Partially blocked static vent from actual experience . .1/ Take off roll - normal indications
2/ Climb - airspeed steadily reducing with height gained, past stall, past zero, to minus 140 kts. VSI close to zero, Altimeter indicating the altitude corresponding the height of the blockage (with a partial blockage it very slowly climbs at a lesser rate than the aircraft.
3/ Cruise - Everything slowly comes back to normal as the pressure equalises to the correct outside pressure.
4/ Descent - The exact opposite to climb, the airspeed indication increased past VNE and right on past zero again. In the circuit 180 kts on final, touched down 140 kts and turned into the taxiway indicating 80 kts (Piper Comanche)
2nd occasion right hand seat in PA31 Navajo IF take off "dont lower the nose any further you have a blocked static vent, hit your alternate static source)
This I teach my IFR students. .If you pre think this scenerio prior to an IFR takeoff, and can immediatly put your hand (eyes closed for practise) on the alternate static source, then you should live to tell the tale.