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YOU have just completed a mission and landing data is well within tolerances. The landing is uneventful. However, the airplane continues past the end of the runway and over a cliff!
How many times after reading such an introduction have you seen the words "This situation is fictional, but it could have happened"? Well, this one really did happen!
The scene is a northern base with typical crosswinds, snow and temperatures hovering near freezing. It's dark and a bird is approaching a 10,000 foot runway. Reported weather is 2000 scattered, 7-mile visibility, crosswinds with a 10-knot gust and the runway condition is: ice on runway, runway condition reading (RCR) 16 patchy. The computed landing roll is 4500 feet including gust factors. There are no aircraft malfunctions; tech data is within tolerances; and everything appears normal through touchdown; but hang on - as everyone knows, large airplanes can (like Cinderella) turn into pumpkins after midnight; and everyone also knows that pumpkins have ineffective braking systems. This particular pumpkin had an actual landing roll that continued beyond the end of the runway. The RCR was actually 05.
To insure a thorough understanding of the braking effectiveness of pumpkins, we have to be familiar with the RCR's and the conditions that can cause them to change. How many know that an RCR reading is an average of several readings? The resultant figure given to the pilot comes from a series of tests at intervals along the runway 20 feet either side of the runway centerline. The problem is that these tests are conducted in a vehicle traveling between 20 and 30 mph. Now if you are flying a "six-pax" or a station wagon not over 30 mph, you have it made and you can stop reading now. If however, you are flying a large jet, especially one without thrust reversers, you may want to press on.
On bone dry hard surfaces, the decelerometer is extremely accurate and will correctly depict the stopping capability. On all dry surfaces, including hard packed snow and ice, the decelerometer remains accurate. With the addition of moisture to the surface, whether it is in the form of standing water, falling rain or slush, the problem becomes critical. At this point speed becomes the key factor. The speed regime for RCR checks is below the hydroplaning speed for the vehicle which is approximately 45 mph.The threshold of hydroplaning is never reached during RCR tests so its effect is not recorded when using the decelerometer. We also must remember that hydroplaning is like he Takhli Trots; easy to get and hard to get rid of. The threshold of hydroplaning can accurately be determined (VH+ square root of P, where P equals tire pressure), and during acceleration the bird will be hydroplaning at that speed. However, during deceleration you will not necessarily stop hydroplaning as you drop below the command speed.
Propanol is another factor which can greatly alter the RCR. It is an old friend when it comes to eliminating ice on the runway, but like wonder drugs, there can be some bad side effects which may put you in a real bind. Propanol applied to the runway will break up ice formations and allow the runway to be swept clean. After cleaning, the runway surface retains enough moisture, even though it appears to be dry, that snow and blowing snow will stick to the surface. The snow which is now adhering to the runway and the moisture retained from the propanol combine to form slush which can reduce the RCR to a 04. If propanol is used on your runway, be alert to the possibility of slush when snow is falling after such an application. This condition can exist for some time following the application of the propanol.
Having reflected on some of the problems involved in recording RCR's and how they can change, we can now examine these variations due to water or slush. This information is contained in the flight manual under hydroplaning; however, figure 1 may help you to understand the prose and let you see just how the engineers came up with RCR figures of 09 and 04 for wet and slush covered runways. The shaded areas indicate the variations in RCR due to water or slush on the runway. The solid diagonal line represents the maximum deviation from a dry runway condition (dashed line) that can exist when water or slush is present. These variations are the result of hydroplaning and can reduce a dry runway (RCR 23) to an RCR of 09 when water is added and to an RCR of 04 when slush is added to the surface. I say "can" reduce effective RCR's since this is supposed to be the worst condition you should encounter. It is possible that actual braking action may be somewhat better than the worst condition. The 09 and 04 figures are representative and are designed to provide an adequate safety margin. This chart is the basis for our tech data and is the result of aircraft actual tests. "
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