What is different between DH and MDA and DA?!
Join Date: Oct 2007
Location: Wherever I go, there I am
Age: 43
Posts: 807
Likes: 0
Received 0 Likes
on
0 Posts
DH and DA are different terms for the same thing - Decision Height or Decision Altitude. They are used for Precision Approaches (ILS) to denote the altitude which, upon reaching during the final approach, if the runway environment is not visual a missed approach must be conducted.
A MDA or Minimum Descent Altitude is the minimum altitude you are allowed to descend to on a non-precision approach. On a NPA the plate will describe the point at which you can descend to the MDA and then it will describe the point at which you must complete a missed approach, incidentally known as the Missed Approach Point.
Essentially, the DH/DA is the Missed Approach Point because you must go around at that altitude and you cannot fly along at it searching, while a MDA does allow you to fly at the altitude until either seeing the runway through a break in cloud or until you reach the missed approach point.
At the end of the day though, they are similar in that it is the lowest altitude you can fly to on an approach - its just the type of approach that denotes whether its aa DH/DA or MDA.
A MDA or Minimum Descent Altitude is the minimum altitude you are allowed to descend to on a non-precision approach. On a NPA the plate will describe the point at which you can descend to the MDA and then it will describe the point at which you must complete a missed approach, incidentally known as the Missed Approach Point.
Essentially, the DH/DA is the Missed Approach Point because you must go around at that altitude and you cannot fly along at it searching, while a MDA does allow you to fly at the altitude until either seeing the runway through a break in cloud or until you reach the missed approach point.
At the end of the day though, they are similar in that it is the lowest altitude you can fly to on an approach - its just the type of approach that denotes whether its aa DH/DA or MDA.
Join Date: Oct 2010
Location: Wayne Manor
Posts: 1,517
Likes: 0
Received 0 Likes
on
0 Posts
FAA CFR 14 1.1
the FAA is your friend, they're there to help.
Decision altitude (DA) is a specified altitude in an instrument approach procedure at which the pilot must decide whether to initiate an immediate missed approach if the pilot does not see the required visual reference, or to continue the approach. Decision altitude is expressed in feet above mean sea level.
Decision height (DH) is a specified height above the ground in an instrument approach procedure at which the pilot must decide whether to initiate an immediate missed approach if the pilot does not see the required visual reference, or to continue the approach. Decision height is expressed in feet above ground level.
Minimum descent altitude (MDA) is the lowest altitude specified in an instrument approach procedure, expressed in feet above mean sea level, to which descent is authorized on final approach or during circle-to-land maneuvering until the pilot sees the required visual references for the heliport or runway of intended landing.
Decision height (DH) is a specified height above the ground in an instrument approach procedure at which the pilot must decide whether to initiate an immediate missed approach if the pilot does not see the required visual reference, or to continue the approach. Decision height is expressed in feet above ground level.
Minimum descent altitude (MDA) is the lowest altitude specified in an instrument approach procedure, expressed in feet above mean sea level, to which descent is authorized on final approach or during circle-to-land maneuvering until the pilot sees the required visual references for the heliport or runway of intended landing.
Join Date: Jun 1999
Location: In the SIM
Posts: 976
Likes: 0
Received 0 Likes
on
0 Posts
Farid,
Firstly I assume you are a trainee pilot and have a grounding on these terms.
One thing I would like to add, with respect to DA and DH.
+TSRA is along the right lines, however, DA and DH are used specifically in CAT 1 and CAT 2/3 approaches respectively.
Most airlines have their own approval for low vis ops (CAT 2/3) and have specified minima for these approaches and we refer to these in our operations manuals.
It should be noted that this info comes, as has been said, as a height, and is based on the RAD ALT, to which an RVR restriction is imposed. A DA is usually referred to as the CAT 1 minima, with the associated RVR restriction minimum 550m and the equivalent altitude which equates to 200ft AGL.
Cheers.
Firstly I assume you are a trainee pilot and have a grounding on these terms.
One thing I would like to add, with respect to DA and DH.
