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Pilots should keep in mind slippery/contaminated runway advisory information is based on an assumption of uniform conditions over the entire runway. This means a uniform depth for slush/standing water for a contaminated runway or a fixed braking coefficient for a slippery runway. The data cannot cover all possible slippery/contaminated runway combinations and does not consider factors such as rubber deposits or heavily painted surfaces near the end of most runways. With these caveats in mind, it is up to the airline to determine operating policies based on the training and operating experience of their flight crews.
One of the commonly used runway descriptors is coefficient of friction. Ground friction measuring vehicles typically measure this coefficient of friction. Much work has been done in the aviation industry to correlate the friction reading from these ground friction measuring vehicles to airplane performance. Use of ground friction vehicles raises the following concerns:
.  
the measured coefficient of friction depends on the type of ground friction
measuring vehicle used. There is not a method, accepted worldwide, for
correlating the friction measurements from the different friction
measuring vehicles to each other, or to the airplane's braking capability.

.  
most testing to date, which compares ground friction vehicle performance
to airplane performance, has been done at relatively low speeds (100
knots or less). The critical part of the airplane's deceleration
characteristics is typically at higher speeds (120 to 150 knots).

 

Copyright . The Boeing Company. See title page for details.
October 31, 2006 FCT 737 (TM)  6.35

737 Flight Crew Training  Manual
.  
ground friction vehicles often provide unreliable readings when
measurements are taken with standing water, slush or snow on the
runway. Ground friction vehicles might not hydroplane (aquaplane) when
taking a measurement while the airplane may hydroplane (aquaplane). In
this case, the ground friction vehicles would provide an optimistic reading
of the runway's friction capability. The other possibility is the ground
friction vehicles might hydroplane (aquaplane) when the airplane would
not, this would provide an overly pessimistic reading of the runway's
friction capability. Accordingly, friction readings from the ground friction
vehicles may not be representative of the airplane's capability in
hydroplaning conditions.

.  
ground friction vehicles measure the friction of the runway at a specific
time and location. The actual runway coefficient of friction may change
with changing atmospheric conditions such as temperature variations,
precipitation etc. Also, the runway condition changes as more operations
are performed.

 

The friction readings from ground friction measuring vehicles do supply an additional piece of information for the pilot to evaluate when considering runway conditions for landing. Crews should evaluate these readings in conjunction with the PIREPS (pilot reports) and the physical description of the runway (snow, slush, ice etc.) when planning the landing. Special care should be taken in evaluating all the information available when braking action is reported as POOR or if slush/standing water is present on the runway.
Copyright . The Boeing Company. See title page for details.
6.36  FCT 737 (TM) October 31, 2006

Factors Affecting Landing Distance (Typical)
737-300 - 737-500

* Landing distance varies with runway condition, wet or dry. Data excludes contaminated runway considerations.

737-600 - 737-900

*Landing distance varies with runway condition, wet or dry. Data excludes contaminated runway considerations.
Copyright . The Boeing Company. See title page for details.
6.38 FCT 737 (TM) October 31, 2006
737 Flight Crew Training  Manual

Wheel Brakes
Braking force is proportional to the force of the tires on the runway and the coefficient of friction between the tires and the runway. The contact area normally changes little during the braking cycle. The perpendicular force comes from airplane weight and any downward aerodynamic force such as speedbrakes.
The coefficient of friction depends on the tire condition and runway surface, (e.g. concrete, asphalt, dry, wet or icy).
Automatic Brakes
Boeing recommends that whenever runway limited, using higher than normal approach speeds, landing on slippery runways or landing in a crosswind, the autobrake system be used.
For normal operation of the autobrake system select a deceleration setting.
Settings include:
.  
MAX: Used when minimum stopping distance is required. Deceleration
rate is less than that produced by full manual braking
　
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