Dont forget to add, that on a NPA you always use time (unless GPS). So, it is not a matter of altitude but a matter of how much time has lapsed since the FAF that determines the missed approachBryan Bernatek.

Bryan,Most NPAs flown by airliners have DME or some other kind of distance check mechanism. This allows a constant descent approach to be made (i.e. flown on VNAV or V/S). Boeing SOPs are to fly to MDA+50ft and go around if visual references are not available. MDA+50ft is treated as a decision altitude (in fact, the manuals name it the 'derived decision altitude'), so it can be flown through, like a decision altitude, whereas an MDA is a hard altitude and must NOT be busted.Cheers,Nick Jones.

Dont forget to add, that on a NPA you always use time (unless GPS). So, it is not a matter of altitude but a matter of how much time has lapsed since the FAF that determines the missed approach

Personally I have not even once used timing on an NPA and was never instructed to, but I agree there might be approaches that require it definately. In fact I believe it is a matter of altitude but then again maybe there are differences among different approaches.

Most NPAs flown by airliners have DME or some other kind of distance check mechanism. This allows a constant descent approach to be made (i.e. flown on VNAV or V/S). Boeing SOPs are to fly to MDA+50ft and go around if visual references are not available. MDA+50ft is treated as a decision altitude (in fact, the manuals name it the 'derived decision altitude'), so it can be flown through, like a decision altitude, whereas an MDA is a hard altitude and must NOT be busted.

Correct. Even more, if you check out some of the latest Jepp charts you will find that, interestingly enogh, many ex-MDAs have been replaced by DAs, and we're talking NPA here, not ILS. Well, I should check against the old charts if the numbers have changed, would be interesting. For exact that reason you mentioned. The 50' margin allows for transition of descent into climb, and hopefully not touch the MDA. Whereas the ordinary DA allows a slight descent thru, like when you take the decision at DA you will lose some more altitude before the missed approach climb has been established. Examiners hate it big time whenever you touch one of those check altitudes or even worse the MDA, that can get you into a whole lot of trouble to say the least.

thanks for the interesting and detailed answers.to say it with the words of the best german soccer player lothar mathäus: "again what learned" ;-)nicolas reich

Correct, but as Andrew pointed out above you might also reference to MSL rather than AGL so especially for small planes that don't even have an RA it's common to use DA and not DH, while it's still a precision approach. Say you fly a CAT I ILS in some simple single engine prop and let's assume a TDZE of 263', and DH 200', then DA would maybe be 463' to make a quick and simple example. This can be baro referenced by the ordinary altimeter.As it's been said before there is also MDH stated on some charts (Jepp does is IIRC), but I've not encountered a practical use for it other than knowing how high you actually are above the ground, then again I haven't seen anyone reference to RA during an NPA but I might be wrong.

Right, and the other key issue here is that you can legally, and may wish to descend rapidly to get to the MDA. An airliner isn't likely to do this. Maybe a turboprop like a J41 in some remote part of the world may want to do this. So, the DA doesn't mean when you reach it, decide in that instance. You'll need a horizontal reference (DME, GPS, Time, whatever) to figure out when to go missed.

Right, and the other key issue here is that you can legally, and may wish to descend rapidly to get to the MDA.

Very true, and sometimes it makes a lot of sense as well, depending on the particular approach. Some MAPs are so early that you will be at the MAP and be yet well above MDA (which obviously is a problem if you have a critically low ceiling) if you had not performed a well-planned and articulate descent. So yes, in fact it might be useful to 'dive' into the MDA after the last check alt if you're in a small agile plane, that is perfectly alright.

So, the DA doesn't mean when you reach it, decide in that instance. You'll need a horizontal reference (DME, GPS, Time, whatever) to figure out when to go missed.

Hm not 100% sure what you mean. As far as I understand it the point as to where the decision has to be taken on an ILS is where DA/DH intersects the GS, so to speak. This marks a definite and fixed point in space, I guess. If that's what you mean by horizontal reference, then I fully agree. For NPAs, yeah the MDA is not quite the decision point but rather the MAP, whatever the altitude be there (see above)... :(

So, the DA doesn't mean when you reach it, decide in that instance. You'll need a horizontal reference (DME, GPS, Time, whatever) to figure out when to go missed.

I would argue it is just the opposite - you need other "horizontal references" (your language) to figure out when to go missed during non-precision approaches only however in case of precision approaches with DA - YES, this is the point to decide, in that instance (and not later). DA in fact defines a point in space, MDA does not.

I happened to be looking at my posts and happened upon Michale and Etienne's responses. And you're both right, I wasn't real clear there, but I was trying to say what Michael articulated better, which is that its with the NPAs that you need to find the MAP using some horizontal refrence . As Etienne pointed out correctly, if you're on a glideslope the DA will be the same spot in space, so usually the DA and the MAP are the same place by definition. (I suppose they could be different, but not on any ILS I've flown.)

This could be helpful to provide further explanation. Be sure to read all the way to the IAP examples:http://www.faa.gov/air_traffic/publications/atpubs/aim/Chap5/aim0504.html