What is the required condition of the aircraft when crossing the runway threshold?

The required condition of the aircraft when crossing the runway threshold is to be at 50 feet Above Ground Level (AGL) at VREF speed. VREF, or reference landing speed, is the speed the aircraft should maintain as it approaches for landing and is crucial for ensuring adequate control and stability during the critical phase of landing. At this altitude and speed, the aircraft is properly configured for landing, and the pilot can effectively manage and control descent, ensuring a smooth transition for touchdown.

Maintaining this specific altitude and speed helps mitigate risks associated with landing, such as stalls or excessive sink rates. The choice corresponding to level flight for approach stability is a necessary condition but does not specify an altitude or speed requirement, which are essential for optimal landing dynamics. Meanwhile, descending at a rate greater than 1000 feet per minute would typically be too steep for a safe approach, and being configured for maximum allowable speed does not align with the objective of preparing for landing, which focuses on controlled and reduced speeds.