What is the fundamental skill of instrument flight that involves continuous instrument observation?

The fundamental skill of instrument flight that involves continuous instrument observation is best described by the concept of cross-checking. In instrument flying, a pilot must consistently monitor various flight instruments to maintain situational awareness and control of the aircraft. Cross-checking refers to the process of scanning and comparing information from multiple instruments to ensure that the aircraft is operating within safe parameters and to detect any discrepancies that require corrective action.

This technique enables the pilot to build a comprehensive picture of the aircraft's performance and navigate effectively regardless of visibility conditions. Continuous cross-checking helps pilots respond promptly to any changes in flight conditions or instruments, thus enhancing safety and efficiency in instrument conditions.

In contrast, the other options—attitude scan, instrument review, and altitude check—while involving aspects of instrument flying, do not encapsulate the broader skill of continuously monitoring and interpreting data from various instruments in a systematic manner like cross-checking does. An attitude scan specifically focuses on a pilot's assessment of the aircraft's orientation, while an instrument review might imply a more infrequent or cursory look at instruments. An altitude check is just one element of what is monitored in instrument flight, rather than representing the comprehensive continuous observation required for effective cross-checking.