Final answer:
A pilot can descend below MDA/DA when visual contact with the runway is established, ensuring a safe landing. The required visibility and identifiable runway references must be met as per FAR 91.175. Option (a) is the correct answer.
Step-by-step explanation:
The question pertains to the regulations for instrument flight rules (IFR) approaches in aviation, specifically about when a pilot can descend below the minimum descent altitude (MDA) or decision altitude (DA). According to the Federal Aviation Regulations (FAR), specifically CFR 91.175, a pilot can only descend below MDA/DA if:
- The aircraft is in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers.
- The flight visibility is not less than the visibility prescribed in the standard instrument approach being used.
- At least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot: the approach light system, the threshold, the threshold markings, the threshold lights, the runway end identifier lights, the visual approach slope indicator, the touchdown zone or touchdown zone markings, the touchdown zone lights, runway or runway markings, or runway lights. Therefore, the correct option from the given choices is (a) When visual contact with the runway is established. Explicit clearance from air traffic control (ATC) is required for IFR flight, but it is not the conditional factor for descending below MDA/DA; it's the pilot's responsibility to ensure that the required visual references are in sight. Descent cannot be initiated solely based on passing the outer marker or at the pilot's discretion without the required visual reference.