Capt S D, I disagree with your statement that you can't descent below MDA at night until on final. The rules specify that at night the MDA must be maintained until the normal approach profile is intercepted. Depending on MDA that intercept can also occur on base or even downwind.
W.r.t the situation of reaching the Straight In MDA and deciding to circle instead of making a missed, I'm of the opinion that both vis & cloud must meet the circling minima for that approach to use the circling procedure - not the SI minima. If it's daytime and I can maintain at least 300' and the weather meets circling minima then I'd circle. I would also climb as much as conditions allow.
If it was at night then I'd commence the missed approach. During the missed if the circling requirements re Wx minima, MDA *and* still in sight of the approach end of the runway then I'd consider circling. Depends on why I couldn't complete the straight in approach. Was I a bit late descending so too high on final to continue or was the runway still occupied or similar?
Both cases presume there isn't a 'no circling' limitation that gets in the way. I always brief the circling procedure if I have any doubt about the approach, even for a straight in. Part of that includes considering how to change from straight in to a circling approach.
Last edited by Tinstaafl; 1st Jul 2010 at 06:29.