Grids may suffer from transient blackouts and brownouts, often due to weather damage. Massive brownouts and blackouts lasted for several weeks, even affecting the water supply. Brownouts occur when the supply power drops below the demand. Brownouts can cause unexpected behaviour in systems with digital control circuits. Also the fast response speed of this method would avoid possible brownouts occurring. In 2008 and 2009, Ethiopia again experienced power cuts and brownouts. Intentional brownouts are used for load reduction in an emergency. CENECO had to resort to load shedding, which included scheduled brownouts. The city would endure brownouts and the subway system would go heat-less. Pilots have compared landing during brownouts to parallel parking an automobile with one's eyes closed.