Algol:
But my question is - since you are normally required to add 50ft to the CDA flown NPA minima, why would it not be required here too? Aren't you likely to sink below the MDA if you commence level-off (for circling) AT the MDA?
Yes, you probably will sink below MDA if you don't commence the level-off until you actually reach the MDA. I would argue that a CDFA is a very different kettle of fish to a non-precision approach followed by circling. The CDFA is designed to mimic an ILS approach - you fly an approach with a constant descent angle to a 'decision altitude' (ie MDA + 50ft). On reaching the 'DA' you either continue the approach and land if visual, or execute a missed approach, taking care not to descend below the MDA. The aircraft does not fly level at any point during the final approach. The purpose of the 50ft buffer is to prevent the aircraft descending below the MDA while executing the missed approach.
The NPA that precedes a circling approach has one purpose; it allows the aircraft to descend to the circling minima for the purpose of executing a circling approach. It might be flown as a constant descent angle approach, but the aircraft can level off at the circling minima and then proceed to the missed approach point before executing the missed approach if not visual. You would certainly start levelling off before you reach the MDA, as you would when levelling off at any altitude, but there is no requirement for a 50ft additive to the MDA.
I agree with vilas regarding the requirement for wind corrections to the downwind timing. The 25kt tailwind component is part of the PANS-OPS design criteria used to define the radius of the circling area. The turn radius used to calculate the circling area is based on the aircraft's TAS plus a 25kt wind factor. The circling area is then defined by a radius that is twice the turn radius plus a fixed straight segment, centred on the runway thresholds. That is somewhat different to saying that the circling procedure includes a 25kt tailwind component; it doesn't. The downwind timing should be corrected for wind to ensure the aircraft turns base at the correct point and remains within the circling area.