Static Port Blockage

If the static port becomes blocked, the static pressure that is present at the time of the blockage is retained inside the airspeed indicator. Normally this pressure is allowed to fluctuate with changes in altitude. When the aircraft climbs, less static pressure is available so less static pressure surrounds the expandable capsule. Normally this static pressure is canceled out by the static pressure inside the capsule, however when the static source is blocked, the two static pressures are no longer equal and will present errors.

With the case of the blocked static port, you see that the static pressure surrounding the expandable capsule is no longer able to equalize with increases and decreases in altitude. What this means is that when the aircraft decends in altitude, the static pressure inside the expandable capsule becomes greater. This greater pressure overcomes the lesser static pressure that surrounds the capsule and leads to a false increase in airspeed indication. *"The airspeed shows an increase because the indicator   assumes that the increase in pressure inside the capsule is   due to an increase in aircraft velocity." (The reverse is true with a gain in altitude.)

Why this is dangerous!

If we look at what the airspeed indicator is telling us when the static port is blocked, we will be receiving false information. A blockage of the static port is usually associated with ice build ups on the aircraft, but this is not always the case. If it is due to ice, we would normally build ice when flying through the clouds. If you are in the clouds, you have no visual reference to the outside world. You rely solely on the aircraft instruments for flight guidance. If you are descending with a static source blocked, the airspeed will increase as you go lower, how might your reaction get you in trouble?

Don't pitch up to slow your airspeed, if you have a static source block!

Your reaction might be to reduce power in order to decrease airspeed. Eventually you will arrive with the power at idle and your only other recourse will be to pitch up to slow down. This is where it gets dangerous. As you continue to descend, the airspeed will continue to increase. If you continue to increase pitch in order to slow your airspeed, eventually you could get into an attitude where you could stall the aircraft. Your airspeed indicator will be of no use to you when it has a static source blockage.

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