Although this might be correct somehow, I don't agree with you saying "the difference...." it's maybe a difference

I wasn't trying to explain the difference between a precision and a non-precision approach - only (a) difference between DA and MDA.

Although this might be correct somehow, I don't agree with you saying "the difference...." it's maybe a difference The difference between a precision approach is that: - a precision approach, you are required to follow carefully a lateral and vertical path. Hence the name precision- a non-precision approach, you are only required to follow a lateral path (a localiser, a VOR, an NDB,.... The vertical path after crossing the FAF, is (within some detailed limits) not important as long as you stay above the MDA. So: after you reach the FAF, you can descend like an eagle, level off to the MDA and fly for miles and miles and miles until you see the runway or reach your MAPt.

Uh, maybe procedures are different, but I strongly disagree. Not a single approach comes to mind where you can dive to the MDA after passing the FAF (not saying there are none, I just haven't come across one recently). In fact the difference by definition between a precision and non-prec approach is that the one OFFERS vertical guidance while the other doesn't, certainly not that on an NPA you have no vertical profile to follow. You just have to figure it out yourself (excluding fancy VNAV/IAN stuff of course).The NPA still has to be flown adhering to all minimum crossing altitudes, and usually you have multiple points after the FAF that you have to cross at such a minimum altitude or higher. Now there are different philisophies of course how to fly that, AFAIK the 'american' way of doing it would be the step-down, meaning you 'dive' towards the next minimum altitude after each checkpoint. While others seem to prefer a continuous descent method, aiming for a roughly constant angle. After the last check point however you can 'dive' down to MDA if you feel so, then continue in level flight to the MAPt. On a side note though many tend to change their procedures and fly an MDA like a DH today. Some carriers will e. g. add say 50' to the MDA and treat that altitude like a DA, without continuing to the MAPt. So when you reach that altitude, either be visual or go around. The added 50' accomodate for the slight drop during G/A initiation, so as to not violate the MDA during the maneuvre. Reason for that I believe is, after all some MAPts are pretty close to the runway at significant altitude, and in a larger aircraft it would simply be impractical to still commence landing from such a late point where you are way too high and the runway if probably way too short to be safe then.

You say you do have the ability to know your exact height above the ground? Then feel free to use either one. Your company policies would dictate the correct method.

I must admit I'm having trouble finding an authoritative source to quote, but it has always been my understanding that you cannot legally use the radio altimeter to determine the decision altitude / height on a CAT I ILS approach. At the point where you are 200' above the runway threshold you are still quite some distance away, and ground slope etc. could cause a significant deviation.

I must admit I'm having trouble finding an authoritative source to quote, but it has always been my understanding that you cannot legally use the radio altimeter to determine the decision altitude / height on a CAT I ILS approach. At the point where you are 200' above the runway threshold you are still quite some distance away, and ground slope etc. could cause a significant deviation.

Correct. That's why CAT II (and higher) plates usually have an exact RA reading, say, 97' or so for CAT II. The missing three feet accomodate for a terrain 'bump' at that very point where DH is attained. This can still be 100' above TDZE. CAT I is baro-referenced just like all NPAs.

Oh I'm sure we could find that specifically in a TERPS manual somewhere. You're only a half-mile or so away from the threshold at the DH - a real crummy place for a mountain. I would guess that if there were a significant (and potentially hazardous) elevation change from the threshold to a point 1/2 mile away the DH would be a little higher than 200'. But, I can't find an authoritative source regarding that either. In fact, when reviewing my former airline's FOM, it just defines the Categories of approaches by the HAT minnima and the only mention of using a radar altimeter is when shooting a Cat II approach and the inner marker is inop, then an RA can be used in lieu of the marker. But, an RA is not required for either a Cat I or Cat II approach. If no RA then the decision altitude is king. By the way, love your tutorial, trisager. Very well done :)

and the only mention of using a radar altimeter is when shooting a Cat II approach and the inner marker is inop, then an RA can be used in lieu of the marker.

Sure, but other parts of the world for instance have no IMs at all. At least we don't over here across the pond. Speakig of IMs, does anyone know an explanation of why some call them airway markers and how they were used in that regard? Something I've always wondered.

