.] This SAFO urgently recommends that operators of turbojet airplanes develop procedures for flightcrews to assess landing performance based on conditions actually existing at time of arrival, as distinct from conditions presumed at time of dispatch. Those conditions include weather, runway conditions, the airplane's weight, and braking systems to be used.
[SAFO 06012, ¶5.e] The landing distance assessment should be accomplished as close to the time of arrival as practicable, taking into account workload considerations during critical phases of flight, using the most up-to-date information available at that time. The most adverse braking condition, based on reliable braking reports or runway contaminant reports (or expected runway surface conditions if no reports are available) for the portion of the runway that will be used for the landing should be used in the actual landing performance assessment. For example, if the runway surface condition is reported as fair to poor, or fair in the middle, but poor at the ends, the runway surface condition should be assumed to be poor for the assessment of the actual landing distance. (This example assumes the entire runway will be used for the landing). If conditions change between the time that the assessment is made and the time of landing, the flightcrew should consider whether it would be safer to continue the landing or reassess the landing distance.
[EASA Air Ops Annex 1 to VIII, §ANNEX IV, ¶CAT.OP.MPA.300] IN-FLIGHT DETERMINATION OF THE LANDING DISTANCE. The in-flight determination of the landing distance should be based on the latest available meteorological or runway state report, preferably not more than 30 minutes before the expected landing time.
[14 CFR 121, §121.195(d)] and [14 CFR 135, §135.385(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 [the dry runway rules] 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 [the dry runway rules] of this section.
7.2.1. When the appropriate weather reports or forecasts or a combination thereof indicate that the runway at the estimated time of arrival may be wet, the landing distance available should be at least 115 per cent of the required landing distance determined in accordance with [dry runway conditions].
A landing distance on a wet runway shorter than that required by 7.2.1 but not less than that required by [dry runway conditions] may be used if the flight manual includes specific additional information about landing distance on wet runways.
Part 1 of Annex 6 applies to commercial aircraft and basically says you will use 115% of dry runway data unless your flight manual gives you data that allows a smaller factor. There does not appear to be a similar requirement for general aviation aircraft.
[EASA Air Ops Annex 1 to VIII, §ANNEX IV, ¶CAT.POL.A.235] Landing — wet and contaminated runways.
When the appropriate weather reports and/or forecasts indicate that the runway at the estimated time of arrival may be wet, the LDA shall be at least 115 % of the required landing distance, determined in accordance with CAT.POL.A.230.
When the appropriate weather reports and/or forecasts indicate that the runway at the estimated time of arrival may be contaminated, the LDA shall be at least the landing distance determined in accordance with (a), or at least 115 % of the landing distance determined in accordance with approved contaminated landing distance data or equivalent, whichever is greater. The operator shall specify in the operations manual if equivalent landing distance data are to be applied.
A landing distance on a wet runway shorter than that required by (a), but not less than that required by CAT.POL.A.230(a), may be used if the AFM includes specific additional information about landing distances on wet runways.
A landing distance on a specially prepared contaminated runway shorter than that required by (b), but not less than that required by CAT.POL.A.230(a), may be used if the AFM includes specific additional information about landing distances on contaminated runways.
For (b), (c) and (d), the criteria of CAT.POL.A.230 shall be applied accordingly, except that CAT.POL.A.230(a) shall not be applied to (b) above.
[14 CFR 91, §91.1037(e)] Unless, based on a showing of actual operating landing techniques on wet runways, a shorter landing distance (but never less than that required by [dry runway rules] 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 or alternate 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 [dry runway rules] of this section.
This applies only the 14 CFR 91 Subpart K Fractional Operators.
[AC 91-79A, appendix 2, ¶2.a.] Part 91 Operational Recommendations.
Preflight planning requirements for part 91 operators are governed by §§ 91.103 and 91.605. It is highly recommended that part 91 operators and pilots calculate predeparture landing distance performance requirements based on the guidance contained in their AFM, and employ the landing assessment process before initiating the approach landing phase of the flight, and consider the factors presented in this AC to avoid a runway overrun.
