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MDA for CONSTANT DESCENT NPA

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MDA for CONSTANT DESCENT NPA

Old 6th Jan 2010, 08:20
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MDA for CONSTANT DESCENT NPA

I know this has been industry-wide practice now for a long time, but can anyone quote me either ICAO or Jeppesen reference to the requirement to add 50 feet to the published MDA for a NPA when flying the now-standard constant descent angle approach technique, to allow for the height loss on a go-around on say a category C airplane?.Also any decent paper on this subject written by ICAO or other professional source explaining the logic behind this practice (for training purposes).Many thanks Pete.
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Old 6th Jan 2010, 08:54
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...when flying the now-standard constant descent angle approach technique,
Now standard, with whom?
At our mob, we still dive and drive....works good, lasts a long time (properly applied, of course).
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Old 6th Jan 2010, 09:08
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Standard in Europe 411.....

Pete, it's in our ops manual and charts (we have company charts from LH Systems)

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Old 6th Jan 2010, 09:10
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NON – PRECISION (NON-ILS) APPROACH :
1. Non-Precision Approach will be carried out in the CANPA
Profile (Constant Angle Non-Precision Approaches) with VNAV
or V/S mode. Detailed description of CANPA is given in Part
‘C’.
2. CANPA procedure does not involve leveling out at MDA &
Missed Approach is initiated after reaching DDA (Derived
Decision Altitude). Levelling out at MDA will only be carried
out in special Approaches, eg. CANARASI R/W 13 at JFK

where special briefing is required.

VNAV APPROACH
“LNAV / VNAV or LOC / VNAV is the preferred method for
accomplishing non-ILS approaches that have an appropriate vertical
path defined on the FMC LEGS page. (V/S may be used as an alternate
method and is given subsequently).
” The A/C is equipped with RNP /
ANP Alerting System. However, VNAV DA(H) will not be used. Use DDA
(Derived Decision Altitude, which is arrived at by adding 50’ to the MDA).
The following should be ensured :-
1) Appropriate Vertical Path defined on the FMC Legs Page is
the same as the published Vertical Path.
2) The Glide Path from FAF to R/W coded in the FMC
Navigation Database, is the same as the published Glide
Path and is between 2.75
0 & 3.770.
3) Atleast one GPS or one DME is operational.
4) Such approaches may be flown provided RNP being used
is equal to or less than RNP specified for the approach.
Following RNP values must be ensured [Prog Page 4
(B777-300ER/LR) & 3 (B747-400) of FMS CDU and can
also be cross checked on ND] :
VOR - DME / VOR / NDB : 0.5

RNAV / RNAV GPS : 0.3
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Old 6th Jan 2010, 09:19
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My understanding is that one may not descend below the MDA , without visual reference ; therefore the habit of adding 50 feet for a Cat C aircraft will hopefully ensure that in the event of a Go Around , initiated at the 50 foot point , the a/c will not go below MDA ( and you will not fail your check ).
With a DH , the missed approach must be INITIATED at or before the DH and a subsequent sag below DH is acceptable.
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Old 6th Jan 2010, 09:35
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pp - we've had this discussion recently here! Have a look here. Note the link to the Jepp info. There is more bedtime reading here As '737jock' says, there is now the option in EU-OPS anyway of establishing a DA for an NPA which will not cause the a/c to descend below a safe obstacle clearance during a g/a. It will be company specific in the OM and some may take a while to change. Note the 'divers and drivers' are required to increase min RVR for their approaches.

As 'backofthedrag' says the whole definition of 'MDA' was that you could not go below it in a g/a and hence the (various) additions for different a/c in different OMs (50' in my lifetime). MDA was an abbreviation for Minimum DESCENT altitude ( not DECISION). Obviously a change to a 'real' DA will need a review of the figures used. It will certainly make like easier at the sharp end.

I believe implementation of the new system has to be sometime in 2011.

Like many things 'European' it is riddled with scope for confusion!

