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MDA and DH

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Guest Boeing747-430

could someone please explain what these two terms mean? i know that they stand for minumum descent altitude and decision height, respectively, but not much after that. what do they mean and how do they differ? thanksadam

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Guest UltraRunner

"Minimum Descent Altitude" is the lowest altitude you can descend to without seeing the ground during a non-precision approach, ie the airport has a NDB and no ILS."Decision Height" is the lowest (Radar) altitude that you can descend to without seeing the runway during a precision approach, ie ILS approach CAT I, II, III.

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Guest UltraRunner

You can descend lower than either of those without seeing the runway or ground, if you have the approach lights you can go down to 100' above the touchdown zone.

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Guest Adverse Yawn

All correct as above. The other difference is the way you fly the approach. There is no 3 deg glideslope, just recommendation which maybe 6 deg. Basically you descnd to MDA as quick as it is safe do so within normal operational limits because this gives the best chance of becoming vis with the terrain permitting you to continue the descent.That is all generally speaking and specific procedures may and often do apply. The plates have the specific details.

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In the UK the Decision Height is defined as the height in a precision approach at which a missed approach procedure must be initiated if the required visual reference to contine the approach has not been established. This more rigorous than the previous definitions.Also in the UK, unless the pilot is an Instrument Rating Holder in Current Practice it's recommended that the DH/MDH is increased to at least 500 ft for a precision approach and 600 ft for a non-precision approach for IMC Rating Holders in Current Practice. Pilots not in Current Practice are advised not to make instrument approaches in bad weather and, if they have no choice, are recommended to add at least 100 ft and more depending on his experience and knowledge.

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The word "Decision Heigt" (This is height above ground) and "Decision Altitude" The altitude for you to decide to land or go around for precision approach (ILS)

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>"Decision Height" is the lowest (Radar) altitude that you can>descend to without seeing the runway during a precision>approach, ie ILS approach CAT I, II, III. DH is defined based on radar altitude only for Cat II and III ILS approaches. For other precision approaches, i.e. ILS Cat I, MLS, PAR...it is a barometric altitude.Also, the DH is the lowest altitude on a precision approach at which the crew may make the decision to land or execute the missed approach. If the decision is to go missed approach, the aircraft will descend below the DH as the missed-approach is initiated, and that is factored into the TERPS criteria used to build the approach. The MDA, on the other hand, is like a glass floor...unlike the DH, the aircraft may not descend below it until the airport environment is in sight. The crew must plan the level off so as to not break the glass floor. As an old flight examiner once told me..."Break the glass, I'll bust your ..."RegardsBob ScottATP IMEL Gulfstream II-III-IV-V L-300Santiago de Chile

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>If the decision is to go missed>approach, the aircraft will descend below the DH as the>missed-approach is initiated, and that is factored into the>TERPS criteria used to build the approach. >>The MDA, on the other hand, is like a glass floor...unlike the>DH, the aircraft may not descend below it until the airport>environment is in sight. The crew must plan the level off so>as to not break the glass floor. As an old flight examiner>once told me..."Break the glass, I'll bust your ...">>Regards>>Bob Scott>ATP IMEL Gulfstream II-III-IV-V L-300>Santiago de Chile>Interesting... I didn't know about that you can descend below the DA.can you do that for the IFR Checkride?

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"... I didn't know about that you can descend below the DA. can you do that for the IFR Checkride? "No! :)Bruce.

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A lot of good info in this thread.As some have already indicated, a DH occurs on a precision approach, and is an ALTITUDE where the approach must be aborted if the "runway environment" is not visible. This includes the threshold, touch-down zone or runway, and identified as the actual object, the markings on the object, or the lights on the object. Other items that qualify are VASI and PAPI. And, of course, the required visibility must be present for the approach, and you must continously be in a position to land.Just as the DH is the missed approach point for a precision approach and is an altitude, the missed approach point for a NON-precision appraoch is a PLACE, and is displaced or coincident with a fix or waypoint. An example is the missed approach for my home airport (KBJC) for the VOR 29R approach, the missed approach point is 1.9 DME from the VOR BJC. This place requires an altitude in 3 dimensional flight, which is the MDA. You must be at or above the MDA, then have the same criteria as above for continuing lower or further without executing a missed procedure. The requirement to be continously in a position to land is more restrictive here than in the precision approach. Sometimes the missed approach point is coincident with the runway threshold, and if you're still at the MDA at that point, you may not be in a position to make a "normal" landing.Bruce.