To ensure that an acceptable landing distance safety margin exists at time of arrival (TOA), the FAA recommends a 15-percent safety margin be applied to the actual airplane landing distance. The 15-percent safety margin is a minimum safety margin to be applied after accounting for all known variables, such as the meteorological conditions, runway surface conditions, landing with a tailwind, airplane configuration and weight, runway slope, threshold crossing height and airspeed, and the timely utilization of ground deceleration devices. Be prepared, know the landing conditions, divert to an alternate, or go-around, but do not risk a runway overrun.
There isn't a lot of regulatory guidance to deal with runway contamination, other than to say add 15% on a wet runway if you are a commercial operator. If the manufacturer provided better guidance, you can use that instead. But even after you've done all that, there is more . . .
The 2005 accident of Southwest Airlines 1248 inspired Safety Alert For Operators (SAFO) 06012 and a new set of guidelines for commercial operators. Even if you are not a commercial operator, the SAFO includes a good discussion of why you should not assume the number in your manual is what you need to land, no matter the weather. Basically, you should be computing your landing distance based on the conditions you expect, then add 15%.
I'm not sure how accurate these factors are, but the point is that each adds to your landing distance.
Photo: Landing distance compound factors, AC 91-97A, Table 1.
So the lesson here is cross the threshold at 50 feet, on speed, with a headwind, use everything you've got to stop, and, even after all that, add an additional factor . . .
1. Purpose. This SAFO urgently recommends that operators of turbojet airplanes develop procedures for flightcrews to assess landing performance based on conditions actually existing at time of arrival, as distinct from conditions presumed at time of dispatch. Those conditions include weather, runway conditions, the airplane's weight, and braking systems to be used. Once the actual landing distance is determined an additional safety margin of at least 15% should be added to that distance. Except under emergency conditions flightcrews should not attempt to land on runways that do not meet the assessment criteria and safety margins as specified in this SAFO.
3. Applicability: a. This SAFO applies to all turbojet operators under Title 14 of the Code of Federal Regulations (14 CFR) parts 121, 135, 125, and 91 subpart K.
5.e.(1) Operators and pilots should use the most adverse reliable braking action report, if available, or the most adverse expected conditions for the runway, or portion of the runway, that will be used for landing when assessing the required landing distance prior to landing. Operators and pilots should consider the following factors in determining the actual landing distance: the age of the report, meteorological conditions present since the report was issued, type of airplane or device used to obtain the report, whether the runway surface was treated since the report, and the methods used for that treatment. Operators and pilots are expected to use sound judgment in determining the applicability of this information to their airplane's landing performance.
We know intuitively that the landing flare consumes distance but something inside us also says that must have been accounted for in the aircraft certification. Well, it was. But it may not be the same idea of a flare as you might think. Remember the guys certifying the airplane are trying to show how the airplane can use short runways so they can sell more of them. Your objective is different. All of this is of critical importance when trying to predict our airplane's braking performance.
Photo: Example landing flare math, from Aim Point vs. Touchdown Point.
[SAFO 06012, ¶5.f.] Airplane flight manual (AFM) landing performance data are determined during flight- testing using flight test and analysis criteria that are not representative of everyday operational practices. Landing distances determined in compliance with 14 CFR part 25, section 25.125 and published in the FAA-approved AFM do not reflect operational landing distances (Note: some manufacturers provide factored landing distance data that addresses operational requirements.) Landing distances determined during certification tests are aimed at demonstrating the shortest landing distances for a given airplane weight with a test pilot at the controls and are established with full awareness that operational rules for normal operations require additional factors to be added for determining minimum operational field lengths. Flight test and data analysis techniques for determining landing distances can result in the use of high touchdown sink rates (as high as 8 feet per second) and approach angles of -3.5 degrees to minimize the airborne portion of the landing distance. Maximum manual braking, initiated as soon as possible after landing, is used in order to minimize the braking portion of the landing distance. Therefore, the landing distances determined under section 25.125 are shorter than the landing distances achieved in normal operations.
