The definition of what constitutes a contaminated runway varies between countries, operators and aircraft manufacturers. Aviation has been in search of a reliable way to quantify what happens to landing distance once a runway becomes contaminated. But we are no closer to a solution today. That isn’t to say we are unarmed in this battle. Pilots do have some tools at their disposal, but there is also a lot of judgment involved. You can develop that judgment by understanding what the authorities mean when they say the runway is contaminated, how that contaminated is reported, how your aircraft performance charts deal with a runway that isn’t dry, and what safety margins are prudent. Then if you follow sound decision-making with recommended approach, landing flare, reverse and braking techniques, you can make a perfect landing on a less-than-perfect runway with far-from perfect information.
Runway Contamination Defined
The Aeronautical Information Manual (AIM) definition: A runway is considered contaminated whenever standing water, ice, snow, slush, frost in any form, heavy rubber or other substances are present. The International Civil Aviation Organization (ICAO) & The European Union Operations Regulations (EU OPS) 1,480 contain what many international organizations are adopting as a standard for identifying runway conditions:
Contaminated Runway–A runway is considered to be contaminated when more than 25% of the runway surface area (whether in isolated areas or not) within the required length and width being used is covered by the following:
Surface water more than 3 mm (0.125 in) deep, or by slush, or loose snow, equivalent to more than 3mm (0.125 in) of water; Snow that has been compressed into a solid mass that resists further compression and will hold together or break into lumps if picked up (compacted snow); or Ice, including wet ice.
- Wet runway: a runway is considered wet when the runway surface is covered with water, or equivalent, less than specified, or when there is sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water.
- 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 that is neither wet nor contaminated, and includes those paved runways that have been specially prepared with grooves or porous pavement and maintained to retain effectively dry braking action even when moisture is present.
Runway Contamination Reported
We know a contaminated runway when we see it, but that contamination isn’t reported consistently around the world. We are often unable to turn those measurements into consistent stopping distance factors. The United States, like most of the world, uses subjective measures based on pilot and ground vehicle reports of braking action. The quality of braking action is described by the terms good, fair, poor, and nil, alone or in combination. Fair is becoming medium to bring the terminology in line with ICAO. These reports can be greatly dependent on aircraft type and a pilot’s subjective judgment. Mechanical and electronic decelerometers are in use at some airports. Experience has shown that results obtained from some types of decelerometers are not accurate on water and slush. As a result, you will probably only see decelerometers at climates where the predominant contamination is snow.
Friction reports given as mu range from 0 to 100, where zero is the lowest friction possible and 100 is the highest. The AIM tells us that braking performance starts to deteriorate with any mu value below 40. But it also cautions that we cannot make any correlation between mu values and the term good, fair, poor, and nil. ICAO Annex 15, Appendix 2, on the other hand, does make such a correlation with its Special Series Notice to Airmen (SNOWTAM).
Many airports throughout Canada are equipped with decelerometers used to obtain an average of the runway friction measurement, which is reported as the Canadian Runway Friction Index (CRFI). The CRFI is graduated from 0 to 1, where anything above 0.8 is considered dry and a rating of 0.5 would be an expected value for a wet runway. While having s concrete number such as the mu can be psychologically reassuring, studies have shown that under certain conditions of thickness and deposits, the friction measurements cannot be relied upon.
Advisory Circular 150/5200-30C notes: Currently, there is no objective type of measurement of runway surface condition that has been shown to consistently correlate with airplane performance in a usable manner to the satisfaction of the FAA. Pilots and airplane operators are expected to use all available information, which include runway condition reports as well as any available pilot braking action reports, to assess whether operations can be safely conducted.
Understanding Aircraft Performance Charts
When contaminated data is given, however, there does seem to be a wide variety in the way charts are presented. Pilots should understand the basis behind each aircraft’s performance numbers before making go/no go decisions. Here are a few questions to ask:
- What constitutes a contaminated runway?
- Do the charts include a safety margin?
- Is the use of reverse thrust included?
- What type of braking is used? (Is there a particular autobraking setting? Should anti-skid protection be employed to its fullest extent?)
Recommended Safety Margins
The FAA Safety Alert for Operators (SAFO 06012) recommends that operators conduct a landing performance assessment as close to the time of arrival as practicable. Additionally the SAFO states crews should add a safety margin of at least 15% to landing distances. Very few manufacturers have added the 15% safety margin to published landing performance numbers.
Compute landing distances using flight manual procedures–always compute landing distances using approved flight manual procedures. You should be aware if these numbers include any kind of dispatch factor, such as the 60% rule or the 80% rule. These rules and their associated landing field lengths are intended departures decisions based on destination conditions, i.e., you cannot takeoff if you don’t expect to be able to land in 60% of the available runway at the destination (unless you have a suitable alternate).
Adjust landing distance for contaminated runway conditions–if your flight manual includes contaminated runway distances, compute them as well. If your flight manual does not have charts or conversion factors for contaminated runways SAFO 06012 includes chart.
