Hi, I have some questions:1- What is the difference between MDA (minimum descent altitude) and DH (decision height)? I also heard the term "minimums", which one it refers to?2- What is the difference between RADIO and BARO mode for the decision height? When to use one instead of another?Thanks!Regards

Without going into detailed FAA instructions, here is the simpe approach to differentiate.MDA = for non-precision approaches such as RNAV, VOR etcDH = for precision approaches such as ILS.http://en.wikipedia.org/wiki/Decision_height

http://en.wikipedia.org/wiki/Radio_altimeter describes both. The radio altimeter reports terrain clearance above ground (AGL) issuing a caution at its set limit. In BARO mode the caution functions as a "bug" slaved to your pressure based altimeter and you set it for the MDH or DH given as MSL in your approach information and it will call out the "minimums" annunciation.On some approaches with radio altimeters used you might see a publication caution noting intermittent terrain warnings due to small "bumps" or other factors which have no major significance. Airline policy for that approach indicates to what degree it would be ignored. Sometimes there can be a false reflection causing that problem.If you have a ground proximity warning system it is the radio altimeter that triggers the warning "pull up" and is always connected to it regardless of the selector switch setting.CAT III approach conditions require a functioning radio altimeter for a near or full autoland procedure in very limited visibility. The radio altimeter also supplies the information for the audio callout of the height above the ground/runway for guided pilot speed and flare control and also the autoland feature.I have attached the CAT I ILS4R KBOS IAP. I have outlined in blue the MSL minimum and in red the above ground minimum. The threshold altitude of 18 determines the difference in the two readings of 218 MSL v.s. 200 AGL. The second attachment is the requirements for autoland of the same runway CAT III autoland approach where the vertical sight of the runway is not available and just a very short forward visibility. Note the absence of altitude minimums declared and the special aircraft requirements and crew requirements of certification. You can't perform this without a radio altimeter as it is a necessary part of the autoland apparatus.

Minimums are the the limits you can fly your aircraft on a instrument approach without the runway insight. Non precision approaches are those which do not use the guidence of a glide scope. These will have minimums that are expressed in horizontal visibility and a Decision Altitude in AMSL (above mean sea level). You cannot even attempt the approach if visibility reported is below the minimums, fly to your alternate. Every approach has an initial fix, and an altitude express as AT OR ABOVE. Since you are a beginner at flying instrument approach, try at hit that fix at the given altitude. Establish yourself on the proper radial or corse from the initial fix, you may have intermediate fixes with other altitudes you can lower to, but once you are passed those you will be heading on final to the airport, when you are at this point you can then descend to the Decision Altitude, you CANNOT go below this altitude untill you can see either the threshold of the runway or guidance lighting for the runway. Then you can descend and proceed to land the plane as if flying VFR. If you continue on final at DA, and reach the beginning of the missed approach fix without ever seeing the runway, you must immediately flying the Missed Approach Procedure (MAP), no if ands or buts, do not try to land, and do not circle to land.Keep in mind that DA is based on local altimeter settings, so if you are using a setting that is not for the local airport but for a nearby larger airport, your minimums will surly go up.On approaches which use a glide scope, these are referred to as ILS, and in its most basic form, you will have a minimums expressed in AGSL (Above Ground Service Level), and this is determined by your radio altimeter. And ILS will have minimums (now refered to as Decision Height, as it uses actual distance from the ground as opposed to an altimeter setting) no lower than 200 ft AGSL. To fly this, fly as you would anyother IAP, except that when you establish on the localizer, then watch for the G/S indicator to come down, and then as it reaches your altitude, lower power until the aircraft is in a suitable descent to keep up with the glidescope, you must not deviate much at all from the G/S indicator, as it is already very sensitive. When you reach DH, you must see the threshold or guidance lights, other wise you must fly the missed approach immediately just like in non-precision.There are also CAT II CAT III (A B and C) ILS which have higher standards of maintenence, interference, and operation, that result in the ability to have DH set to lower than 200. Cat IIs have DH no lower than 100, and CAT III/C have no DH at all, you essentially can land the aircraft without ever seeing the runway at all.Now as far as autoland is concerned, some airliners may have company minimums that you cannot attempt an autoland on any ILS unless it is certified as CAT II or higher, but in reality, a modern AUTOLAND can land the plane on any ILS system. The only requirements are that there is at least one redundant Autopilot system that is powered from a different source than the active one. Essentially you must have two APs, both tuned to the same ILS freq. and course, just incase on fails you can still land the ILS manually after reaching DH. Heavier aircraft have as many as three APs and thus can still autoland if one AP fails.All airliners that I am aware off require the use of Autoland when flying a CAT III/C.Hope this helps you out a bit.

