If you're referring to NPAs flown using the CDFA technique, then an allowance must be included when calculating a DA from the published MDA.
If an NPA is flown using a notional GS, but without the CDFA technique, then it will invariably be above the GS at MDA as power must be applied to level at MDA.
However, if an NPA is flown using a free-descent technique at the maximum descent gradient assumed by procedure design (where permitted), then the aircraft can be levelled at MDA until either landing criteria are achieved or the MAP is reached.
So for large CAT aircraft, the CDFA technique is now normal SOP, as it then means that only one basic SOP is required for non-LVP ops. Whereas for light GA aircraft, either the notional GS or the free-descent technique to MDA may well be preferable.
Those who use the stupid term 'duck and drive' probably have no idea that the maximum free-descent gradient assumed by procedure design is only 25% steeper than the normal notional GS....
Last edited by BEagle; 30th Mar 2015 at 16:57.