A blocked pitot tube is a pitot-static problem that will only affect airspeed indicators.[5] A blocked pitot tube will cause the airspeed indicator to register an increase in airspeed when the aircraft climbs, even though actual airspeed is constant. (As long as the drain hole is also blocked, as the air pressure would otherwise leak out to the atmosphere.) This is caused by the pressure in the pitot system remaining constant when the atmospheric pressure (and static pressure) are decreasing.