What might indicate a need for greater scrutiny if an aircraft is shown on radar with stability issues?

When an aircraft is shown on radar displaying stability issues, altitude deviation is a significant factor that may necessitate closer examination. Altitude deviation refers to an aircraft's failure to maintain its assigned altitude, which can be an indication of potential control problems or maneuvers that aren't consistent with standard flight operations.

Monitoring altitude is critical for air traffic control as variations can lead to close encounters with other aircraft or create unsafe flight conditions. If an aircraft deviates from its assigned altitude while exhibiting stability issues, it could imply that the pilots are struggling to maintain control or that there is a malfunction aboard the aircraft. This would prompt the need for more intense scrutiny to ensure safety and to determine the appropriate responses or interventions.

Flight visibility, radar returns, and aircraft handoff are less indicative of stability issues when compared to altitude deviations. While these factors contribute to the overall situational awareness of air traffic control, they do not directly reflect the aircraft's control and handling capability in the same critical way that altitude deviations do. Hence, altitude deviation is the most relevant indicator in this context for justifying increased monitoring and responsive measures.