What is the primary instrument affected by the proper functioning of the static pressure system?

The primary instrument affected by the proper functioning of the static pressure system is the altimeter. The altimeter relies on atmospheric pressure to determine altitude; it measures the static pressure of the air surrounding the aircraft. When the altimeter is calibrated correctly with reference to the standard atmosphere, it will provide accurate readings of altitude above sea level. If the static pressure system is malfunctioning or blocked, the altimeter can give inaccurate readings, which can be critical during flight operations, particularly during takeoff and landing when altitude awareness is crucial.

Other instruments, while they may also utilize static pressure to some extent, are not primarily dependent on it for their main function. For instance, the airspeed indicator uses a combination of both static pressure and dynamic pressure from airflow. The direction and heading indicators utilize magnetic and gyroscopic principles, which are not reliant on static pressure.