[Gulfstream G550 Aircraft Operating Manual, §13-03-20, ¶1.A.(3)] Landing distances based on 3.5° glide path at 50 feet and 8 FPS sink rate at touchdown.
To convert 8 FPS into something more tangible, multiply by 60. That means you are touching down at 480 feet per minute (fpm). In a G550 your typical ILS final has a descent rate of 600 fpm. So you are barely flaring at all. I've witnessed one or two "no flare" landings in the GV and it isn't pretty, but it works. So there are two lessons here: (1) Learn to touchdown more firmly, and (2) Don't expect to realize the landing distance numbers in your manual.
[AC 91-79A, appendix 1, ¶6.] A proper flare reduces the aircraft’s rate of descent to achieve the desired firm landing. If the flare is extended while additional speed is bled off, additional runway will be used. An extended flare may also result in an increase in pitch attitude, which may lead to a tail strike. A firm landing does not mean a hard landing, but rather a deliberate or positive touchdown at the desired touchdown point. A landing executed solely for passenger comfort considerations, which extends the touchdown point beyond the touchdown zone (TDZ), is not impressive, desirable, or consistent with safety or regulations. It is essential to fly the airplane onto the runway at the target touchdown point.
[Boeing 787-BBJ Flight Crew Operations Manual, §PI.17.7] The reference landing distance is the distance from threshold to complete stop. It includes an air distance allowance from threshold to touchdown associated with a flare time of 7 seconds.
That works out to an average of 429 feet per minute.
[Bombardier BD-700 Airplane Flight Manual, §04-08-16, ¶12] FULL STOP LANDING — Perform partial flare, and touchdown without holding off.
[Cessna Citation X Model 750 Airplane Flight Manual, p.2-310-5] Load Factors / Landing . . . +3.5G / This acceleration represents landing at a sink rate, at touchdown, of 600 feet per minute.
Learn how to flare your airplane to keep it in the touchdown zone, and do this consistently.
Once you touchdown the aerodynamic drag of the aircraft starts the process of slowing the airplane. But you will also have help from thrust reversers (or reverse pitch on the props), ground spoilers (or speed brakes), and the wheel brakes. I've drawn this graph to show a generalized idea of what is effective and when. This comes from a Challenger 605 but the idea works for all aircraft with a few caveats. Bottom line: the thrust reversers are most effective early on and the effectivenes of the wheel brakes depends greatly on the runway condition.
Photo: Retardation forces on landing, from Eddie's notes.
Note that for our example aircraft, the retardation force of the thrust reversers has to wait for them to unlock and spool up. But once that happens, the force goes way up very quickly. The force decreases as the aircraft decelerates and then there comes a time when they must be brought to idle and the retardation force drops off considerably. For this particular aircraft, the thrust reversers are more effective than the wheel brakes at high speeds on wet runways and ice. This isn't true of all aircraft.
We used to always assume reverse thrust was extra; the numbers in our books assumed we didn't have any reverse thrust at all. All that has changed. Some airplanes always assume you have it (such as the BC-700 example shown here). Other assume you have reverse for takeoff but not for landing (such as the G450 example shown here). Of course there are still many aircraft where no thrust reverser credit is ever taken (such as the CL-605 example shown).
[AC 91-79A, appendix 1, ¶5.j.] Thrust Reversers. In the event of an asymmetric deployment, the nosewheel on the ground will aid in directional control. If the thrust reversers deploy asymmetrically, or if the airplane begins to drift due to a crosswind, close the thrust reversers and reestablish directional control. Once the airplane’s track down the runway is reestablished, redeploy the thrust reversers. Use airplane steering in accordance with the AFM procedures.
[Bombardier BD-700 Airplane Flight Manual, §06-01-1, ¶1.A.(7)] The take-off field length data on wet runways are based on both thrust reversers operable and assumes that maximum available reverse thrust is used down to a complete stop.
