When is the pilot required to activate the alternate static source?

The requirement for a pilot to activate the alternate static source arises specifically when the primary static source is blocked. The static source is essential for the operation of various instruments, particularly for altitude and airspeed readings. If the primary source becomes obstructed due to icing, blockages, or other environmental factors, the instruments relying on it may provide erroneous readings, which could jeopardize flight safety. Activating the alternate static source allows the pilot to obtain accurate data from these critical gauges, thus ensuring effective monitoring of the aircraft's performance and facilitating safe operation.

While the other options may describe various flight scenarios, they do not directly relate to the immediate need for activation of the alternate static source. Climbing through a transition altitude, low fuel levels, and descending through controlled airspace do not inherently necessitate switching to an alternate static source unless there are indications that the primary source has failed or become blocked. Therefore, activating the alternate static source is specifically a response to issues affecting the primary static system, establishing its critical role in maintaining accurate instrument readings in the cockpit.