Last edited by BOAC; 6th Jan 2010 at 09:55.
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Old 6th Jan 2010, 10:19
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Surely the "add-on" depends on the a/c type? On the B737-200/300 it was 35 ft (defined I believe somewhere in the AFM). For CAT A types we used 10 ft and then also added the max pressure error correction of 50ft, the latter also specified in the AFM.
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Old 6th Jan 2010, 11:30
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My company does not add this 50' to an MDA. In some cases, and these are the cases where the weather is on the limit, during this 50' you will see the rwy and a G/A is not necessary. If not, there is still enough margin to safely execute the G/A. I had already tons of NPA's where this was the case.
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Old 6th Jan 2010, 11:35
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My company does not add this 50' to an MDA. In some cases, and these are the cases where the weather is on the limit, during this 50' you will see the rwy and a G/A is not necessary. If not, there is still enough margin to safely execute the G/A. I had already tons of NPA's where this was the case.
So you're saying that you initiate a G/A AT MDA and knowingly go below MDA because there is "enough saftey margin"?
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Old 6th Jan 2010, 12:12
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The DDA is required for CANPA.
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Old 6th Jan 2010, 12:18
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My company does not add this 50' to an MDA.
- see what I mean about confusion. As written there that company is operating illegally. A minimum is a minimum!
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Old 6th Jan 2010, 12:34
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knowingly go below MDA because there is "enough saftey margin"?
I realise this will cause an uproar, but realistically 50 feet is going to make stuff-all difference in terms of flight safety in a go-around from an MDA on a non-precision approach. The lowest MDA I know of is probably around 350 feet and if for decades the normal published MDA has been used from where a go-around is commenced (whether dive and drive or a constant angle descent) I doubt if there are hundreds of aircraft spread over the countryside simply because they went below a MDA by 50 feet on an instrument go-around. Isn't it all really a storm in a tea cup? Having said that I realise there is a question of legality.
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Old 6th Jan 2010, 12:43
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The MDA has been designed decennia ago when aircraft were still driven by props. In these times, you descended to MDA and kept that altitude until visual or at the MAP nowadays, nearly all companies do a CDA. based on the DME/altitude readings on the profile. it is therefore considered that an MDA is in fact a DA.
As previously mentioned, the DA and MDA will be the same in the new procedures. No need to add this 50'.
Operating illegally?! Don't make me laugh! Our OM is approved by a major EASA state and my company is well known for its good standards.
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Old 6th Jan 2010, 12:53
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Now standard, with whom?
At our mob, we still dive and drive....works good, lasts a long time (properly applied, of course).
Dive and drive is stone-age. Is "your mob" the "fly it like a 200 mob?"

LNAV/VNAV or CANPA is much more stable/safer way to go. Drive and drive "not applied properly" = GPWS warning now and then.

Last edited by Rapid D; 7th Jan 2010 at 01:35.
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Old 6th Jan 2010, 13:16
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Hi despegue,

What aircraft are you operating?

If you are using dive and drive to MDA then you must have commenced the level off before so that you didn't sink below MDA. (at MDA + 50 say?)

We can't satisfy the stable approach criteria with the level segment below 500 feet AGL so we must use a continuous descent profile.
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Old 6th Jan 2010, 13:17
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Even LNAV/VNAV is old now, the newfangled thing for boeings is IAN. Everything is flown with identical procedures and indications, be it ILS, GLS, or any kind of NPA.

That said we do not add 50ft to the MDA, however we have to initiate the go around latest 50 feet above an MDA, if a DA is used instead no added safety margin is needed. Sadly the 50' above an MDA thing is put into a small note that is very easy to miss, simply bad presentation in our present manuals.
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Old 7th Jan 2010, 01:57
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Thanks

PT6 and BOAC, thanks kindly for that info.I know most operators have the procedure in their ops manuals also AFM has the actual 'aircraft allowance' but where else is this CANPA information published.Sorry im currently working somewhere I cant access a Jeppesen manual, and my current employer has never heard of this stuff (dont laugh), perhaps 411A is the DFO here, and has banned any discussion of the dreaded "constant descent NPA", cheers Pete.
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Old 7th Jan 2010, 02:23
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Hi, guys I understand diference exist on a DA and MDA but I never see in a paper those 50 feet that you are talking about, and the initial post is asking about a reference to find these matter.
toby
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Old 7th Jan 2010, 03:36
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Dive and drive is stone-age
Yup, and it suits this stone age guy just fine, thank you.

Quote:
My company does not add this 50' to an MDA.

- see what I mean about confusion. As written there that company is operating illegally. A minimum is a minimum!
Illegally...nonsense. They don't add 50 feet, and apparently don't descend below MDA without visual reference.
We do exactly the same at our mob.
50 feet is for the uninformed.
Or those that can't fly level at MDA.
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Old 7th Jan 2010, 07:25
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Originally Posted by 411
and apparently don't descend below MDA without visual reference
- read his post again?
My company does not add this 50' to an MDA. In some cases, and these are the cases where the weather is on the limit, during this 50' you will see the rwy and a G/A is not necessary.
- and in others? Yes - he could well descend below M Descent A without visual reference. I cannot see how you arrive at your conclusion!
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