What happens to the altimeter if the static source is blocked while the pitot tube remains open?

When the static source becomes blocked while the pitot tube remains open, the altimeter's function is affected due to the relationship between static pressure and altitude. The altimeter relies on static pressure to determine the aircraft's altitude, as it measures the difference between the ambient external pressure and the pressure inside the instrument.

If the static port is blocked, the altimeter cannot adjust to changes in the atmospheric pressure outside the aircraft, which occurs as the aircraft climbs or descends. As a result, the altimeter becomes "frozen" at the altitude it was indicating at the moment the static source was blocked; it will not respond to actual changes in altitude. This characteristic is critical for pilots to understand, as it can lead to dangerous situations if they rely on incorrect altitude information when the static port is obstructed.

The other potential outcomes, such as showing a steady climb or descent, do not occur because without a clear static reference, the altimeter simply cannot update and responds to changes in altitude inaccurately, leading it to display the last known altitude steadily without fluctuation.