Pitot Tube Blockage

If the pitot tube becomes blocked, only the Airspeed Indicator will be affected (The Altimeter and Vertical Speed Indicator depend on static vents). The Airspeed Indicator will act like an altimeter. At altitudes above where the pitot tube became blocked, the Airspeed Indicator will display a higher-than-actual airspeed which will increase steadily as altitude increases; At lower altitudes, the Airspeed Indicator will display a lower-than-actual airspeed which will decrease steadily as altitude decreases. If only the ram air hole is clogged, the pressure in the line will vent out the drain hole, causing the airspeed indication to drop to zero.

•If both the ram air port and the drain hole become blocked, the AI acts like an altimeter (If you climb, the airspeed indicator will show an increase in speed and if you descend it will show a reduction in speed. When both the ram and drain holes are blocked, it's like blowing up a balloon and then sealing it off with a knot. The previously inflated air becomes trapped inside and will hold a constant amount of pressure against the diaphragm. If the airplane starts to climb, the pressure outside will become less and the air on the static side of the diaphragm will escape out the static port. This means that there is less static pressure to push back the ram air. With less pressure to hold it back, the pent-up ram pressure bulges the diaphragm farther and this moves the airspeed indicator's needle to a faster value. How can a pilot detect this? Most aircraft cannot climb and accelerate at the same time. If you see a climb on the VSI and a simultaneous increase in airspeed, something is not right. On the other hand, if the airplane descends, more static pressure will enter the static side, providing additional force to push back the ram air inside. This means less bulge of the diaphragm and less speed shown on the indicator. Apply pitot heat any time you suspect an instrument conflict between speed and climb.

If static only freezes, AI only accurate at altitude where it froze (climbs read lower / descents read higher)

If flying in icing conditions, the blockage is likely due to ice and the Pitot Heat should be turned on immediately.

Static Port Blockage

At altitudes above where the static ports became blocked, the Airspeed Indicator will display a lower-than-actual airspeed which will continually decrease as altitude increases. At altitudes below where the static ports became blocked,the Airspeed Indicator will display a higher-than-actual airspeed which will increase steadily as altitude decreases.

The Alternate Static Source should be turned on which will draw static pressure from inside the cockpit. However, static pressure inside the airplane is usually lower than outside static pressure because of the venturi effect of the outside air flowing over the cockpit. The Altimeter will read higher than actual altitude, Indicated Airspeed (IAS) will read faster than normal and the Vertical Speed Indicator will momentarily show a climb. REMEMBER—FROM HIGH TO LOW, LOOK OUT BELOW (the same effect occurs when switching to the alternate static source inside the cockpit that is in a lower pressure area). •If the ram air port gets blocked but the drain hole remains open, the drain hole acts like another static port, equalizing the static pressure on the other side of the diaphragm (coming from the actual static port)--Air from the outside static port arrives on one side of the diaphragm and air from the drain hole arrives at the opposing side. Both have the same outside pressure so the diaphragm will not budge in either direction. No budge means no airspeed reading.