Which instrument is primarily affected during a power outage?

The turn coordinator is primarily affected during a power outage because it relies on electrical power to operate its internal components. This instrument uses an electrically-driven gyroscope to measure and display the rate of turn and ensure that the aircraft is flying coordinated turns. In the event of a power failure, the turn coordinator may not provide accurate information, rendering it inoperative.

On the other hand, instruments like the altimeter, heading indicator, and airspeed indicator may not be as directly impacted by a loss of electrical power. The altimeter, for instance, operates using barometric pressure; hence, it does not require power to function. The heading indicator, although it can rely on a gyroscope, can also be a simple magnetic compass variant, which operates without electricity. The airspeed indicator operates based on differential pressure and also does not require electrical power to provide readings. Thus, the turn coordinator stands out as the instrument most critically affected by a power outage.