Runway excursions (REs) are among the most common event categories of accidents in air transport operations. According to the European Aviation Safety Agency’s (EASA’s) latest safety review, “There were 100 runway excursion accidents and serious incidents at EASA aerodromes between 2008 and 2012.”
These are events in which an aircraft either veers off the runway surface or overruns the end of the runway. Most REs are caused by improper approaches that lead to aircraft control issues after touchdown.
The threshold crossing height, airspeed, descent rate and angle on the approach are usually involved. Sometimes strong, gusty crosswinds, tail wind and/or runway friction are involved (ASW, 7/13, p. 43). Once the aircraft touches down, its deceleration capability and flight crew actions also play a role. So most REs are associated with multiple factors.
If you change one factor, an RE might be avoided. Runway conditions, although not a primary cause of REs, are often a contributing factor. Compromised runway conditions make it more difficult for the flight crew to overcome the problems produced by an unstable approach. A recent study by Boeing (ASW, 11/12, p. 8) indicated that 94 percent of REs occurred on non-dry runways.
After the tire tread on the landing gear makes effective contact with the surface of the runway, friction between the two allows the pilots to decelerate the aircraft while maintaining control. Any “contaminant” that gets between the tire and the runway surface can lessen the frictional bond. This leads to longer stopping distances and, at worst, runway excursions. Any type of debris, such as rubber particle buildup from tires, can be problematic. Water from rain is one of the most common contaminants. In colder climates, winter weather elements such as frost, snow, slush and ice can also greatly affect runway friction.
How winter runway conditions are determined and reported to pilots remains a focus of ongoing study and debate. Many airports periodically measure runway friction with specialized devices for decision making by air traffic control and, in some countries, advice to pilots. The resulting so-called Mu (coefficient of friction) value generated can range from 100 (U.S. value) or 1.00 (International Civil Aviation Organization value) for the highest friction to 0 for the lowest friction. Any value less than 40 on an operational runway should be recorded and the information passed along to incoming aircraft pilots.
Values less than 20 would result in closing the runway. More commonly, runway braking action is reported by pilots who already have landed; this is usually considered the most reliable indicator of runway conditions at the time of landing. A pilot who deems that the “braking action conditions” are less than good is expected to fill out a runway condition report and provide a pilot report.
Pilots should keep in mind that runway friction can change very quickly when precipitation is occurring, sometimes within minutes. A major problem in reporting runway conditions by pilots is their subjective nature. Last year, the U.S. Federal Aviation Administration (FAA) also stopped recommending that airports provide runway friction measurements to pilots when snow or ice is on the runway, citing inconsistencies in the measurement process (ASW, 11/12).
Finally, at some locations, airport personnel themselves have been authorized to make the determination of runway friction and pass the information along to pilots.
To further assist pilots in assessing U.S. runway conditions, the FAA classifies runways as being “dry,” “wet” or “contaminated.” “Dry” would seem self-explanatory, but a damp runway, one which appears discolored but not reflective, is considered “dry.” Damp runways often are the result of dew or very light rain, or represent the final drying stage of a previously wet runway.
A “wet” runway is sufficiently moist to appear reflective, but is still not considered “contaminated.” Braking action is reduced, but conditions are still acceptable. A 1/8-in (3-mm) depth of water is considered the threshold for “hydroplaning” or “aquaplaning,” in which a layer of water comes between the tire and the pavement surface, and loss of directional control can result. Keep in mind that water depths less than 1/8 in can still reduce friction and increase stopping distance even without loss of control. Also, sometimes runways can be described as “flooded.” In these cases, large areas of standing water are visible.
Because rainwater is a common contaminant of runways, a number of methods are used to reduce its negative impact on friction. The asphalt or concrete used for the runway is specially textured. The primary purpose of the macrotexture or roughness of the runway surface is to provide a path for water to escape from beneath the tires.
Pavement texture makes a significant difference in the friction of a wet surface. A rough macrotexture provides much better friction in wet conditions. The microtexture, the fine-scale roughness or feel of the surface, helps break through the residual water film left. Grooves also are often cut into the runway surface to facilitate drainage. The FAA standard groove is ¼ in (6 mm) deep and ¼ in wide, and the grooves are spaced 1½ in (38 mm) apart. Runways also are sloped or crowned for better drainage.
The goal is to keep water levels on the runway below the height of the textured surface, thus eliminating standing water. The drainage ability of a runway is affected by the cross slope, the surface texture, wheel ruts and two weather factors — crosswinds and rainfall intensity. The direction of the wind can help or hinder water flow off the runway. But one of the most important factors is the intensity of the rainfall. The key question for airport engineers is, at what rainfall rate would the drainage capacity of a runway be overwhelmed and the water level on the pavement exceed the texture depth of the surface, leading to standing water?
Unfortunately, with all of the variables, there is no way to directly correlate specific rainfall rates with resulting runway conditions. We can, however, make some general statements.
Convective showers or thunderstorms can generate rainfall rates that compromise runway conditions. An inch (25 mm) of rain in 15 minutes or less is not that unusual. The critical 1/8 in of water can fall within a minute or two. Convection associated with tropical cyclones also is known for producing excessive amounts of rain. The more typical winter storms and fronts without convection usually are associated with rainfall rates of less than 1 in per hour.