[Bombardier BD-700 Airplane Flight Manual, §06-01-1, ¶1.A.(7)] ACTUAL LANDING DISTANCE ON CONTAMINATED RUNWAYS — The following charts are used to obtain the actual landing distance on contaminated runways, for a slat OUT / flap 30° landing, for a given value of airplane weight, airport pressure altitude, reported wind, landing approach speed (VREF) increment and number of operable thrust reversers.
[CL605 AFM, §03-01-1, ¶2.] No operation should be predicated on the use of thrust reversers.
[Gulfstream G450 Airplane Flight Manual, §05-02-10, ¶1] Field Length Limited Performance. No reverse thrust credit was taken for accelerate-stop distances computed for dry runways; however, the use of reverse thrust is recommended to reduce the braking distance and the kinetic energy absorbed by the brakes. Wet runway accelerate-stop distances are calculated assuming the deployment of one or both thrust reversers.
[Gulfstream G450 Airplane Flight Manual, §05-11-20, ¶3.B.(3)] Deploy thrust reversers as required. Note that reverse thrust credit is not shown in the landing distance charts, but the use of reverse thrust will result in distances less than those computed and significantly improve brake wear characteristics.
We all know that hydroplanning occurs when the tire no longer has enough contact with the surface so as to maintain a one-to-one movement of rubber against that surface. In other words, the tire slips. And once it starts slipping, you have less stopping power until you stop the slipping. If you really want to understand that, a little physics lesson is in order . . .
We briefly covered Mu (μ) above and learned that it is the coefficient of friction. You can diagram a changing μ against the force applied to the objects where the friction is being measured. The higher the μ the better the friction and your stopping power. You can think of the "F" in the chart above as the amount of friction you are asking from your brakes. The harder you press the higher the "F" you are asking for. Notice that the μ, your stopping force, goes up and up to a point. Once the tire breaks free from the surface you have slippage and the stopping force goes down. The only way to get the stopping force back is to back off on the brakes. That isn't ideal, of course. So the key to success when it comes to stopping on a wet or contaminated runway, is to get your braking up to but short of that point static friction turns to kinetic friction. That's something your anti-skid brake system can do better than you.
[AC 91-79A, appendix 3, ¶11.a.] Hydroplaning Causal Factors. Hydroplaning is a condition that can exist when an airplane has landed on a runway surface contaminated with standing water, slush, and/or wet snow. Hydroplaning can have serious adverse effects on ground controllability and braking efficiency. The three basic types of hydroplaning are dynamic, reverted rubber, and viscous. Any one of the three can render an airplane partially or totally uncontrollable anytime during the landing roll. Hydroplaning is a significant factor that should be considered in determining stopping or steering capability on a contaminated or otherwise flooded runway. Hydroplaning depends on wheel speed and tire pressure. The key item to remember when the airplane is hydroplaning is that the tires are no longer in contact with the runway, although the airplane is not airborne. Though at high speeds, aerodynamic drag can provide a significant decelerating force. Reverse thrust/propeller reversing, if installed, provides nearly all the decelerating force when hydroplaning since the brakes are essentially ineffective. These characteristics are included in the distances presented in the contaminated runway supplemental data when provided by the airplane manufacturer.
[AC 91-79A, appendix 3, ¶11.a.(1)] Dynamic hydroplaning is related to tire inflation pressure. Data obtained during hydroplaning tests have shown the minimum dynamic hydroplaning speed (VP) of a tire to be 8.6 times the square root of the tire pressure in pounds per square inch (psi). For an airplane with a main tire pressure of 24 pounds, the calculated hydroplaning speed would be approximately 42 knots (kts). It is important to note that the calculated speed referred to above is for the start of dynamic hydroplaning. Once hydroplaning has started, it may persist to a significantly slower speed depending on the type being experienced.
