Which instrument should be observed continuously for extended periods?

Observing the Attitude Direction Indicator (ADI) continuously for extended periods is vital because it provides essential information regarding the aircraft's orientation in three-dimensional space—specifically its roll and pitch attitudes. Maintaining proper attitude is crucial, especially in instrument flight, where visual references outside the aircraft may be limited. The ADI helps pilots ensure that they remain in a safe flight attitude, aiding in recovery from unusual situations and maintaining situational awareness during various flight conditions.

In contrast, while other instruments like the Distance Measuring Equipment (DME) and Vertical Speed Indicator (VSI) offer important information, they do not require the same continuous attention due to their less critical nature in maintaining immediate situational awareness and aircraft control. The Primary Flight Display (PFD) with an Electronic Horizontal Situation Indicator (EHSI) is comprehensive and includes multiple flight parameters, but consistent close monitoring of specific instruments like the ADI remains more crucial for attitude control, hence the preference for its continuous observation.