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MAPt/DA - LOWG

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MAPt/DA - LOWG

Old 2nd Aug 2011, 17:52
  #41 (permalink)  
 
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FS,
I know the math has been worked many times...
RWY 1089+53TCH= 1142
[email protected]=467
= 1609 so this corresponds to the GRZ 1600 shown on one part of the LIDO and Jepp charts

Why all of the charts show the GP altitude at GRZ of 1600 (3.14 GPA)with a min at the same point as 1500 (2.4GPA) with the ILS/lighting at 3 GPA.....

Austro Control GmbH is really the entity to contact about all of this...lets see what they have to say...
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Old 2nd Aug 2011, 17:57
  #42 (permalink)  
 
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FlightPathOBN: The table you provided is for circling approaches. Minimal obstacle clearance (MOC) for all aircraft is 246 ft for a non-precision approach with FAF.
I understand, but this chart seems to combine the circling approach parameters, so I am wondering if that was a leftover

The VEB ROC is not fixed for the final segment, granted the 246 is a minimum (the infamous 200 foot ROC plus 46 feet of momentary descent, where that surface meets the ground, but the surface tapers to a 500 foot ROC at the FAF as the intermediate segment ROC is 500'.

Last edited by FlightPathOBN; 2nd Aug 2011 at 18:37.
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Old 2nd Aug 2011, 20:02
  #43 (permalink)  
 
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This reason for the 1600 has to do with the continuous descent approach. From the below numbers it is obvious that you can not descent from 2300 to 1500 and then to the threshold in a straight (continous) line:

If you cross the NDB at 2300 feet and pass the VOR at 1600 feet the profile angle is 3.14 degs like Jeppesen writes on the chart. Roughly the same as from the NDB at 2300 feet to the threshold + 53 feet. Here the angle is 3.12 degs - or practically the same. The difference is probably due to rounding of numbers.

However:
If you cross the NDB at 2300 feet and pass the VOR at 1500 feet the profile angle is 3.59 degs. After the VOR the profile angle now becomes very shallow with only 2.42 degs to reach the threshold at 53 feet.

Only quick reference I could find on google to post is this old Jeppesen Bulletin:
http://jeppesen.com/download/briefbu...l_JEP_02_B.pdf

Here Jeppesen writes that the procedure altitudes are recommended (like hvogt already said ) and "The Procedure Altitudes have been established to accommodate a stabilized descent profile on a prescribed descent angle on the final approach course." Hence, no guarantee that you will reach your minimum in time.


Looking from a practical point of view, in this particular instance, the distance travelled to descent 100 feet is less than 0.33 nautical miles in round numbers (100 feet pr. min. / 600 feet pr. min = 10 seconds => 10/60 x sixty factor 2 (120 knots) = 0.33). The fix tolerance for a DME is 0.25 nm. Add to that the slant range (albeit very little in this case), and you practically will not notice the difference. All very academic. Disregard.

This procedure would be quite the adventure with a CAT C or CAT D aircraft.
I flew this procedure in a 737 only a few weeks ago with rain and a cloud base around 1000 feet. And it is no more adventurous than other non-precision approaches. Coming from the North ATC will give you a downwind vector and descent you to 3500 feet. Then a base and an intercept heading along with a clearance for the VOR approach. Configure to flaps 5/ speed 5 during the base turn. At 6 nm and established, you may start a normal 3 degs'ish descent to cross the NDB at 2300 (in a continuous descent) and then down to 1500 feet DA. All very straight forward.

With regard to the MAPt, you have to descent a little below the Jeppesen recommended path if cloud base is at minimum.

Last edited by cosmo kramer; 3rd Aug 2011 at 00:17.
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Old 2nd Aug 2011, 21:05
  #44 (permalink)  
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or fly a little past the MAPt
- not an option, I believe.
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Old 2nd Aug 2011, 23:53
  #45 (permalink)  
 
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Cosmos,

Thanks for the real time update...I am curious, flying this NPA, what is your vertical guidance?
If flying baro-vnav, at what temp would you consider this NA?

an example of approach procedure design, precision, non-precision, temperatures, and all of the surfaces considered...


