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posted 30th October 2003 13:18
I wrote this piece on MU readings/runway contamination for our in house publication......
RUNWAY FRICTION (MU) MEASUREMENTS:
In the course of reviewing a recent industry runway excursion (U.S), it has become apparent that there is a major discrepancy between airports and the methods used in measuring and reporting runway friction values. The purpose of this article is to make crews aware of these discrepancies and to be cautious in using reported “MU” values as a primary indication of braking action.
In an effort to standardize with I.C.A.O., the FAA has begun to report braking action in a quantifiable way, using the “MU” value. MU values range from 0 to 100, where 0 is the lowest friction value and 100 is the theoretical maximum friction value obtainable. There is an “estimated” correlation between MU values and the historical braking action reports of good, fair, poor, and nil.
MU readings are normally provided, where available, to the crew by ATC in the form of NOTAM, ATIS, or tower. One measurement is taken on each third of the runway, resulting in a report for three segments of the runway. Normally MU values are reported only when values fall below an MU of 40, this being the baseline number where braking performance deteriorates.
All of this sounds good and quite scientific in its approach, however there have proven to be some major discrepancies in how these values are measured, charted, and reported. There are two primary ways to measure an MU value. One is by the “Continuous Friction Measuring Equipment” (CFME), and is the preferred method. An example of this is the Saab Friction Tester (the car that looks like it is dragging a bicycle tire behind it). The other method of measuring MU is using either an electric or mechanical decelerometer. There are multiple manufacturers of these devices, and their measurements, scales and charts are only guaranteed to provide a “nearly equivalent” correlation to MU values.
Other limitations of using an MU value as a primary indication of runway friction are:
(1) Runway friction measurements are taken only on the center section of the runway (10 feet from centerline for narrow body operations, 20 feet for wide body operations), so the tested area is only a small part of the total runway surface.
(2) If the runway MU values are measured by decelerometer, its readings are affected by the deceleration characteristics of the vehicle it is mounted to. There is no standard. (The same can be said of aircraft – different aircraft types have different deceleration characteristics).
(3) The airport operator shoulders the responsibility of conducting and reporting runway friction tests to ATC. Data once again shows inconsistencies between how different airport operators conduct these tests and report their results.
(4) The CAA of Great Britain has issued a bulletin (FODCOM 17/2001) to eliminate reporting of (measured) runway friction in slush and thin deposits of wet snow. “The Authority is aware that the runway friction measuring machines currently available do not give a reliably accurate reading in conditions of slush or thin deposits of wet snow”.
In preparing for this article, I consulted information from the FAA (AIM, regulations, and advisory circulars), CAA (U.K.), CTA (Canada), our own and other airline FOM’s, Jeppesen “J-Aid”, and material from the manufacturers of various friction measuring devices. The main consistency I found among these materials was their inconsistency.
This is not to say that runway friction measurements should be totally disregarded during contaminated runway operations. It is just to emphasize that there are limitations to relying on what appears to be, by way of measurement and numerical values reported, a “scientific” approach to measuring surface friction. In the real world, braking action and directional control are affected by numerous other factors that are aircraft specific such as aircraft geometry, tire pressures, CG, brake pack wear, etc. As mentioned in one of the referenced materials, braking action reported as poor by an F-100 may be good to a B-757.
The nature of our operation presents some interesting challenges in this regard. Our next day operation results in our aircraft sometimes being the first transport category aircraft in hours to use a contaminated runway, with the lack of recent braking action reports from other transport aircraft. An MU value may be the only indication provided to the crew in these circumstances, and as indicated above, these values are sometimes suspect. It is in these circumstances that we become part line pilot/part experimental test pilot.
Make sure to provide ATC with a braking action report under these circumstances or any time you experience degradation from reported conditions.
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