Figure 11-15.  Takeoff distance chart.  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 Chart Supplement U.S. (formerly Airport/  Facility Directory).  [Figure 11-17]  Water on the runways reduces the friction between the tires  and the ground and can reduce braking effectiveness. The  ability to brake can be completely lost when the tires are  hydroplaning because a layer of water separates the tires from  the runway surface. This is also true of braking effectiveness  when runways are covered in ice.  When the runway is wet, the pilot may be confronted with  dynamic hydroplaning. Dynamic hydroplaning is a condition  in which the aircraft tires ride on a thin sheet of water rather  than on the runway’s surface. Because hydroplaning wheels  are not touching the runway, braking and directional control  are almost nil. To help minimize dynamic hydroplaning,  some runways are grooved to help drain off water; most  runways are not.  Water on the Runway and Dynamic Hydroplaning  Figure 11-16.  An aircraft’s performance during takeoff depends greatly on the runway surface.  11-13 