Under what circumstance must gear be extended early?

Extending the gear early during an unstable approach is a critical safety measure. An unstable approach can lead to a variety of issues, including inadequate descent rates or misaligned landing trajectories, which increases the risk of a runway excursion or hard landing. Lowering the landing gear sooner in this scenario provides the pilot with additional drag, helping to reduce the aircraft's speed and sink rate. This change in configuration allows for a more controlled descent and better visibility of the runway, ultimately contributing to a safer landing.

In this case, the other scenarios are less relevant for the early extension of the gear. In a climb, the aircraft typically does not require gear to be down since it is gaining altitude. Executing a stall is a situation that is usually practiced under controlled conditions, and gear position would not be early extended as it might introduce variables that complicate stall recovery. Cruising at altitude maintains a clean airframe configuration for optimal performance and efficiency, so there is no need to extend the gear in this phase of flight.