Refer to figures.

The pitot-static instruments usually are very reliable. Gross errors almost always indicate blockage of the pitot tube, the static port, or both. Blockage can be caused by moisture (including ice), dirt, or even insects.

PITOT BLOCKAGE

The airspeed indicator is the only instrument affected by a pitot tube blockage.
If the pitot system becomes completely clogged and the static system remains clear, the indicator reacts as an altimeter, showing an increase in airspeed when climbing and a decrease in speed when descending. As the descent is made, the static pressure would increase against the diaphragm causing it to compress, thereby resulting in an indication of decreased airspeed. The opposite would happen in a climb.

STATIC BLOCKAGE

If the static system becomes clogged the airspeed indicator continues to react to changes in airspeed, because raim air pressure is still being supplied by the pitot tube, but the readings are not correct. The ASI will reverse the error that occurs with a blocked pitot, it underreads in a climb and overreads in a descent. Because the altimeter determines altitude by measuring ambient air pressure, any blockage of the static port freezes the altimeter in place and makes it unusable. The Vertical Speed indicator freezes at zero, because its only source of pressure is from the static port.

• After verifying a blockage of the static system, an alternate source of static pressure should be used. If the aircraft is not equipped with an alternate static source, the glass of the vertical speed indicator can be broken to allow ambient air pressure to enter the static system. This, however, makes the VSI unserviceable.
• If, while in level flight, it becomes necessary to use an alternate source of static pressure vented inside the aeroplane, the altimeter might read a little higher and the airspeed a little faster than normal, and the vertical speed indicator might show a momentary climb.