Last edited by FlightPathOBN; 3rd Aug 2011 at 00:04.
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Old 3rd Aug 2011, 00:10
  #46 (permalink)  
 
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I did not mean to sanction indiscriminately flying past MAPt in other procedures. As I wrote, in this exact example, I thought the distance to be so small that it in practical world would be negligible = inside tolerances.

However, I correct myself regarding the fix tolerance after digging up ICAO doc 8168.
While the posted DME tolerance is correct, it was of course wrong that I based the calculations on the DME and not the VOR itself.
When overheading a VOR station the tolerance is actually only "nm = 0.033 x height (in 1000 of feet)". Or in this case 0.033 x 0.412 = 0.014 nm.

Of course it's not correct to wait to observe an increasing DME distance when already having a "from indication". So I admit that the go-around must be initiated immediately upon a "from indication" and hence, the only legal choice is to go below the recommended profile in case the cloud base is at minimum. I edited that above and thank you for making me refresh that point.
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Old 3rd Aug 2011, 00:28
  #47 (permalink)  
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Response from Jeppesen:

I believe that this is actually a problem with the design of the procedure. AIP Austria does not designate an actual descent angle (or gradient) for the CDFA on this procedure. This leaves it up to Jeppesen to build the descent angle for the navigational database, using ARINC-424 coding rules. There is also no designated TCH, so per our rules the TCH from the ILS approach is borrowed (53). Drawing an angle between the 53 TCH for Rwy 35C and the source-driven final altitude of 2300, the aircraft will cross the MAP (GRZ VOR) at close to 1600, not 1500. So, in the pure sense, the recommended descent band is correct. But following the EU-OPS required CDFA (as published by Jeppesen using ARINC-424 coding rules), you will never get to the 1500 DA by the MAP. You will always be 100 high. In reality, in order to have a procedure which is built in accordance with ARINC-424 and EU-OPS, the straight-in minimums should probably be 1600.

I will send a request to our analysts in the Frankfurt office about getting an inquiry off to the Austria AIS.
Basically they confirmed my initial thesis that you actually can't reach DA if you fly the approach "by the book".

Originally Posted by cosmo_kramer
So I admit that the go-around must be initiated immediately upon a "from indication" and hence, the only legal choice is to go below the recommended profile in case the cloud base is at minimum.
Isn't more accurate indication of overflying the VOR the increasing groundspeed on the DME, which isn't affected by the cone of silence? This should be especially valid at greater heights, where the diameter of cone of silence (or its horizontal component) is larger. Nevertheless, I'm being picky, usually you get a from indication quite fast, so you'd still be within limits.
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Old 3rd Aug 2011, 00:35
  #48 (permalink)  
 
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FlightPathOBN:

In a modern aircraft with path vector symbology it seems to me that the charted (and presumably database) descent angle can be used to arrive at a DA of 1,500 at the VOR. Lacking required visual references the missed approach is executed at the VOR using the DA concept. But, if the runway is in sight (or at least the approach end) adjust the path vector to achieve a shallower descent gradient from DA to the runway.

This seems perfectly safe and acceptable especially since it is a charted "fly visual to airport" segment from the VOR to the runway.
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Old 3rd Aug 2011, 00:37
  #49 (permalink)  
 
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Originally Posted by Flying Stone
Again interesting, it seems that only the AIP chart shows that you are 1500ft when overhead GRZ VOR, LIDO shows 1600ft,
As I explained earlier, that is because the "legal" minimum for the last segment GRZ NDB to GRZ VOR is 1500ft. This cannot be achieved using a constant angle descent from LENIZ without clipping the 2300ft step, so the "end" of the LIDO CDA at the MAPt is 1600ft.

the only legal choice is to go below the recommended profile in case the cloud base is at minimum.
Correct.