Hydroplaning is a function of the water depth, tire pressure and speed. Moreover, the minimum speed at which a non-rotating tire will begin to hydroplane is lower than the speed at which a rotating tire will begin to hydroplane because a build up of water under the non-rotating tire increases the hydroplaning effect. Pilots should therefore be aware of this since it will result in a substantial difference between the take-off and landing roll aircraft performance under the same runway conditions. The minimum speed, in knots, at which hydroplaning will commence can be calculated by multiplying the square root of the tire pressure (PSI) by 7.7 for a non-rotating tire, or by 9 for a rotating tire.
Gently "kissing" the runway with the tire increases the chances it will not be rotating when it finally makes rubber-to-runway contact and therefore increasing the likelihood of hydroplaning.
These are good numbers to know for each aircraft you fly. For example, a typical Gulfstream's main gear tire pressures will be around 190 psi. That means you can expect to begin hydroplaning around 125 knots and will not regain friction until 106 knots. The nose gear is typically about 135 psi, which means directional control via the nose wheel can be suspect around 105 knots.
When hydroplaning occurs, the aircraft's tires are completely separated from the actual runway surface by a thin water film and they will continue to hydroplane until a reduction in speed permits the tires to regain contact with the runway. This speed will be considerably lower than the speed at which hydroplaning commences. Under these conditions, the tire traction drops to almost negligible values, and in some cases, the wheel will stop rotating entirely. The tires will provide no braking capability and will not contribute to the directional control of the aircraft. The resultant increase in stopping distance is impossible to predict accurately, but it has been estimated to increase as much as 700 percent. Further, it is known that a 10-kt crosswind will drift an aircraft off the side of a 200-ft wide runway in approximately 7 sec under hydroplaning conditions.
Back in the days when large aircraft didn't have anti-skid braking, or the anti-skid systems were rudimentary, we had to handle our brakes with care. We worried about locking up the wheels, blowing the tires, and then having to rely on the remains of each wheel for stopping power. These days, most aircraft will allow you to mash down on the brakes and let the electrons worry about all that. In fact, your aircraft's certified stopping distance may depend on that. So, what else can you do to maximize braking effectiveness? Put the nose wheel down, deploy devices that "spoil" the lift and increase the weight placed on the brakes. And do all this as soon as you can.
[AC 91-79A, appendix 1, ¶5.c.] Non-Turbojet Airplanes Braking After Touchdown. Putting maximum weight on the wheels after touchdown is an important factor in obtaining optimum braking performance. During the early part of rollout, some lift continues to be generated by the wing. After touchdown, the nosewheel is lowered to the runway to maintain directional control. During deceleration, the nose may pitch down due to braking, transferring the weight from the main wheels to the nosewheel. This does not aid in braking action, so back pressure is applied to the controls without lifting the nosewheel off the runway. This enables directional control while keeping weight on the main wheels. Careful application of the brakes is initiated after the nosewheel is on the ground and directional control is established. Maximum brake effectiveness is just short of the point where skidding occurs. If the brakes are applied so hard that skidding takes place, braking becomes ineffective.
[AC 91-79A, appendix 1, ¶5.d.] Braking. A distinction should be made between the procedures for minimum landing distance and an ordinary landing roll with considerable excess runway available. Minimum landing distance will be obtained by creating a continuous peak deceleration of the airplane; that is, extensive use of the brakes for maximum deceleration. On the other hand, an ordinary landing roll with considerable excess runway may allow extensive use of aerodynamic drag to minimize wear and tear on the tires and brakes. If aerodynamic drag is sufficient to cause deceleration, it can be used in deference to the brakes in the early stages of the landing roll. For instance, the use of aerodynamic drag is applicable only for deceleration to 60 to 70 percent of the touchdown speed. At speeds less than 60 to 70 percent of the touchdown speed, aerodynamic drag is so slight as to be of little use, and braking must be utilized to produce continued deceleration. Since the objective during the landing roll is to decelerate, the powerplant thrust should be the smallest possible positive value (or largest possible negative value in the case of thrust reversers).