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>"... I didn't know about that you can descend below the DA.>can you do that for the IFR Checkride? ">>No! :)>>Bruce.>>>>DOH!ROFLMAO... :)

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> Sometimes the missed approach>point is coincident with the runway threshold, and if you're>still at the MDA at that point, you may not be in a position>to make a "normal" landing.>>>Bruce.>>You know Bruce..I believe it was in this forum someone had said.. in most cases for GA pilots the threshold is more than sufficiant to start a normal descent.They usually have a reasonable length of runway where you have ILS approaches and you are flying a single engine prop or a twin prop.Manny

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Guest Adverse Yawn

You mustn't fly below the DH without viz, but if you choose to go around at DH (reaosnable) in a heavy then you will almost certainly sink below it. In fact it has been known for an aircraft's mains to kiss the runway during a go-around.

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>You mustn't fly below the DH without viz, but if you choose>to go around at DH (reaosnable) in a heavy then you will>almost certainly sink below it. In fact it has been known for>an aircraft's mains to kiss the runway during a go-around.But the fact still remains, I can't use that explanation on my IFR Checkride while flying a Seneca I.LOL :)

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To expand slightly on your reply, the MAPt is a place but a place whose definition which can vary with the type of non-precision approach.

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Hi Manny,Yes- that's right, if there's sufficient runway length remaining. This of course depends on both the landing requirements of the aircraft (taking the current ground speed and configuration into account), and the runway length from the threshold that is available for landing.Interestingly, in my King training material, reference was made to what constitutes a "normal" landing. Is there such a thing? The regulation indicates that one must be continously in a position to do a "normal landing". King deduced that having to be in a forward slip to make it down and be able to stop would not be "normal". So, I guess there's a degree of judgement there :)How's the IR going? Bruce.

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>"... I didn't know about that you can descend below the DA.>can you do that for the IFR Checkride? ">>No! :)>>Bruce.>Well, actually, yes.Here's what's in FAR 91.175c:Operation below DH or MDA. Where a DH or MDA is applicable, no pilot may operate an aircraft, except a military aircraft of the United States, at any airport below the authorized MDA or continue an approach below the authorized DH unless -- As long as you're not "continuing the approach" below the DH (assuming requirements for landing not met) i.e. you're in the process of executing the missed, ops below DH are permissible. So if engine spool lag or other performance factors are in play, it's absolutely possible (and legal) to be below the DH during the early stages of a missed approach.The other exception is for EFVS-equipped acft like the G-V and G-550, which may descend below the DH but not below 100' AGL by use of enhanced visual systems (FLIR camera pic on the HUD in this case).CheersBob ScottATP IMEL Gulfstream II-III-IV-V L-300Santiago de Chile

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Guest Adverse Yawn

No you can't. Instant fail (or at best a partial pass) :(

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Yes Ed.I found it took me once on an ILS with my CFII, and part way down the GS he told me that it had failed and it was now a LOC approach. Of course he had noted that I had not started the timer at the FAF! I knew to descend to the MDA, and what the MDA was, then he said "now, how do you know where the MAP is?". One of those defining moments that never gets forgotten :)Bruce.

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I'll help out in another "IFR Thing". When shooting an ADF Approach, leave the Ident on all the time. If the ADF fails during the approach you would have no way to notice it unless the morse code signal stopped.Regards,Ed

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Good one Ed. We could go on and on. Another one I recall is that, when a distraction occurs, the last instrument you should look at before dealing with the distraction is the AI. And it's the first one to look at after you return to instruments from the distraction.We could develop some tips and tricks for instrument flight- there's enough of us around here to do that...... :)Bruce.(I'd better shut up now :) )

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