- Runway Condition-Wet Runway/Dry Snow– Reported Braking Action–Good–Factor to Apply to (Factored) Dry Runway Landing Distance* =0.9
- Runway Condition–Packed or Compacted Snow, Reported Braking Action–Fair / Medium, Factor to Apply to (Factored) Dry Runway Landing Distance = 1.2
- Runway Condition–Wet Snow, Slush, Standing water, Ice, Reported Braking Action–Poor, Factor to Apply to (Factored) Dry Runway Landing Distance = 1.6
- Runway condition–Wet Ice, Reported Braking Action–Nil, Landing is prohibited
*you will need a factored dry runway landing distance. If your flight manual provided unfactored distances, multiply these by 1.667 to obtain factored dry runway landing distances.
If for example, your unfactored dry distance is 2,400 ft, your factored dry distance will be 4,000 feet and your contaminated landing distance on packed snow will be 4,800 feet.
Add the SAFO 06012 safety factor–multiply your contaminated runway landing distance by 1.15 to determine a contaminated runway landing distance with the 15% safety factor. In the example, the 4,800 feet becomes 5,520 feet.
If your flight manual numbers say no go, do not land– if the contaminated runway landing distance with the safety factor exceeds the runway available, you are done considering this runway. You should either hold until conditions improve or find another runway.
If flight manual numbers say go, but are close, consider CRFI numbers–if the available runway length greatly exceeds the computed landing distance, you are good to go. Just make sure the conditions are equal to or better than what you used in your computations when you show up on final approach. If the available runway length exceeds the computed runway landing distance but only by a small margin, you could use the Canadian Runway Friction Index method. If you don’t have reported CRFI, you will need a way to convert the reported surface condition to a CRFI equivalent.
Friction measurements on Each Third of Runway and Friction Measuring Device [Measured or Calculated Coefficient / Estimated Surface Friction]
- 0.40 and above / Good — 5
- 0.39 to 0.36 / Medium/Good — 4
- 0.35 to 0.30 / Medium — 3
- 0.29 to 0.26 / Medium / Poor — 2
- 0.25 and below / Poor — 1
- 9 -unreliable / Unreliable –9
(When quoting measured coefficient, use the observed two figures, followed by the abbreviation of the friction measuring device used. When quoting an estimate, use single digit.)
When the runway is contaminated there is a very good chance the weather is less than perfect, so you are going to need the perfect approach. Remember that flying a stabilized approach that arrives over the runway threshold at just the right speed and descent rate, and aligned with the runway centerline will improve your odds for the work yet to come.
Your aircraft’s demonstrated landing performance is based on very specific landing techniques that are not designed for passenger comfort or applause, but rather to turn the aircraft from an air to a ground vehicle expeditiously and within the touchdown zone of the runway. If you do not normally land this way, you need to practice in the simulator or resign yourself to add hundreds, if not thousands, of feet to your computed landing distances. You should research your aircraft flight manuals for the correct technique. On contaminated runways you must make a positive landing. Many aircraft rely on the pilot arriving over the threshold at a specific speed, reducing thrust and touching down at a specific speed. This precludes any kind of exaggerated flare if flown by the book. Flare distance can be the largest variable in landing distance if not flown properly. No matter how it is specified, you need to follow the manufacturer’s procedure if you expect to achieve the manufacturer’s performance numbers.
If your aircraft calls for full reverse thrust after touchdown, that’s what you need to apply. Most thrust reverser systems are most effective at higher speeds and become mere noisemakers at low speeds. All of it, as soon as you can, is a good mantra when it comes to reverse thrust. You also need to understand how this will impact your computed landing distances. With some aircraft, reverse thrust is an added bonus and only makes things better. With others, partial or full credit may be taken for reverse thrust.
Braking should be accomplished as dictated by the aircraft flight manual. In general, brakes on aircraft without antiskid systems should be applied firmly to achieve maximum braking effectiveness. This point occurs just prior to the point where wheel skidding occurs. If skidding does occur, release brake pressure to stop skidding and reapply brake pressure with a little less force. Maximum braking is normally accomplished on aircraft with antiskid braking by holding maximum brake pressure and allowing the antiskid system to operate. Letting up on the brakes defeats the purpose of the antiskid system. Pilots may notice a pulsating feeling in their feet as the antiskid cycles; this is considered perfectly normal for most systems.
The Perfect Landing
Many of us have trained our entire careers for those silken landings that prompt passengers to ask, “Are we down yet?” How can you be a pilot and not have your ego fed by the applause in the back when the first sign of landing is the reverse thrust? We have to train ourselves and our passengers, that a perfect landing is one that follows the perfect approach, is on speed in the touchdown zone of the runway, and ends with the airplane exiting the runway in a controlled manner. We have to train to do this on the best, dry runways so that the result is exactly the same on the worst, contaminated runways. You’ve probably heard this your entire career and may have even preached it. But now you have to practice what you preach.
Excerpts from: Contaminated Runways, by James Albright, B & CA Sept 2014