circling minima
 

once visual cues are aquired, when a pilot is "legally" allowed to start the descent from minima during circling ? (on downwind, base or final ?)

My question because If the visibility is marginal you should stay very close to the runway to mantain it in sight and therefore need to anticipate the descent from minima (downwind) .

The answer is simple. If you cannot see the ground in front of you (night) over which your flight path will take you, then never descend below the circling MDA until on final approach. At night it is impossible to judge obstacle or terrain clearance unless you are over water. The critical obstacle that determines the MDA is rarely displayed on the instrument approach chart and spot heights are not always shown. If you decide to descend below the published circling MDA on downwind or base in an attempt to fly a comfortable profile then obstacle clearance becomes entirely your own responsibility. The lawyers will have you if you live to tell the tale. And rightly so.

If it means the MDA will make it impossible to descend until on final and you will then be too high for a stabilised approach, then simply divert or try an alternative approach. Rationalisation is potentially dangerous.

PANS-OPS Vol 1

7.3.3 Minimum descent altitude/height (MDA/H)
When the OCA/H is established, an MDA/H is also specified to allow for operational considerations. Descent below MDA/H should not be made until:
a) visual reference has been established and can be maintained;
b) the pilot has the landing threshold in sight; and
c) the required obstacle clearance can be maintained and the aircraft is in a position to carry out a landing.

it means with circling minima of 1500 agl you should turn on final 5 miles !

If in your senario of a low-vis circling approach, if you can't keep the runway in sight in order to position yourself to make a safe and stable approach, then divert. Thats why there is an alternate.