What happens if the static port becomes blocked?

When the static port becomes blocked, the airspeed indicator (ASI) will function incorrectly because it relies on static pressure to determine airspeed. The ASI measures the difference between dynamic pressure (from the pitot tube) and static pressure. If the static port is blocked, the static pressure remains constant and does not reflect changes in altitude or air pressure, leading to inaccurate readings.

While this blockage can have an effect on other instruments, the ASI is primarily affected as it does not have the ability to compensate for trapped static pressure. The vertical speed indicator (VSI) will show a lack of rate of climb or descent but may still initially indicate a slight movement before stabilizing inaccurately. The altimeter will also read incorrectly, but it may still be functional depending on the pressure at which it was last calibrated.

An alternate static source could be used to bypass a blocked static port, rather than being the only option after the blockage occurs. Knowing this is essential for a pilot in assessing and responding to an instrument malfunction during flight.