MDA

[Airbus Commander 563]

As I previouisly stated, the MDA in the 737 should be set in the FCU as the cleared altitude. IE, say the MDA is 800' , then set 800' in the FCU.... Simples...

[PIC007 82]

3 pages on MDA/DH?? wow! You folks have a lot of free time! In the US, it's pretty simple...MDA is for non precision approaches (no GS) and DH is for precision approaches. Someone may have mentioned that already, I didnt wade through every post. All based on altimiter settings with local baro pressure. MDA you can descend immediatly to that published altitude, DH you follow a glide slope to that altitude, MDA is normally higher than a DH, both guarentee obstruction clearance.

[badderjet 181]

Someone may have mentioned that already, I didnt wade through every post.

Oh, then you probably should have. Please don't bust our discussion, we were already past the point agreeing on what approach has MDA, DA and DH... LOL (j/k)

[PIC007 82]

Oh, then you probably should have. Please don't bust our discussion, we were already past the point agreeing on what approach has MDA, DA and DH... LOL (j/k)

You are right, but the ones I did read still sounded confused, my bad :-)

[badderjet 181]

LOL nm

[masneoquil 66]

MDA you can descend immediatly to that published altitude, DH you follow a glide slope to that altitude, MDA is normally higher than a DH, both guarentee obstruction clearance.

Isn't MDA the TDZE + DH? If thats correct when you reach the published MDA youi should also reach DH, don't you? Here is a CAT I and CAT II and IIIA into SAEZ.

[badderjet 181]

Isn't MDA the TDZE + DH? If thats correct when you reach the published MDA youi should also reach DH, don't you?

Uhhh, no. MDA is MDA, DH is DH. You use either of them, not both.You could however say TDZE+MDH=MDA, or TDZE+DH=DA. That would probably make sense. Heights are above ground (AGL), altitudes are above sea level (MSL). Now if you add the terrain elevation (which is MSL) to a height, you again get an altitude (MDA or DA). In your first example you see a DH (AGL) of 200'. TDZE is 62'. Now 62'+200' is 262', which is your DA (MSL). As for the LOC approach, we have an MDH (AGL) of 398'. Plus the terrain elevation at runway threshold of 62' sums up to 460', which is the MDA (MSL). The second example is a but different (but has been discussed at length as well, see above), because we're dealing with CAT II+ here, so we're definately radio referenced here. DH is 100' as is standard for a CAT II. Now that is above TDZE, don't forget. Plus runway elevation, 162' MSL as DA. However, to be at 162' MSL at the very point where you reach DH, it's not obvious how high you're above ground then (NOT above TDZE! But above the ground when passing DH, which obviously is in front of the runway!). So we need to know the exact terrain elevation at that very point. If we substract that from our DA of 162', we'd obviously come up with an RA of 102'. So your radio referenced DH will be 102', because you'll be 102' over the terrain at the time you are 100' above TDZE. If that makes any sense. The chart format is usually as you see it in your example - it says DA(H) and that means the first number is the DA, the one in parantheses is DH. So 262'(200') means DA is 262', while DH is 200'. Same principle for MDA(H). And RA obviously is self-explanatory. Always radio referenced, and always at the very point where you reach your minimum, obviously NOT above TDZE, at least not necessarily (we don't have forward looking RAs yet ).

[masneoquil 66]

Uhhh, no. MDA is MDA, DH is DH. You use either of them, not both.You could however say TDZE+MDH=MDA, or TDZE+DH=DA. That would probably make sense. Heights are above ground (AGL), altitudes are above sea level (MSL). Now if you add the terrain elevation (which is MSL) to a height, you again get an altitude (MDA or DA). In your first example you see a DH (AGL) of 200'. TDZE is 62'. Now 62'+200' is 262', which is your DA (MSL). As for the LOC approach, we have an MDH (AGL) of 398'. Plus the terrain elevation at runway threshold of 62' sums up to 460', which is the MDA (MSL). The second example is a but different (but has been discussed at length as well, see above), because we're dealing with CAT II+ here, so we're definately radio referenced here. DH is 100' as is standard for a CAT II. Now that is above TDZE, don't forget. Plus runway elevation, 162' MSL as DA. However, to be at 162' MSL at the very point where you reach DH, it's not obvious how high you're above ground then (NOT above TDZE! But above the ground when passing DH, which obviously is in front of the runway!). So we need to know the exact terrain elevation at that very point. If we substract that from our DA of 162', we'd obviously come up with an RA of 102'. So your radio referenced DH will be 102', because you'll be 102' over the terrain at the time you are 100' above TDZE. If that makes any sense. The chart format is usually as you see it in your example - it says DA(H) and that means the first number is the DA, the one in parantheses is DH. So 262'(200') means DA is 262', while DH is 200'. Same principle for MDA(H). And RA obviously is self-explanatory. Always radio referenced, and always at the very point where you reach your minimum, obviously NOT above TDZE, at least not necessarily (we don't have forward looking RAs yet ).

