A traditional NPA (some of them still have that option) has a step down procedure. In this case you level off and continue to the MAP, where you decide to go around or not. In this case you would not loose ALT, so you won't go below your MDA. With these CDAs of nowadays it gives you the possibility to shoot a NPA faster, easier and with less noise. Only problem is, that you will not level of towards the MAP. When you initiate the go around you will indeed go below the MDA.
As it stands for MINIMUM DECENT ALT, you can not bust it unless you continue with the approach. The 50' is a safe buffer.
MDA on NPA's is slowly being replaced by DA at a lot of EASA member-state airports, sometimes resulting in slightly higher minima's.
In some cases, 50' makes the difference of becoming visual or doing a G/A. This is why I am not a fan of VNAV NPA approaches combined with an MDA in marginal weather. Use the V/S and put the MDA in the MCP. Keep slightly low in order to ensure level flight at the Decision Point. (Decision Point is the point where on a normal ILS, you would end up at minima considering a standard glide angle of 3°). If no contact, go around. (and NO there is NO requirement to fly level up to the MDA, this is a big misunderstanding by some collegues). HOWEVER, and this I must emphasise: FOLLOW your COMPANY SOP. This is a personal note and in my present company VNAV NPA's are not done.
MDA means Minimum Descent Altitude. You cannot descend below this altitude unless adequate visual reference is achieved.
DA means Decision Altitude. This is the altitude that the decision is made to go around. You will descend slightly below this altitude in a go around.
In my company, when conducting a NPA there is an auto call-out of "Hundred above" when there is 100 feet to go to the MDA. We count to two seconds and if not visual, push the thrust levers forward and go around. From the call of "Hundred above" and two seconds thereafter, we ensure that at no time we descend below the MDA.
Keep slightly low in order to ensure level flight at the Decision Point. (Decision Point is the point where on a normal ILS, you would end up at minima considering a standard glide angle of 3°). If no contact, go around. (and NO there is NO requirement to fly level up to the MDA,
If you note the Jeppeson charts, many have just recently been converted to an MDA. ICAO RNP 9905 uses MDA, while FAA 8260.52 uses a DA. It is not due to precision, but multi-variant performance considerations. With RNP baro-vnav, the approach can be anywhere from 2.5 to 3.1 degrees on a 3 degree approach, hence an MDA...
RNP transition to GBAS final, uses of course, a DA.
Last edited by FlightPathOBN; 26th Jun 2011 at 18:44.
When conducting approach other than ILS, there is no need to add 50ft to MDA as long as a CDA is being used. This is relatively new proc at my current company. We were doing it 15 years ago at my previous company.