Gents, For Visual, Non-precision and ILS Cat I approaches, pilots use the baro altimeter as the Decision Altitude(DA).<pre style="line-height: 17px; white-space: normal; ">Cat II to Cat IIIB approaches, pilots use the radio altimeter to determinethe DH. This will be the height above the runway.</pre>Here are some approaches and their corresponding DA's + visibility: <pre style="line-height: 17px; white-space: normal; ">Approach Decision Height Visibility Visual 1000ft 3.6km Non-precision* 400ft 1.6km ILS Cat I 200ft 550m ILS Cat II 100ft 300m ILS Cat IIIA 50ft 200m ILS Cat IIIB 15ft 75m ILS Cat IIIC 0ft Nil </pre><pre style="line-height: 17px; white-space: normal; ">* A Non-precision approach is one without any electronic glideslopeguidance. Typical non-precision approaches are VOR/DME, NDB and Localizeronly approaches.</pre>Hope this helps

Found this: http://www.airbus.com/fileadmin/media_gallery/files/safety_library_items/AirbusSafetyLib_-FLT_OPS-SUPP_TECH-SEQ01.pdf baro for Cat I only in a <gasp> Airbus no RA unless a Cat II period. Not a civil aviation authority, but its the closest thing I've found to an authority yet. I think IMs have gone the way of the Dodo in many places. I know Middle Markers were all being decommissioned several years ago here in the states - or at least not fixed once they broke. In the states, airway markers were also referred to as Fan Markers, I believe, and they went out in the 50's and 60's. Just another way to know where you were along an airway in the pre-DME days.

Why not just the DH ?Afterall the height of the airport/runway is all that really matters.

Tell that to those aircraft that crashed not respecting their MDA and smacked into maintains etc or went down just a bit more to "have a quick look"!

By the way... We all know that all precision approaches have vertical guidance. But, are all approaches with electronic ground-based vertical guidance precision approaches - if not what types have vertical guidance but are not precision? Hint: They are not non-precision either.

Correct. That's why CAT II (and higher) plates usually have an exact RA reading, say, 97' or so for CAT II. The missing three feet accomodate for a terrain 'bump' at that very point where DH is attained. This can still be 100' above TDZE. CAT I is baro-referenced just like all NPAs.

Agree, but you are not saying that CAT I is a NPA, are you? Bert Van Bulck

Radio Minimums for CAT II/IIIBaro minimums and MDA for CAT I and NPAs. Simple as.[unless of course specified on the specific aerodrome chart, which takes precedence over afore mentioned standards] Rónán O Cadhain Snr.

Agree, but you are not saying that CAT I is a NPA, are you? Bert Van Bulck

CAT I is not an NPA.

Found this: http://www.airbus.co..._TECH-SEQ01.pdf baro for Cat I only in a <gasp> Airbus no RA unless a Cat II period. Not a civil aviation authority, but its the closest thing I've found to an authority yet.

Actually, once an *cough* Airbus check captain wanted to convince me a CAT I ILS used MDA and not DA. Obviously it turned out this was a direct result of them placing their CAT I DA in the MDA field in their FMS. Apparently for 'bus pilots the following rules apply: MDA = their baro referenced field, and DH = their radio referenced field in the FMS. Quite an interesting design flaw IMHO (correct me if I'm wrong). So for their CAT I DA, just because it's baro referenced, they placed that into the MDA field. After years of using this system, it obviously makes people think that CAT I has an MDA and no DA. Was quite an interesting discussion though. Thanks for the IM/AM insight BTW! EDIT

Agree, but you are not saying that CAT I is a NPA, are you?

No, of course I'm not. I dearly hope it didn't come across that way. EDIT2 Oh yes, just re-read my phrasing. CAT I "just like all other NPAs" yeah right *facepalm*. Correct, thanks for the heads-up. Bad wording on my part. Was meant to say CAT I and all NPAs are baro referenced, if you will.

Edited September 29, 201114 yr by badderjet

Actually, once an *cough* Airbus *cough* check captain wanted to convince me a CAT I ILS used MDA and not DA. Obviously this was a result of them placing their CAT I DA in the MDA field in their FMS. Apparently for 'bus pilots MDA = their baro referenced field, and DH = their radio referenced field in the FMS. Quite an interesting design flaw IMHO (correct me if I'm wrong). So for their CAT I DA, just because it's baro referenced, they placed that into the MDA field. After years of using this system, it obviously makes people think that CAT I has an MDA and no DA. Was quite an interesting discussion though. Thanks for the IM/AM insight BTW! EDIT No, of course I'm not. I dearly hope it didn't come across that way.

DA and MDA are near interchangable for all intents and purposes and it's considered to be good airmanship to put the DA value into the FMGS as the MDA, [At least in my operation it is]

By the way... We all know that all precision approaches have vertical guidance. But, are all approaches with electronic ground-based vertical guidance precision approaches - and what are they?

Euh.... don't know exactly which discussion you want to launch here but I'm looking forward to it. You refer to ILS/MLS?You can fly a visual approach while using the ILS signal as a reference for accuracy...?Or is a PAR approach a precision approach? Bert Van Bulck