Austro Control GmbH is really the entity to contact about all of this...lets see what they have to say...
Hardly necessary. It's pretty obvious.
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Old 3rd Aug 2011, 01:36
  #50 (permalink)  
 
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In a modern aircraft with path vector symbology it seems to me that the charted (and presumably database) descent angle can be used to arrive at a DA of 1,500 at the VOR.
The ARINC-424 coding does not work for this procedure, even if you were try to force this into 424 coding, it would disco in the database.

Quote:
Austro Control GmbH is really the entity to contact about all of this...lets see what they have to say...
Hardly necessary. It's pretty obvious
Jepp response...

I will send a request to our analysts in the Frankfurt office about getting an inquiry off to the Austria AIS.
Stone is correct in questioning this procedure.
The 1500 at GRZ (2.4 GPA) is not supported by any criteria, obstacle assessment areas, or even simple math. the lights are at 3 degrees, the 34:1 surface and 20:1 surfaces do not provide the required ROC for this 2.4 approach, there is no obstacle assessment that provides ROC protection for the 1500 2.4 gpa surface.
If flying NPA, with baro-vnav, the temp limits are not shown, for when the 2.4 GPA ROC meets limits??? what is the limit...ground surface?
Show me the ICAO surface that provides 200' ROC. 246 ROC. or 250ROC at 1500.

Last edited by FlightPathOBN; 3rd Aug 2011 at 02:15.
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Old 3rd Aug 2011, 22:11
  #51 (permalink)  
 
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This seems perfectly safe and acceptable especially since it is a charted "fly visual to airport" segment from the VOR to the runway.
Not sure if I agree...as this runway in question has an ILS...

The surfaces used for an approach, 34:1 and 20:1 are used for obstacle clearance. These surfaces originate at runway endpoint, while the GPA is a function of the standard 50' TCH.
The 20:1 surface, 2.86, is used for the visual portion of an approach.
The 34:1 surface, 1.68 is the absolute obstacle surface for the approach.
The 50:1 surface, is used for on-airport appurtenances, such as the VOR ,and lighting.
The minimum of 2.5 GPA for non precision due to temperature limitations of baro-vnav is the absolute minimum the obstacle surfaces allow.
The VOR approach with a 1500' min or 2.42 GPA, falls outside any criteria for obstacle evaluation.

This is not to say that an exemption or variance has been provided for with a more detailed obstacle/terrain assessment. It would be difficult given general assumptions of a 200' AAO, but I would allow for that to be mitigated with restrictions...Rad-alt may possibly get one to the VOR min, but there is no definition on the plate as to the method of altitude calculation, or temperature limit...
Again, AustoControl has some questions to answer...
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Old 4th Aug 2011, 07:59
  #52 (permalink)  
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the only legal choice is to follow the legal procedures ergo to initiate the go around at the DA or the MAP whichever comes first as much as to follow the published profile. The glitch in the procedure design isn't any of airman's concern.
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Old 4th Aug 2011, 23:17
  #53 (permalink)  
 
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the only legal choice is to follow the legal procedures ergo to initiate the go around at the DA or the MAP whichever comes first as much as to follow the published profile.
so your only advice is to follow the legal choice...did you notice that the DA and the MAPt are both 1500?
While the GPA places you at 1600 for the same point?

So you should use 1500 with a GPA of 2.42, and of course, since this is an MDA(H), you feel its okay to drop below this for your missed approach point..

are you a groundskeeper?
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Old 5th Aug 2011, 07:21
  #54 (permalink)  
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FPO, I'd rather you read the post carefully. Clearly you're not a pilot coz you're brainstorming way too much for a simple matter. We're operators not designers therefore I follow the published profile and perform a go around as per rule. In this case at the VOR, basta. I couldn't care less if it's 1600 ft and 100 ft above the DA. Rules are rules, over.
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