+TSRA is along the right lines, however, DA and DH are used specifically in CAT 1 and CAT 2/3 approaches respectively.
Most airlines have their own approval for low vis ops (CAT 2/3) and have specified minima for these approaches and we refer to these in our operations manuals.
It should be noted that this info comes, as has been said, as a height, and is based on the RAD ALT, to which an RVR restriction is imposed. A DA is usually referred to as the CAT 1 minima, with the associated RVR restriction minimum 550m and the equivalent altitude which equates to 200ft AGL.
Cheers.
Thread Starter
Join Date: Apr 2011
Location: Tehran
Age: 42
Posts: 17
Likes: 0
Received 0 Likes
on
0 Posts
thank you ,
since i saw many videos from different airliners i have another question
why pilots never use DA/DH?!even on ILS approaches they always input MDA?
i bet you i saw 59 (airbus)videos about world airtours at ALL OF THEM pilots input MDA for minimum altitude approach?
would you please explain that to me?
thank you in advance
since i saw many videos from different airliners i have another question
why pilots never use DA/DH?!even on ILS approaches they always input MDA?
i bet you i saw 59 (airbus)videos about world airtours at ALL OF THEM pilots input MDA for minimum altitude approach?
would you please explain that to me?
thank you in advance
Join Date: Jun 1999
Location: In the SIM
Posts: 976
Likes: 0
Received 0 Likes
on
0 Posts
Farid,
This is just the way it is presented on the PERF page on the FMGC with respect to the Airbus. When flying a CAT 1 ILS the DA goes in the MDA field. When you are flying a CAT 2 or CAT 3 approach just below it there is a DH field, where you put in the RAD ALT DH. What ever the pilot inputs into this field, it will appear on the pilot primary flight display. It also drives the synthetic voice for the pilot i.e. '100 above' and 'Minimums' etc.
Hope this helps.
This is just the way it is presented on the PERF page on the FMGC with respect to the Airbus. When flying a CAT 1 ILS the DA goes in the MDA field. When you are flying a CAT 2 or CAT 3 approach just below it there is a DH field, where you put in the RAD ALT DH. What ever the pilot inputs into this field, it will appear on the pilot primary flight display. It also drives the synthetic voice for the pilot i.e. '100 above' and 'Minimums' etc.
Hope this helps.
Join Date: Oct 2007
Location: France
Posts: 749
Likes: 0
Received 0 Likes
on
0 Posts
Farid, an other minimum to keep you busy: the Circle To land MDA(H).
When you read the explanations above in this topic, get a jeppesen approach chart to understand what is is about. It is important to have a visual. Don't be confused by what you have to input on the Airbus system.
I fly Boeing and we manually set the minimums, and call ourself the missed appoach point for a non precision approach.
Be very familiar with the minimums, be sure you really understand them and can visualize them, it may save your life one day, and it may help you for a pilot interview.
When you read the explanations above in this topic, get a jeppesen approach chart to understand what is is about. It is important to have a visual. Don't be confused by what you have to input on the Airbus system.
I fly Boeing and we manually set the minimums, and call ourself the missed appoach point for a non precision approach.
Be very familiar with the minimums, be sure you really understand them and can visualize them, it may save your life one day, and it may help you for a pilot interview.
Why do it if it's not fun?
Join Date: Jul 2001
Location: Bournemouth
Posts: 4,779
Likes: 0
Received 0 Likes
on
0 Posts
And let's not even begin to discuss Jeppesen's decision to publish a "DA(DH)" for non-precision approaches in Europe, where the minima are calculated in accordance with EU-OPS 1.430 Appendix 1 (New).
FFF
-------------
FFF
-------------
Join Date: Aug 2008
Location: North America
Age: 64
Posts: 364
Likes: 0
Received 0 Likes
on
0 Posts
Allekum Salam - Farid!
When flying an ILS (Instrument landing system) you will be tracking a localizer and a glideslope. The localizer is a ground based navigation beam designed to line you up with the center of the runway. the glideslope (also a ground based navigation beam) is a vertical beam (usually 3 degrees) designed to provide a stable and safe vertical descent path to the runway.
