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What's The Difference Between DH And MDA?  
User currently offlineModesto2 From United States of America, joined Jul 2000, 2786 posts, RR: 5
Posted (10 years 4 months 6 days 11 hours ago) and read 28888 times:

As the topic line implies, what's the difference between decision height and minimum descent altitude? When is each parameter applicable? Also, please share your insight about any other "heights" including alert height, etc...

10 replies: All unread, jump to last
 
User currently offlineSushka From United States of America, joined Nov 1999, 4784 posts, RR: 14
Reply 1, posted (10 years 4 months 6 days 11 hours ago) and read 28870 times:

Decision Height is used for ILS approaches while Minimum Descent Altitude is used for non precision approaches, (LOC, VOR, ILS G/S out)


Pershoyu Spravoyu Litaki!
User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 2, posted (10 years 4 months 6 days 9 hours ago) and read 28823 times:

Dear Modesto2 -
xxx
Altitude and heights are different "elevation" measurements (like - levels).
In the USA and many countries, it is MDA and DA (not MDH or DH)...
xxx
Altitude is in reference to MSL...
With heights, you read "0" on touch down...
Suggest you do some research on QNH and QFE altimeters here in the forum...
Tons of pages in Tech.Ops.
xxx
Happy contrails  Smile
(s) Skipper


User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4190 posts, RR: 37
Reply 3, posted (10 years 4 months 6 days 4 hours ago) and read 28713 times:

Just a correction.... we call it DH in the USA.


Chicks dig winglets.
User currently offlineNormalSpeed From , joined Dec 1969, posts, RR:
Reply 4, posted (10 years 4 months 6 days 4 hours ago) and read 28715 times:

To add to what has been posted already...

Decision Height (now referred to as Decision Altitude, hence the "DA" abreviation) is a point on an ILS approach where a decision must be made whether to continue the approach and land, or execute a missed approach. If the pilot has "adequate visual reference," i.e. the runway or runway lights in sight when at DA (on glideslope, of course), then the pilot may land. If not, then the pilot must execute the missed approach procedure.

MDA, or Minimum Descent Altitude, is a bit different. Since no electronic vertical guidance (glideslope indication) is provided on a non-precision approach (by definition), a minimum altitude, or MDA is published. The MDA guarantees obstacle clearance between the Final Approach Fix (FAF--roughly equivalent to the outer marker on an ILS approach) and the missed approach point (MAP). MAP is where the pilot makes the continue/missed decision on a non-precision approach. The pilot will simply descend to the MDA, and fly to the Missed Approach Point. (The MAP may be a DME fix, or it could be determined by timing and groundspeed.)

Other important heights:

TCH - Threshold Crossing Height, for an ILS approach (or non-precision with VNAV), the height that the aircraft will fly over the runway threshold when on glideslope.

TDZE - Touchdown Zone Elevation, this is important to know because the pilot may descend to 100 feet above the TDZE even if the runway itself is not in sight, even though the DA may be a higher altitude. If the approach lighting is in sight, the pilot may continue the descent to 100 feet above TDZE. TDZE is defined as the highest usable portion of the first 3000' of runway.

Well, let me know if you need clarifications.

'Speed


User currently offlineNormalSpeed From , joined Dec 1969, posts, RR:
Reply 5, posted (10 years 4 months 6 days 4 hours ago) and read 28708 times:

"Just a correction.... we call it DH in the USA."

Actually, we call it DA(H) in the USA.

'Speed


User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 6, posted (10 years 4 months 6 days 4 hours ago) and read 28701 times:

Oh my gosh... the explanation again...
xxx
NormalSpeed gave a correct wording and print of the DA(H)...
Decision altitude (height)...
We will speak strictly "USA" here...
xxx
A normal ILS is flown to 200 feet DH (above threshold elevation).
If that elevation is, say, 213 feet MSL...
That ILS would have printed minimums as - DA(H): 413' (200')...
xxx
XSFUgimpLB41X (wow, a tongue twister - or finger twister you are)
Most people call that "413 feet" in the above example a DH decision height.
It is a decision altitude, really, but I agree with you, 75% of USA pilots erroneously say DH.
xxx
Suppose we now deal with a real DH...
Radio altimeters are used for Cat.II approach minimums...
They measure a HEIGHT above the ground below the aircraft.
If it is, and let's assume absolutely flat terrain, the DH will be generally 100' DH...
Your decision to land will occur at RA 100' (height). You will not look your normal altimeter.
That normal altimeter indicates altitude, it (would) read something like 313 ft at time of DH on Cat.II
But your decision is based on RA (and DH), not on a DA on Cat.II -
xxx
Suggest you research what I have discussed many times about QNH and QFE in the past.
A QNH altimeter setting (like in USA) gives altitudes.
A QFE altimeter setting (sometimes used outside USA) gives heights above TDZE.
A QNE altimeter setting (cruise levels) is 29.92 - 1013.2 - 760 gives levels.
xxx
Happy contrails  Smile
(s) Skipper



User currently offlineAAR90 From United States of America, joined Jan 2000, 3465 posts, RR: 47
Reply 7, posted (10 years 4 months 6 days 1 hour ago) and read 28634 times:

It is a decision altitude, really, but I agree with you, 75% of USA pilots erroneously say DH.

Probably more than that now that AA no longer uses QFE procedures. Vast majority of FO's I see haven't got a clue what the differences are or why it is important. Me? I miss the standardization QFE [and "heights"] provided.  Crying



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineRalgha From United States of America, joined Nov 1999, 1614 posts, RR: 6
Reply 8, posted (10 years 4 months 6 days 1 hour ago) and read 28616 times:

The MDA guarantees obstacle clearance between the Final Approach Fix (FAF--roughly equivalent to the outer marker on an ILS approach) and the missed approach point (MAP).

Not always, there is often another step down after the FAF.



09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 9, posted (10 years 4 months 6 days 1 hour ago) and read 28621 times:

Dear AAR90 -
xxx
I have to use QFE a few times occasionally, it is an alternate procedure for us...
Part of our standard briefing before we go, or on approach.
Russia, China still uses it. Worse, with them it is metric altitudes.
Like you had that 3rd (QNH altimeter) with AA, we install a metric altimeter.
xxx
But I have had QFEs here and there in Europe. With PanAm we knew how to use them, in case that is what we got. You at AA standardized on always using QFE. So did Eastern. The only objection I have with QFE in the USA, ATC did not know what the heck that animal is...
xxx
Honest, I think that both should be known by all pilots (and ATC).
xxx
Happy contrails  Smile
(s) Skipper


User currently offlineNormalSpeed From , joined Dec 1969, posts, RR:
Reply 10, posted (10 years 4 months 6 days 1 hour ago) and read 28617 times:

"Not always, there is often another step down after the FAF."

Yeah, I was just trying to keep it simple.

'Speed


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