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Why is a remote altimeter setting less restrictive than a local altimeter setting?  Rate Topic 
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 Posted: Sat Jul 8th, 2006 09:58 pm
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rkaplan
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An interesting question was asked recently by one of our accelerated IFR rating students at Flight Level Aviation:


Take a look at the VOR 29 approach to Quakertown (UKT).  There is a VDP at CKZ DME 4.  There is a note that ‘VDP NA when using Pennridge altimeter setting."

As I understood VDPs, they are essentially extensions to the MDA where regardless of whether you can see the runway or not, you cannot descend below the MDA until you get to the VDP. So, how does one ‘NA’ a VDP? Without the VDP, seems to me I could descend to the airport normally at whatever point I might choose. That’s less restrictive than the VDP, and it doesn’t make any sense to me to have a less restrictive approach when using a less accurate (more remote) altimeter.

So, what am I missing?




Attachment: uktvor29.jpg (Downloaded 29 times)



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 Posted: Sat Jul 8th, 2006 10:03 pm
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rkaplan
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rkaplan wrote: That’s less restrictive than the VDP, and it doesn’t make any sense to me to have a less restrictive approach when using a less accurate (more remote) altimeter.



That is an excellent question.

A stepdown fix shared between a primary and remote altimeter setting can be raised to a higher altitude in order to preserve obstacle avoidance criteria.  But this cannot be done with a VDP because the VDP is not there for obstacle avoidance but rather to guide the pilot as to when a normal descent may be commenced -- if you move the VDP closer to the runway to accommodate a worst-case remote altimeter setting error then the VDP will no longer be accurate and instead will place the pilot at a position where a faster-than-normal descent rate will be required.  
 
As a result, TERPS prohibits publishing a VDP based upon a remote altimeter setting.  The pilot instead needs to rely on visual clues in order to determine when to begin a normal descent profile.
 
Think of this as similar to a Localizer approach vs. an ILS approach to the same runway.  On the Localizer approach, you are permitted to descend earlier than you are permitted to descend on the ILS; in a way that is more "permissive" but it also makes the approach less accurate and more challenging.


Here is the relevant text from TERPS:
 
 
253. VISUAL DESCENT POINT (VDP) (applicable to straight-in procedures only). When dual minimums are published, use the lowest minimum descent altitude (MDA) to calculate the VDP distance. PUBLISH A VDP FOR ALL STRAIGHT-IN NONPRECISION APPROACHES except as follows:
• Do not publish a VDP associated with an MDA based on part-time or full time remote altimeter settings.
• Do not publish a VDP located prior to a stepdown fix.
• If the VDP is between the MAP and the runway, do not publish a VDP.

 
 
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For further insights into approach plate design, I highly recommend reviewing the TERPS document, which may be found online here:
 
http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs400/afs420/policies_guidance/orders/media/TOTALCHG19.pdf
 
 



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Richard Kaplan, CFII
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