Is the MAP protected?
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Is the MAP protected?
I don't know how much pilots rely on their Radar Altimeters at the MAP, which made me wonder if the MAP is protected from changes that might affect RadAlt readings? For example someone digs a 30 foot crater 
Anyway, are there any provisions for that? Or is it insignificant?
Anyway, are there any provisions for that? Or is it insignificant?
There should be a level area on the ground prior to MAPt on all precision approaches; it's laid down as a specific distance for Cat 11/111 ILS, but I forget what that distance is! This ensures radalt gives a correct reading prior to MAPt.
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Smurfjet
Are you sure you should be using radalt to determine MAP (I assume you mean where MAP = DH/DA on a Precission Approach?)
Yes, there is protection built-in for CATII/III approaches; but I think (within the UK &, as far as I know, worldwide) for everyhting else procedure design is based on an assumption of using pressure altimeter; so MAP is represented as a distance from touchdown (or fix)... and for precision approaches it is the defined as the "point of intercept of the glidepath..."
Radalt has its uses, but picture the scene: An airfield sits atop a hill, at 400ftamsl. Your minima is 200ft. But, in the approach to the runway is a valley where the land drops to 200ftamsl, rising steeply towards the threshold. You're on an instrument approach, and looking at the radalt (which gives you height above terrain NOT Height above Airfield). Now please tell me exactly what indication your radalt gives you to tell you when you've reached the MAP or Decision Altitude?
Are you sure you should be using radalt to determine MAP (I assume you mean where MAP = DH/DA on a Precission Approach?)
Yes, there is protection built-in for CATII/III approaches; but I think (within the UK &, as far as I know, worldwide) for everyhting else procedure design is based on an assumption of using pressure altimeter; so MAP is represented as a distance from touchdown (or fix)... and for precision approaches it is the defined as the "point of intercept of the glidepath..."
Radalt has its uses, but picture the scene: An airfield sits atop a hill, at 400ftamsl. Your minima is 200ft. But, in the approach to the runway is a valley where the land drops to 200ftamsl, rising steeply towards the threshold. You're on an instrument approach, and looking at the radalt (which gives you height above terrain NOT Height above Airfield). Now please tell me exactly what indication your radalt gives you to tell you when you've reached the MAP or Decision Altitude?
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First, thank you all for posting here.
You assume correctly, and no I am not sure, I do not fly professionally. I discussed this with 'real' pilots and the conclusion is that it is illegal to use RadAlt for that purpose. I will try to find the legal text of the regulator to see what it says.
400 ft.
The whole discussion started while I was writing a text about cold temperature altitude correction, and I wanted to point out that the correction MUST NOT be applied to the DH which, I believed it to be a RadAlt reading.
From the discussions with the pilots, I realized that DH and DA are used to mean the same thing for them in the sense that all that matters is the Decision at the BaroAltitude, x-checked with Radalt, but it was my understanding that DA is based on BaroAlt, and DH is the RadAlt, can you confirm/deny this?
I do not have any approach plates handy at the moment, I assume the case you describe above will have the DA of 600 ft with and below it the DH (400ft) indicated in brackets corrected for Terrain or just (200ft)?
Yes, there is protection built-in for CATII/III approaches; but I think (within the UK &, as far as I know, worldwide) for everyhting else procedure design is based on an assumption of using pressure altimeter; so MAP is represented as a distance from touchdown (or fix)... and for precision approaches it is the defined as the "point of intercept of the glidepath..."
Radalt has its uses, but picture the scene: An airfield sits atop a hill, at 400ftamsl. Your minima is 200ft. But, in the approach to the runway is a valley where the land drops to 200ftamsl, rising steeply towards the threshold. You're on an instrument approach, and looking at the radalt (which gives you height above terrain NOT Height above Airfield). Now please tell me exactly what indication your radalt gives you to tell you when you've reached the MAP or Decision Altitude?
Radalt has its uses, but picture the scene: An airfield sits atop a hill, at 400ftamsl. Your minima is 200ft. But, in the approach to the runway is a valley where the land drops to 200ftamsl, rising steeply towards the threshold. You're on an instrument approach, and looking at the radalt (which gives you height above terrain NOT Height above Airfield). Now please tell me exactly what indication your radalt gives you to tell you when you've reached the MAP or Decision Altitude?
The whole discussion started while I was writing a text about cold temperature altitude correction, and I wanted to point out that the correction MUST NOT be applied to the DH which, I believed it to be a RadAlt reading.
From the discussions with the pilots, I realized that DH and DA are used to mean the same thing for them in the sense that all that matters is the Decision at the BaroAltitude, x-checked with Radalt, but it was my understanding that DA is based on BaroAlt, and DH is the RadAlt, can you confirm/deny this?
I do not have any approach plates handy at the moment, I assume the case you describe above will have the DA of 600 ft with and below it the DH (400ft) indicated in brackets corrected for Terrain or just (200ft)?
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DH and MDA
Hi there,
I've never heard of a DA, may be you're confused between DH and MDA. The first one is use with precision appch, mainly ILS or MLS (which i've never seen). The DH is decison height at which you must start a missed appch if you don't have visual with the proper ground reference (it can be the runway or the proper light). the MDA is pretty different , it's the minimum descent altitude and it's used with non-precision appch (ndb, ndb/vor, vor etc). The main difference is that you don't necesarily have to initiate a missed apch upon reaching this altitude with no visual to the ground. Depending on the appch you may descend to this altitude and then keep on flying til a certain DME point or any other MAP. For example you may descend to the MDA within 7 NM from threshold and the MAP could be at 1 DME from threshold. It will never be the case for a DH because following the glide and the loc will bring you to a unique point : the MAP.
