What visual reference must be identified to descend below MDA/DA?

To descend below Minimum Descent Altitude (MDA) or Decision Altitude (DA) during an instrument approach, it is crucial to identify specific visual references that provide assurance that a safe landing can be made. One of the primary visual references is the runway threshold lights. These lights indicate that the runway is sufficiently visible and that the pilot is in a position to safely land the aircraft.

When threshold lights are visible, it means that the pilot has a clear indication of the runway's position, which is essential for making a safe landing. The presence of appropriate visual references, such as threshold lights, outlines the ability to proceed with the descent and assures compliance with regulations requiring visual contact with the runway environment.

Other options—while they are related to flying safety—do not fulfill the criteria for proceeding below MDA/DA. For example, flight visibility refers to the general visibility conditions, which may or may not include adequate references for landing, and sky condition and cloud base give information on the weather but do not provide direct visual cues for a safe approach and landing. Thus, verifying threshold lights is a critical step in deciding to descend safely.