(1) There are three primary forces available for deceleration during the rollout process: wheel braking, aerodynamic drag, and reverse thrust/propeller reversing, if available. Spoilers are designed to reduce lift and transfer aircraft weight to the landing gear. Deployment of drag devices is most effective at higher speeds and is not affected by runway surface conditions. In all cases, the pilot must ensure the automatic deployment of the deceleration devices occurs. If not, the pilot must immediately manually deploy the devices to minimize the time lost to achieve as close as possible the assessed landing distance.
(2) Timely deployment of spoilers will increase drag by 50 to 60 percent, but more importantly, deployment of the spoilers increases wheel loading by as much as 200 percent in the landing flap configuration. This increases the tire-to-ground friction force, making the maximum tire braking forces available. Many airplanes with autospoilers installed require weight on wheels to deploy the spoilers, which reinforces the requirement for a positive touchdown, as a soft touchdown can delay the automatic deployment. Spoiler deployment immediately after touchdown provides the greatest benefit.
(3) When minimum landing distances are considered, braking friction forces predominate during the landing roll and, for the majority of airplane configurations, braking friction is the main source of deceleration when the runway is dry.
[AC 91-79A, appendix 1, ¶5.e.] Wet and Contaminated Runway Surface. When the runway is wet or slippery, reverse thrust (if the airplane is equipped) may be the dominant deceleration force just after touchdown, and throughout the deceleration if the runway has poor or worse braking conditions. As the airplane slows down, the wheel brakes become the dominant deceleration force. When the runway length is limited, for airplanes equipped with an antiskid system, maximum wheel braking should be applied immediately after touchdown. For airplanes without an antiskid system, slow back pressure should be applied to the yoke such that it will not raise the nose of a nose gear airplane for aerodynamic braking while maximum braking that will not cause skidding is applied. In all situations, braking should be maintained until the airplane slows to a safe taxi speed for the conditions.
[AC 91-79A, appendix 1, ¶5.f.] Autobrakes. If the airplane is equipped with autobrakes, manufacturers recommend the use of the autobrakes for all landings on contaminated runways. Autobrakes are applied earlier in the landing roll, and to the level selected by the pilot(s) or flightcrew for the anticipated landing condition, and runway available, and provide the most efficient and timely use of braking action. However, the pilot must ensure that if the autobrakes do not engage upon touchdown, then manual braking must be applied.
[AC 91-79A, appendix 1, ¶5.g.] Antiskid. Application of brakes is different for airplanes equipped with a functioning antiskid braking system than for airplanes without such a system.
(1) For airplanes without an antiskid system, brakes should be applied progressively throughout the deceleration process, and the pilot must recognize the point that wheel skid occurs. Maximum braking effectiveness occurs just prior to the point where wheel skidding occurs. However, should a skid occur, releasing brake pressure can stop skidding and then maximum braking can be reestablished until the deceleration process is completed. Pilots should be aware that a skid is most likely at higher speeds and at that point may not be perceptible.
(2) For airplanes with an antiskid system, to achieve the benefits of antiskid, the brakes must be applied firmly throughout the deceleration process. When maximum braking is required, it is accomplished by holding maximum brake application pressure and allowing the antiskid system to operate. Letting up on the brakes (unless required to regain directional control) defeats the purpose of the antiskid system. The pulsation caused by the modulation of the brake pressure by the antiskid system indicates that the antiskid system is operating normally, although the pulsation may be disconcerting to the pilot.
If you have any experience in jet fighter aircraft or something similar (fast, high angle of attack, prefers landing in a crab), you may be tempted to reduced the wear on your brakes and even improve your stopping distance by leaving the nose up in the air a bit longer to "aero brake." The theory in smaller "fighter" type aircraft is they have very small brakes and the aircraft is fully controllable with its nose up in the air to offer the relative wind the most resistance. This is unsafe in a large, transport category aircraft where directional control below VMCA depends on a three-point attitude. Besides, it is unnecessary and counterproductive. Unnecessary because your brakes are more capable. Counterproductive because those brakes work best with as much weight on them as possible. So: unsafe, unnecessary, and counterproductive. So don't do it, okay?
