Well now, it depends on how you fly the NPA, the speeds used, and the configuration standard.
For example, in the Lockheed L1011, non-precision approaches as normally flown at Vref+20, with the approach flap setting...22 degrees.
Runway in sight, and descent required for landing, landing flaps are selected (33 degrees) and the speed reduced toward Vref plus any additives.
This is for both straight in and circling approaches.
Using this technique, the aeroplane can be flown right down to MDA, and if the runway is not seen, go-around thrust is applied, a climb commenced, and the landing gear is retracted.
No need to reset the flaps, as they are already at the go-around setting...22 degrees.
The problem arrises when you have slow spooling engines, and landing flaps are selected prior to having the runway in sight.
In these cases, a descent is possible below MDA while going around, IF not done properly.
In short, the 50 foot additive to MDA is for folks who don't know how to do non-precission approaches properly in the first place.
