Runway surfaces vary widely from one airport to another.
The runway surface encountered may be concrete, asphalt,
gravel, dirt, or grass. The runway surface for a specific airport
is noted in the Airport/Facility Directory (A/FD). Any surface
that is not hard and smooth will increase the ground roll
during takeoff. This is due to the inability of the tires to roll
smoothly along the runway. Tires can sink into soft, grassy,
or muddy runways. Potholes or other ruts in the pavement
can be the cause of poor tire movement along the runway.

Obstructions such as mud, snow, or standing water reduce the
airplane's acceleration down the runway. Although muddy
and wet surface conditions can reduce friction between
the runway and the tires, they can also act as obstructions
and reduce the landing distance. [Figure 10-15] Braking
effectiveness is another consideration when dealing with
various runway types. The condition of the surface affects
the braking ability of the airplane.

The amount of power that is applied to the brakes without
skidding the tires is referred to as braking effectiveness.
Ensure that runways are adequate in length for takeoff
acceleration and landing deceleration when less than ideal
surface conditions are being reported.

The gradient or slope of the runway is the amount of change
in runway height over the length of the runway. The gradient
is expressed as a percentage such as a 3 percent gradient. This
means that for every 100 feet of runway length, the runway
height changes by 3 feet. A positive gradient indicates the
runway height increases, and a negative gradient indicates the
runway decreases in height. An upsloping runway impedes
acceleration and results in a longer ground run during takeoff.
However, landing on an upsloping runway typically reduces
the landing roll. A downsloping runway aids in acceleration
on takeoff resulting in shorter takeoff distances. The opposite
is true when landing, as landing on a downsloping runway
increases landing distances. Runway slope information is
contained in the A/FD. [Figure 10-16]