Continuing to descend, the aircraft reached its minimum descent altitude (MDA) of 2800 feet – the lowest altitude to which descent is authorized on final approach or during circle-to-land maneuvering in execution of a standard instrument approach procedure where no electronic glideslope is provided (the airport was not equipped with an instrument landing system) – but both pilots said that they still had no visual reference to the runway or its approach lighting system because of the thick fog.