1-TO-T-38A-1, p. 2-12] After touchdown, continue to increase back pressure on the stick to obtain the highest possible nose-high attitude without flying the aircraft off the runway. Just prior to loss o elevator authority, lower the nosewheel to the runway. After the nosewheel is lowered to the runway, a single, smooth brake application should be used to stop. This technique could increase landing distance as much as 50 percent from that computed from the landing distance chart in Part 7 of Appendix.
Certification rules don't explicitly cover what it takes to maintain control on the ground, other than during takeoff. Even VMCL is really a speed for flight, just prior to landing . . .
(6) Go-around power or thrust setting on the operating engine(s).
So, given the lack of certification rules, I encourage you to check your aircraft's procedures. But lacking those, if you are flying a multi-engine aircraft consider that the size of your vertical fin was designed primarily to help you maintain lateral control in the event of an engine failure. Many of the fighters (and the T-38 example) are licensed as "multi-engine limited to centerline thrust." Chances are your airplane has no such limitation. That means your vertical fin will act as a very large weather vane, turning you into the wind at a rate your rudder might not be able to counteract. Try this in the simulator with a crosswind: after landing keep the nose up for as long as you can and see how much time it takes to hit the grass. You might be surprised.
Most aircraft enter the flare with a nose up pitch and positive angle of attack on the wings. The relative wind hits the wing bottom first, having the effect of pushing the nose of the aircraft upward. As the main gear touch, the momentum of the aircraft continues downward and the aircraft tends to pivot around the main gear, throwing the nose gear downward. We quite naturally add back pressure to cushion the nose wheel's landing. (For many aircraft, it is a landing all it own, distinct from the main gear landing.) From the stand point of braking efficiency, however, all this is good. The relative wind helps add weight to the wheels and thereby improve the efficiency of the brakes. Why is that?
Remember the equation Ff= μ FN? If not, see Mu (μ), above. The key point is that so long as you don't exceed the μ, the more N (downward force, or the weight of the airplane), the more Ff (braking force) you have available to use.
There are three possibilities when it comes to the performance data in your books when it comes to landing on a runway that is less than dry: (1) The data doesn't exist, (2) The data for a wet runway is nothing more than a 15% additive, or (3) The data is a result of actual tests.
You should know where the data in your manual came from. Only then can you approach everything about this topic with a sound foundation.
[SAFO 06012, ¶5.i.] Manufacturer-supplied landing performance data for conditions worse than a dry, smooth runway is normally an analytical computation based on the dry runway landing performance data, adjusted for a reduced airplane braking coefficient of friction available for the specific runway surface condition. Most of the data for runways contaminated by snow, slush, standing water, or ice were developed to show compliance with European Aviation Safety Agency and Joint Aviation Authority airworthiness certification and operating requirements. The FAA considers the data developed for showing compliance with the European contaminated runway certification or operating requirements, as applicable, to be acceptable for making landing distance assessments for contaminated runways at the time of arrival.
[SAFO 06012, ¶7.d.] If wet or contaminated runway landing distance data are unavailable, the factors in Table 2 should be applied to the pre-flight planning (factored) dry runway landing distances determined in accordance with the applicable operating rule (e.g., sections 91.1037, 121.195(b) or 135.385(b). Table 2 should only apply when no such data are available. The factors in Table 2 include the 15% safety margin recommended by this guidance, and are considered to include an air distance representative of normal operational practices. Therefore, operators do not need to apply further adjustments to the resulting distances to comply with the recommendations of this guidance.
Figure: Multiplication factors to apply to the factored dry runway landing distances when the data for the specified runway condition are unavailable, from SAFO 06012, Table 2.
* The factored dry runway landing distances for use with Table 2 must be based on landing within a distance of 60% of the effective length of the runway, even for operations where the preflight planning (factored) dry runway landing distances are based on landing within a distance other than 60% of the effective length of the runway (e.g., certain operations under part 135 and subpart K of par t91). To use unfactored dry runway landing distances, first multiply the unfactored dry runway landing distance by 1.667 to get the factored dry runway landing distance before entering Table 2 above.
