The holes in the sensors for pitot and static pressure detection must be fully open and free from blockage. Partially or completely blocked sensor holes will give erratic or zero readings on the flight instruments.
If the (unheated) sensor holes in the static system become blocked (possibly with ice), the altimeters will appear to freeze at their present altitude, the Vertical Speed Indicators (VSI) will indicate level flight and the Air Speed Indicators (ASI) will either over-read or under-read.
An alternate static source control valve is located below the instrument panel to the lower right of the left-hand-seat pilot’s control wheel, to the left of the gear lever. It is difficult to see due to its location, but normally the valve handle feels like it points to the right. Rotating the valve to point towards the pilot allows the static source to be taken from inside the cabin and should restore the readings on the altimeter, ASI and VSI.
Selecting alternate static source will result in slightly different readings dependent on pressure conditions in the cabin. Airspeed, heating and ventilation settings and the status of the storm window can influence cabin pressure. However, a correction card (calibrated with the storm window close and the cabin heater AIR INTAKE set to open) is available by the pilot’s left knee.