I differ from Scott in the following areas:Note on the KBOS minimums I posted the table shows both MSL and AGL for both ILS and LOC (no GS) conditions. The no GS block has higher minima both in altitude and visibility.Not all ILS approaches are certified for CAT II or the variations of CAT III. The runway and ILS equipment must have certain qualifications to be certified.http://en.wikipedia.org/wiki/CAT_III#ILS_categoriesWhile not mentioned in the referenced article I think runway width plays a factor in CAT IIIA and CAT IIIB certification. I also believe that uneven terrain prior to the DH position for CAT III types plus in certain areas false returns concerning the radar altimeter play a part in CAT III runway certification.I have not downloaded the latest KBOS plates but at that time only runway 4R had CAT II and CAT III plates published. One runway being CAT II or CAT III certified does not guarantee all runways are certified for those extended minima regardless of aircraft and crew certification. This current listing:http://flightaware.com/resources/airport/KBOS/proceduresshows still only 4R can go beyond CAT I procedures.

I differ from Scott in the following areas:Note on the KBOS minimums I posted the table shows both MSL and AGL for both ILS and LOC (no GS) conditions. The no GS block has higher minima both in altitude and visibility.Not all ILS approaches are certified for CAT II or the variations of CAT III. The runway and ILS equipment must have certain qualifications to be certified.http://en.wikipedia.org/wiki/CAT_III#ILS_categoriesWhile not mentioned in the referenced article I think runway width plays a factor in CAT IIIA and CAT IIIB certification. I also believe that uneven terrain prior to the DH position for CAT III types plus in certain areas false returns concerning the radar altimeter play a part in CAT III runway certification.I have not downloaded the latest KBOS plates but at that time only runway 4R had CAT II and CAT III plates published. One runway being CAT II or CAT III certified does not guarantee all runways are certified for those extended minima regardless of aircraft and crew certification. This current listing:http://flightaware.com/resources/airport/KBOS/proceduresshows still only 4R can go beyond CAT I procedures.

No difference there. I didnt intend to imply that GS would require a higher minimum in terms of AGSL. GS is the most precise instrument approach available today, so any ILS approach will have lower minimums compared to LOC, RNAV, VOR, NDB, or GPS/GNSS approaches.As far as ruwway width, that is something I have never heard. I was taught that the higher the CAT of an ILS, the more reliable the signal must be, and there is a lighting requirement for the runway as well. As far as terrain goes, this I am not sure about, but since every approach plate I have seen in the US has a 10nm cirlce in which circling is allowed at a lower altitude than the IF, and since the vertical profile, especially on a CAT II or III will have DME to determine the MAF, false readings on the radio altimeter should not bother the pilot, as he knows that it is when the radio call outs shout minimums and the DME is coincident, then you know you have reached minimums for runway in sight.\I hope I am not rambling, but when I start talking about IAPs, I just cant stop. I love to fly them, and I love to talk about them

Same here :)

-------------------snip----------------I hope I am not rambling, but when I start talking about IAPs, I just cant stop. I love to fly them, and I love to talk about them

For those interested there is an article by Mike Ray, retired United Airlines captain, in the February/March 2010 issue of Computer Pilot devoted to the radio altimeter. He is the author of the PC Sim series books on airliner operations. You can see sample viewings on his site:http://www.utem.com/manuals.htmlhttp://www.utem.com/arrivals.htmland be sure you look at the red banded manuals dedicated to PC Simmers which have much lower prices, not the checkride manuals for real world pilots which are more expensive. I have the Boeing 700 series book which in hand with the PMDG documentation really makes a good combination to help digesting and explaining all that material. You must know basic flying techniques and the aircraft systems. These manuals are about operating the aircraft using its facilities and being consistent with practical techniques to agree with civil authority and airline operating procedures. Items like reserve fuel, decisions on icing and runway conditions, and other policies are covered besides operating the aircraft power and navigation equipment and performing departures, en-route, and arrival procedures.His humorous cartoons lighten the burden of reading all that information and accent the important items. His method of offering practice flights at different levels of complexity first using primary equipment followed by more advanced use of navigation equipment allow each level of sophistication to sink in.