NOTE: These factors assume maximum manual braking, autospoilers (if so equipped), and reverse thrust will be used. For operations without reverse thrust (or without credit for the use of reverse thrust) multiply the results of the factors in Table 2 by 1.2. These factors cannot be used to assess landing distance requirements with autobrakes.
[Cessna Citation CJ2+ AFM, p. 4-5] All takeoff and landing performance is based on a paved, dry runway.
115 percent of the two-engine horizontal takeoff distance from start to a height of 35 feet above runway surface.
The engine-out accelerate-go distance to 15 feet.
The Dry Takeoff Field Length.
There is a similar statement for landings on wet runways on page 7-4.
[Gulfstream G550 AFM, §5.1, p. 5.1-1] Both dry and wet runway performance is included.
Going through the charts, it appear most of their wet runway performance number is better than a 1.15% adjustment, as is permitted by regulatory agencies when "based on a showing of actual operating landing techniques on wet runways."
Some manufacturers have stepped up to the plate in performance software by adding Runway Condition Codes. The following shots, for example, come from the Gulfstream performance application, PlanePerformance.
Have you ever landed 23 knots fast? Yeah, me too. How about with a tailwind. Yup, same here. How about on a runway that is just barely long enough? I've done that a few times. Now, what about all three of those combined into one landing? I've never done that. I am betting the Citation pilots on N262Y shown here bet they never did either. Just prior to the landing the PIC asked the SIC what he thought. The SIC said, "It's up to you." That turned out to be the wrong answer.
[NTSB ERA11FA001] The pilots touched down at an excessive airspeed (23 knots above Vref), more than 1,200 feet down a wet 4,305-foot-long runway, leaving about 3,100 feet for the airplane to stop. According to manufacturer calculations, about 2,710 feet of ground roll would be required after the airplane touched down, assuming a touchdown speed at Vref; a longer ground roll would be required at higher touchdown speeds. Although a 2 knot crosswind component existed at the time of the accident, the airplane's excessive airspeed at touchdown (23 knots above Vref) had a much larger effect on the outcome of the landing.
Landing on anything less than a dry runway ought to set into motion an additional set of precautions in your normally cautious pre-landing decision making. You need to get relevant and recent weather and go through your airplane's performance numbers to make sure it will all work out, on paper at least. On a wet runway, you need to add 15% to the dry runway numbers if your manufacturer doesn't provide wet runway data. If the runway is grooved, you might be able to consider it "effectively dry" if your manufacturer permits this. But once the surface gets much deeper than 0.1 inch or slush is involved, that "effectively dry" runway may not be after all. You will have a number of safety factors to consider prior to departure if you are a commercial operator. Commercial operators will also have to add 15% to landing distance numbers once en route; Part 91 operators should be doing this too. You should also remember that the way you flare the airplane, apply reverse thrust, and brake the airplane have a big impact on your stopping distance. Many pilots get most of this wrong.
One of the problems with the way most of us pilots approach this subject is the way we look at accident reports of pilots who failed to stop on the confines of a runway. "I would never do that." While some of these pilots were careless and had no business in their cockpits, some were quite professional and thought they were doing everything just right. The problem is that we don't have precise tools to measure the runway's condition and our aircraft' braking effectiveness. It behooves us to do everything by the book and, even then, look at everything to do with stopping on a wet or contaminated runway with a skeptical eye.
Callaghan, John J., Slippery When Wet: The Case for More Conservative Wet Runway Braking Coefficient Models, AIAA Aviation Forum, 13-17 June 2016, Washington, D.C.
NTSB Aviation Accident Final Report, ERA11FA001, 10/01.2010, Manteo, NC.

References: §25
 § 139
 §4
 §121
 §135
 §91
 §13
 §04
 §06
 §06
 §03
 §05
 §05
 §5