Page 111 - Planning and Design of Airports

Air craft Characteristics Related to Airport Design 81

criteria limit the gradient to a maximum of 1½ percent. Information
provided by aircraft manufacturers in flight manuals is based on uni-
form gradient, yet most runway profiles are not uniform. In the
United States aircraft operators are allowed to substitute an average
uniform gradient, which is a straight line joining the ends of the run-
way, as long as no intervening point along the actual path profile lies
more than 5 ft above or below the average line. Fortunately most run-
ways meet this requirement. For airport planning purposes only, the
FAA uses an effective gradient. The effective gradient is defined as
the difference in elevation between the highest and lowest points on
the actual runway profile divided by the length of the runway. Stud-
ies indicate that within the degree of accuracy required for airport
planning there is not very much difference between the use of the
average uniform gradient and effective gradient.
Condition of Runway Surface
Slush or standing water on the runway has an undesirable effect on
aircraft performance. Slush is equivalent to wet snow. It has a slip-
pery texture which makes braking extremely poor. Being a fluid, it is
displaced by tires rolling through it, causing a significant retarding
force, especially on takeoff. The retarding forces can get so large that
aircraft can no longer accelerate to takeoff speed. In the process slush
is sprayed on the aircraft, which further increases the resisting forces
on the vehicle and can cause damage to some parts. Considerable
experimental work has been conducted by NASA and the FAA on the
effect of standing water and slush. As a result of these tests, jet opera-
tions are limited to no more than ½ in of slush or water. Between ¼
and ½ in depth, the takeoff weight of an aircraft must be reduced
substantially to overcome the retarding force of water or slush. It is
therefore important to provide adequate drainage on the surface of
the runway for removal of water and means for rapidly removing
slush. Both water and slush result in a very poor coefficient of brak-
ing friction. When tires ride on the surface of the water or slush the
phenomenon is known as hydroplaning. When the tires hydroplane,
the coefficient of friction is on the order of wet ice and steering ability
is completely lost. Hydroplaning is primarily a function of tire infla-
tion pressure and to some extent the condition and type of grooves in
the tires. According to tests made by NASA, the approximate speed
at which hydroplaning develops may be determined by the following
formula:

0.5
V = 10 p (2-6)
p
where V is the speed in miles per hour at which hydroplaning devel-
p
ops and p is the tire inflation pressure in pounds per square inch.
The range of inflation pressures for commercial jet transports var-
2
ies from 120 to over 200 lb/in . Therefore the hydroplaning speeds