I've never heard of a DA, may be you're confused between DH and MDA. The first one is use with precision appch, mainly ILS or MLS (which i've never seen). The DH is decison height at which you must start a missed appch if you don't have visual with the proper ground reference (it can be the runway or the proper light). the MDA is pretty different , it's the minimum descent altitude and it's used with non-precision appch (ndb, ndb/vor, vor etc). The main difference is that you don't necesarily have to initiate a missed apch upon reaching this altitude with no visual to the ground. Depending on the appch you may descend to this altitude and then keep on flying til a certain DME point or any other MAP. For example you may descend to the MDA within 7 NM from threshold and the MAP could be at 1 DME from threshold. It will never be the case for a DH because following the glide and the loc will bring you to a unique point : the MAP.
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clr4takeoff thanks for coming in...
The following applies to Canada, but I'd like to hear from other places.
CAP GEN abbrivation page, look for DA. When I read your post I suspected you might be right, but I finally looked at the CAPs to clear things up.
Now while you have the CAP GEN (assuming its in the library
) Page 1 of Definitions:
And to keep things in perspective, same page:
I understand the difference of minima that exist between PA and NPA (thanks for explaining anyway), what I really want to clear up/point out/confirm is the difference between DA and DH. Actually is there a difference?
When I use terms to explain or communicate I'd like to be accurate, and while discussing this it appeared to me that their is no real distinction between the 2 terms for pilots. i.e. the sample I spoke with refered to it as DH for all practical purposes and thats it.
If you look at the Approach Chart legend (CAP GEN) the only time DA is mentioned is for an LNAV/VNAV approach, DH is indicated for the ILS.
It is not explained clearly enough for me, so I seek superior (more experienced) minds for an explanation
The following applies to Canada, but I'd like to hear from other places.
Now while you have the CAP GEN (assuming its in the library
) Page 1 of Definitions:
DECISION ALTITUDE OR DECISION HEIGHT (DA/DH): Means an altitude or height specified in the Canada Air Pilot or the route and approach inventory at which a missed approach procedure shall be initiated during a precision approach or an approach procedure with vertical guidance, if the required visual reference necessary to continue the approach to land has not been established.
MINIMUM DESCENT ALTITUDE (MDA): A specified altitude referenced to sea level for a non-precision approach below which descent must not be made until the required visual reference to continue the approach to land has been established.
When I use terms to explain or communicate I'd like to be accurate, and while discussing this it appeared to me that their is no real distinction between the 2 terms for pilots. i.e. the sample I spoke with refered to it as DH for all practical purposes and thats it.
If you look at the Approach Chart legend (CAP GEN) the only time DA is mentioned is for an LNAV/VNAV approach, DH is indicated for the ILS.
It is not explained clearly enough for me, so I seek superior (more experienced) minds for an explanation
Last edited by Smurfjet; 4th Nov 2006 at 02:21. Reason: Typos
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Radalt has its uses, but picture the scene: An airfield sits atop a hill, at 400ftamsl. Your minima is 200ft. But, in the approach to the runway is a valley where the land drops to 200ftamsl, rising steeply towards the threshold. You're on an instrument approach, and looking at the radalt (which gives you height above terrain NOT Height above Airfield). Now please tell me exactly what indication your radalt gives you to tell you when you've reached the MAP or Decision Altitude?
Luton is a case in point, 526ft up on top of a hill with valleys either side. We got Cat III approval before Cat II when we managed to prove there was the required no. of seconds steady radalt descent. Even so, this warning is on the approach plates:
Due to the sloping terrain in the approach area
of ILS CAT II operations, the rate of radio
altimeter height reduction prior to the THR will
be approximately double the normal rate.
At the heights, and therefore range, of DA/DH on a Cat II the terrain is almost certainly to be within the aerodrome boundary so the airport will (should
) have control over who digs holes where. Outside the boundary, within the UK, there is the safeguarding process which ensures the airport is notified in advance of any proposed development that might have a bearing on operations.
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clr4takeoff
DA = DH + TDZE... Granted, the term is seldom used in North America, where DH is used to talk about DA and/or DH.
Smurfjet
True, pilots don't use the radalt as a primary means of defining DA. However: many GPWS systems use the radalt and the DH manually set in by the pilot (e.g. 200ft) to give the 'Minimums! Minimums!' call. Thence if somebody digs the crater you are talking about before the runway AND the pilot does not properly monitor his altimeter, then the automatic GPWS Minimums call will occur 30ft below DA.
DA = DH + TDZE... Granted, the term is seldom used in North America, where DH is used to talk about DA and/or DH.
Smurfjet
True, pilots don't use the radalt as a primary means of defining DA. However: many GPWS systems use the radalt and the DH manually set in by the pilot (e.g. 200ft) to give the 'Minimums! Minimums!' call. Thence if somebody digs the crater you are talking about before the runway AND the pilot does not properly monitor his altimeter, then the automatic GPWS Minimums call will occur 30ft below DA.
