Your aircraft’s airspeed indicator, altimeter, and vertical speed indicator get their reference static pressure from a small hole, or port, in the skin of the aircraft at a location where the airflow is not disturbed. Combined with impact air pressure—from air striking the airplane because of its forward motion—transmitted from the pitot tube, static pressure allows proper operation of the pitot-static system. Errors in the ASI, altimeter, or VSI normally indicate blockage of the pitot tube, static port(s), or both. The culprit usually is moisture, dirt, or insects.
There may be a static port on only one side of the fuselage, or on both; sometimes the static port is part of a combined pitot-static tube. Be sure to check the static port during preflight, especially if you’re the first to fly an airplane after maintenance—it’s not uncommon for it to be covered with a piece of tape while the airplane is being washed. Blocked or partially blocked static or pitot openings should be cleared by a mechanic; blowing into the openings could damage the instruments.
How do you know if the static port is blocked? If the altimeter and vertical speed indicator needles don’t move while you climb or descend, the port’s probably blocked. Your airspeed indicator also may not read correctly.
To fix this problem, activate the alternate static air source, if the airplane is equipped with one. If not, in an emergency—say you’ve entered instrument meteorological conditions—break the glass on the VSI. That will restore basic functionality, although the instruments’ readings won’t be exactly correct.
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