Final answer:
In aviation, the circling Minimum Descent Altitude (MDA) is the lowest altitude an aircraft can descend to during a non-precision instrument approach without visual reference to the runway. The pilot can descend below the circling MDA only after establishing visual reference to the runway and meeting visibility requirements.
Step-by-step explanation:
In aviation, the term MDA stands for Minimum Descent Altitude. It is the lowest altitude that an aircraft can descend to during a non-precision instrument approach without visual reference to the runway or runway environment. On the other hand, circling MDA is the minimum descent altitude specific to circling approaches, which are used when the final approach course and landing runway are not aligned. The circling MDA is usually higher than the straight-in MDA to provide additional obstacle clearance during the circling maneuver.
Regarding the question of when you can descend below the circling MDA, you are allowed to descend below the circling MDA only after you have established visual reference to the runway and runway environment, and have the required visibility and cloud clearance as specified in the applicable regulations. This means that the pilot must have adequate visual cues and be able to maintain visual contact with the runway and surrounding environment throughout the maneuver.
For example, if you are flying a circling approach and you spot the runway environment at a point higher than the circling MDA, you can continue the approach and descend below the circling MDA as long as you can maintain visual contact with the runway and meet the visibility and cloud clearance requirements. However, if you descend below the circling MDA and lose visual reference, you must execute a go-around and climb back to a safe altitude while repositioning for another attempt.