MDA

[Lawyer+Pilot 126]

Just pull up when you hear birds chirping outside.

[trisager 2]

Just pull up when you hear birds chirping outside.

You've been flying into too many ORBX airstrips

[ARM505 0]

Just a small point - there is no fast slew in the real plane as somebody asked for above. Surprisingly, it's not actually a rotary controller (inasmuch as it isn't rotated completely around like say, the altitude selector). It's spring loaded to the centre position, and holding it to one side increases/decreases the set value. Weird, isn't it. It does accelerate somewhat though (and cause you to overshoot the desired value), but I'll be honest - I'm not sold on Mr. Boeings new rotary controllers. Although you can use your mouswheel in FSX, it works well. Better than the real plane actually.

[rocketfs 2]

As far as I know (please correct mei f i'm wrong) MDA = barometric altitude (cat I and above)DH = radio altimerte (cat II and III)

I don't agree with that: MDA: Non-precision approachDH(A): precision approach. A Cat I ILS approach is a precision approach and has a DH(A).Sometimes there is a Minimum Descent Altitude (MDA) published on the same chart but that is for the llz aproach (GP INOP). A Non-Precision Approach has a Final Approach Fix (FAF), MDA and a Missed Approach Point (MAPt), a precision approach has a final approach point (FAP) and DH (with secondary reference point: marker or DME (or more?...)) Flying a non-precision approach: after the FAF, you can immediately descent to your MDA (as long as your descent is not more then 15% degrees (or percent, I don't remember)) and continue to fly at that altitude until you see the runway or reach the MAPt. I'm from Europe, so ICAO based but I guess this applies also to U.S.Also, I'm talking about ICAO regulation, not internal operator regulation or noise or.... Bert Van Bulck

[rocketfs 2]

Sort of. Decision Height (DH) and Altitude (DA) are for precision approaches. You must have a radar altimeter to use DH. MDA is used for non-precision approaches.

"You must have a radar altimeter to use DH." Don't agree. For a ILS CAT I approach, you typically use barometric altitude measurement and not radar altitude. Otherwise the terrain in front of the runway must be "flat" for a much larger distance, which is expensive, sometimes unrealistis and for these minimums not needed. Bert Van Bulck.

[masneoquil 66]

I think also that: 1- MDA is for CAT I and non.precision 2- DH is for CAT II and IIIa 3- CAT IIIb and CAT IIIc doesn't have a any DH because they're autoland.

[Chrjs 61]

I don't agree with that: MDA: Non-precision approachDH(A): precision approach. A Cat I ILS approach is a precision approach and has a DH(A).Sometimes there is a Minimum Descent Altitude (MDA) published on the same chart but that is for the llz aproach (GP INOP). A Non-Precision Approach has a Final Approach Fix (FAF), MDA and a Missed Approach Point (MAPt), a precision approach has a final approach point (FAP) and DH (with secondary reference point: marker or DME (or more?...)) Flying a non-precision approach: after the FAF, you can immediately descent to your MDA (as long as your descent is not more then 15% degrees (or percent, I don't remember)) and continue to fly at that altitude until you see the runway or reach the MAPt. I'm from Europe, so ICAO based but I guess this applies also to U.S.Also, I'm talking about ICAO regulation, not internal operator regulation or noise or.... Bert Van Bulck

Thanks for the correction/clarifictaion. but i think we now have some sort of confusion of the terms. we all agree that for non-precision approaches we use MDA, which is a barometric altitude. Also, that for CAT II and III we use DH, which is radio altitude. SO the only question would be how the numbers for CAT I are called.

[Chrjs 61]

I guess Bert is right with the DH for Cat I apporaches. here a statement of the official Instrument Procedures Manual of the FAA: "The lowest authorized ILS minimums, with all required ground and airborne systems components operative, are: - CAT I - Decision Height (DH) 200 feet and Runway Visual Range (RVR) 2,400 feet (with touchdown zone and centerline lighting, RVR 1800 feet (p. 5-51) So DH applies also to CAT I. But when do we really speak about DA? Is DA and MDA the same concerning the approach phase?

[trisager 2]

A CAT I ILS system typically lets you descend to 200' height above terrain. However the published minimum is barometric - this is what is called the decision altitude (DA). The difference between the minimum descent altitude (MDA) on a non-precision approach and the decision altitude for a CAT I ILS is that you may on no account descend below MDA unless you have the visibility required for landing. The DA, however, is the point where you must decide - you may be going below DA after you initiate a go-around.

[Geargrip 0]

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

[rocketfs 2]

I guess Bert is right with the DH for Cat I apporaches. here a statement of the official Instrument Procedures Manual of the FAA: "The lowest authorized ILS minimums, with all required ground and airborne systems components operative, are: - CAT I - Decision Height (DH) 200 feet and Runway Visual Range (RVR) 2,400 feet (with touchdown zone and centerline lighting, RVR 1800 feet (p. 5-51) So DH applies also to CAT I. But when do we really speak about DA? Is DA and MDA the same concerning the approach phase?

"So DH applies also to CAT I. But when do we really speak about DA? Is DA and MDA the same concerning the approach phase?" DH is DA - runway elevation. example: runway is at 1000' and DA is 1200', then the DH is 200' Bert Van Bulck

[badderjet 181]

Just a small point - there is no fast slew in the real plane as somebody asked for above. Surprisingly, it's not actually a rotary controller (inasmuch as it isn't rotated completely around like say, the altitude selector). It's spring loaded to the centre position, and holding it to one side increases/decreases the set value. Weird, isn't it. It does accelerate somewhat though (and cause you to overshoot the desired value), but I'll be honest - I'm not sold on Mr. Boeings new rotary controllers. Although you can use your mouswheel in FSX, it works well. Better than the real plane actually.

Hey Simon, funny, I actually reported this most minor issue to Tabs a couple weeks ago and wondered if such small thing would ever get a fix. So far it didn't make it though, speaking of HF4. Still a rotary, as is the BARO (QNH) knob. Maybe in some other future SP.

[Geargrip 0]

Thats right, Bert. DH is DA-TDZE on every plate I've looked at today. KSEA ILS 34R. DA=572, TDZE=372, DH=200

[Spin737 102]

Wow, you guys make this complicated. If the chart says RA, use RADAR, if not use BARO. Minimums are minimums. Or minima are minima, if you prefer.

[rocketfs 2]

A CAT I ILS system typically lets you descend to 200' height above terrain. However the published minimum is barometric - this is what is called the decision altitude (DA). The difference between the minimum descent altitude (MDA) on a non-precision approach and the decision altitude for a CAT I ILS is that you may on no account descend below MDA unless you have the visibility required for landing. The DA, however, is the point where you must decide - you may be going below DA after you initiate a go-around.

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. Bert Van Bulck