Weather radar is useful in determining rainfall rates because the colors used on the meteorologist’s standard radar display represent the strength of the signal return, which is directly related to rainfall intensity. Green areas indicate less than 0.1 in (2.5 mm) per hour. Yellow areas mean up to 0.5 in (12.7 mm). Orange shows 1 in or more. Reds and then purples indicate rainfall rates of 2 in (5 cm) or more per hour.
Winter-type precipitation in the higher latitudes is even worse in terms of contaminating a runway. According to the FAA, “a contaminated runway has more than 1/8 in of slush, snow, or compacted snow, ice, or frost covering more than 25 percent of the required length and width of its surface.” It takes very little time, sometimes only minutes, for 1/8 in of frozen precipitation to accumulate. And unlike rainwater, frozen precipitation will not drain off. The methods used to drain water from runways will be of little use in these situations. Only air or ground surface temperatures above freezing will create melting from below and runoff.
Although frost is included among runway contaminants, its typical depth, less than 0.04 in (1 mm), is usually less of a problem. Frost also differs from other cold weather contaminants because, technically, it’s not precipitation that falls from clouds. Frost is a frozen deposit of water vapor as ice crystals on a surface that occurs when the surface temperature falls to freezing and reaches the dewpoint. It usually occurs under clear skies and light to calm winds. Rime ice, which is deposited when clouds with below-freezing temperatures move across a surface, is similar.
Winter precipitation types include snow, sleet and freezing rain. Each has its own unique properties. Snow comprises ice crystals. The consistency of snow is a function of temperature. With temperatures near freezing, the snow is usually wet and heavy, the good packing snow of snowballs and snowmen. At colder temperatures, the snow becomes lighter and drier and not as compactable.
As any skier can tell you, the consistency of the snow affects how the surface of the ski moves over it. Skiers use different waxes on the bottom of their skis to go faster in different conditions. This same principle applies to snow on runways. Cold, dry snow has more grip, at times becoming almost sticky. Wet, mushy snow has a higher water content and greatly reduces traction. When temperatures on the ground are above freezing, the snow on the ground can contain a percentage of liquid water that produces slush. Slush also greatly reduces friction.
Snowfall rates are important in determining how fast the coefficient of friction of a runway can be affected. In convective snow events (ASW, 10/10, p. 18), snowfall rates can approach 4 in (102 mm) per hour. At this intensity, a runway can be compromised within minutes. Snowfall rates also are critical in marginal situations when air temperatures are near, or even above, freezing and/or when the runway surface itself has above-freezing temperatures. In these situations, snow can still accumulate if the fall rate exceeds the melting rate. In other words, when the snow is piling up on top faster than it is melting from below, it can accumulate regardless of temperature.
Sleet, which is composed of small ice pellets, can accumulate quickly, but its granular nature makes it less of a problem for aircraft deceleration.
Clearly, the worst winter precipitation for runway excursion risk is freezing rain or glaze. In this case, liquid water droplets fall to earth and freeze on contact with any surface that has temperatures below 32 degrees F (0 degrees C). This can leave a layer of sheer ice on any paved surface. Often the layer of ice has some water on top of it.
Studies have shown that wet ice produces the most dangerous runway surface conditions. Four or five times the stopping distance that would be required on dry pavement can be needed. Wet ice on a runway often leads to unacceptable braking conditions and closing the runway.
To deal with winter precipitation, airports utilize a number of tools. For snow, old-fashioned snowplows and snowblowers help keep runways open. In addition, runway sweepers with rotary steel bristle brushes remove remaining snow. Unfortunately, these are very labor intensive and often slow processes. And if snowfall rates are too great, it becomes physically impossible to remove the snow fast enough to keep the runways clear and the airport must close until the snowfall lessens.
In icing situations, runways may be treated with sand, like highways. Or deicing/anti-icing chemicals can be sprayed on runways just as they are sprayed on aircraft to remove ice and/or temporarily prevent surface icing.
Some airports opt to treat runways ahead of time with “freezing-point depressant” chemicals to prevent ice accumulation. The cost of the chemicals and environmental concerns are factors in their availability as a mitigation, however. Easily the most ambitious recent attempt at solving the winter precipitation problem is with a heated runway system such as the one at the Denver International Airport. Geothermal energy also shows promise.
Edward Brotak, Ph.D., retired in 2007 after 25 years as a professor and program director in the Department of Atmospheric Sciences at the University of North Carolina, Asheville.
References
Daniels, Jeffrey M. “Aircraft Performance and Flight Deck Basics.” Presented at the 23rd Annual Schedulers and Dispatchers Conference, San Diego, California, U.S. January 2012.
International Civil Aviation Organization. Runway Surface Condition Assessment, Measurement, and Reporting. 2008.
International Federation of Air Line Pilots’ Associations. Runway Safety Manual. 2009.
Roginski, Michael. “Manufacturers Perspective — Runway Friction and Aircraft Performance.” Presented at Asociación Latino Americana y Caribeña de Pavimentos Aeroportuarios Seminar of Airport Pavements, Panama City, Florida, U.S., September 2012.