An ILS has ground based vertical guidance, it is called a precision approach.
When flying the ILS to runway 29 in Teheran as you come closer to the runway you will reach an altitude where you must see the runway before you are allowed to continue to land. That altitude where you have to make the decision whether you continue to land; or discontinue the approach, is often referred to as "minimums". The minimums for a precision approach are called DA(H) Decision Altitude or Decision Height. The DA(H) is an altitude read on the altimeter where the pilot must make a decision: do I have the required runway in sight and can I safely land. If the answer is "yes", then at the DA(H) the pilot continues the flight and lands. If the answer is "NO", then the pilot discontinues the approach and flies to a safe altitude (Missed approach).
A precision approach has a DA(H) associated with it. In many cases the DA(H) is 200' above the runway touchdown zone.
An instrument approach to a runway without a vertical glide path is called a non-precision approach. Examples would include: a localizer only approach or a VOR approach (there are others as well).
When flying a non-precision approach; as you get closer to the runway you are allowed to descend to a lower altitude with the intent of being able to visually see the runway so as to land. But with a non-precision approach there is no vertical guidance provided by a ground based navigation aide.
While flying a non-precison approach the lowest altitude where the pilot may descend without first seeing the runway of intended landing is called the MDA (minimum descent altitude).
With a precision approach minimums are DA(H)
With a non-precision approach minimums are the MDA.
With a non-precision approach minimums are the MDA.
As with most things in aviation there is usually "more" to the subject. For example CAT II & Approaches, minimums based on radar altimiters, PAR approaches (nothing ground based) and LNAV-VNAV based RNP-RNAV approaches....so there is always "more to the story". I have tried to be simple yet correct in my explanation. The above is from a U.S. pilot with the acknowledgement that in other parts of the world differences will occur.
Khodafas Farid.
Respectfully,
Northbeach
Join Date: Oct 2007
Location: France
Posts: 749
Likes: 0
Received 0 Likes
on
0 Posts
Interesting:
To add something not too formal: the difference between MDA and DA(H):
-the first one is a line,
-the second one is a point (yest I know this is more poetry than formal explaination).
An other way to say it: DA(H) is an Altitude (Height), while MDA is an Altitude plus a Distance (refer to your approach charts).
Basically:
-Precision approach: DA(H): Altitude (Height)
-Non-precision: MDA: Altitude plus distance (you have to reach a minimum altitude BEFORE a minimum distance (or at the same time for some SOPs... But this is an other story). Have a look at MAP (missed approach point) to understand. Because you have to bear in mind that MDA is not the counterpart of DA(H) if you don't include MAP in term of decision/action.
Of course, as North Beach wrote there is much more to say...
When flying a non-precision approach; as you get closer to the runway you are allowed to descend to a lower altitude with the intent of being able to visually see the runway so as to land. But with a non-precision approach there is no vertical guidance provided by a ground based navigation aide.
While flying a non-precison approach the lowest altitude where the pilot may descend without first seeing the runway of intended landing is called the MDA (minimum descent altitude).
With a precision approach minimums are DA(H)
With a non-precision approach minimums are the MDA.
While flying a non-precison approach the lowest altitude where the pilot may descend without first seeing the runway of intended landing is called the MDA (minimum descent altitude).
With a precision approach minimums are DA(H)
With a non-precision approach minimums are the MDA.
-the first one is a line,
-the second one is a point (yest I know this is more poetry than formal explaination).
An other way to say it: DA(H) is an Altitude (Height), while MDA is an Altitude plus a Distance (refer to your approach charts).
Basically:
-Precision approach: DA(H): Altitude (Height)
-Non-precision: MDA: Altitude plus distance (you have to reach a minimum altitude BEFORE a minimum distance (or at the same time for some SOPs... But this is an other story). Have a look at MAP (missed approach point) to understand. Because you have to bear in mind that MDA is not the counterpart of DA(H) if you don't include MAP in term of decision/action.
Of course, as North Beach wrote there is much more to say...