Thanks Etienne for the detailed explination, and yes, i was aware that i can't use DA and DH at the same time. I was just saying....

[Lawyer+Pilot 126]

Shocking, the number of US pilots on here who are confusing these rules. I think I may have to call the FSDO on these guys. Hehehehe

[JakobF 19]

Two types of approaches; precision and non-precision. Precision Approach

Vertical guidance is provided via the glideslope and the decision to land is made at the decision height (DH). You're riding down the glideslope get to that decision height and look out the window. If you can see the "runway environment" you can decide to land. Now, that's a lot to do at an exact altitude - so generally you're sneaking peeks outside at DH+100 all the way down to the actual DH and you may actually descend a little below the DH as you go missed. A decision height applies to precision approaches. BUT the DH is also the missed approach point (MAP). You may use either a decision altitude (barometic altitude above sea-level) or a decision height (height above the touchdown zone). I know of nothing in the FARs that dictates which one you have to use other than when thinking about how the aircraft is equipped. No radar altimeter? Then you have to use the barometric altitude. 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.

Non-precision Approach

On a non-precision approach you make your decision at a geographical place over the ground which is the...you guessed it MAP. There is no DH because there is no vertical guidance on a non-precision approach. The altitude you will probably be at as you approach the MAP would be the mimimum descent altitude (which you've probably been at for quite some time as you feel your way to the MAP waiting for the proper DME or a certain time to elapse. As you work you way down the approach course there may be several step down fixes where you get down to the next minimum altitude until you're at the MAP and the MDA (if you choose). Remember, it's a minimum. There is nothing that says you can't arrive at the MAP 200 feet above the MDA. That's up to you. However, if you don't see the airport environment you have to go missed no matter what your altitude is at the MAP. In this case, the MDA would be set as a barometic altitude since that altitude is calculated as a height above the airport elevation (which may be significantly different than the height above the ground at the MAP. That's (only) one reason why the minimums are quite a bit higher. You can almost hear the dramatic organ music playing as you plod along at your final approach speed with the flaps and gear hanging out (usually accumulating ice) leaning forward as if that will enable you to see the airport before the timer in your lap gets to 2:45. Or was that 2:54?...

So, Chrjs, when speaking about using a Decision Altitude or an Minimum Descent Altitude they could be considered similar in that they are both barometric altitudes. Sorry for the long winded explanation. It's been a while since I've taught and I guess I had some pent-up rambling to take care of. Before someone rips my explanation apart, keep in mind that I purposely left some things out, like if you just see some runway approach lights on an ILS how much further you can proceed yadda-yadda-yadda. Or how on a precision approach visibility is really the determining factor not a ceiling and so on. But, hopefully this will help someone. Tim

I was about to write the same as GearGrip, although I haven't taught, I am an educated commercial pilot. Well guys, if you have some spare time here is a good reading, the bible of IFR. And for you who, know what I talk about then yes it's the almighty ICAO Doc 8168... ;) It covers everything you will encounter flying ICAO IFR! And yes it's confusing when both FAA and ICAO has different ways to look upon this.. So here you are guys, happy reading! :)

http://www.icao.int/icaonet/dcs/8168/8168_v1_5ed_amend_14_advanced.pdf

[skyrider 0]

Tom, Excellent! Thanks for your response- I think it finally makes sense. I'm finding quite a bit of information as I research various SOP's on the topic. Not so much use during the approach phase, but how it is used during the peparture phase and even at times during cruise, e.g., the MSA for a particular segment in case of emergency descent. Again, thanks for the repsonses! JW

For departure you can use the 'mins' selector to dial in your acceleration altitude. When you set up the flight deck for the approach you can reset it to the DH/MDA. Cheers, Doug

[Jeffrey L. Whitaker 0]

Doug, Excellent advice. Thanks for your input. I'm sure much of this is pilot preference (save the obvious setup for approach). I'd love to get my hands on an SOP so I could see all these nuances. JW