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Procedure N/A

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Instrument Rating

I see on some approaches the words "Procedure N/A from V-123 Westbound". My guess is that there is some magical number of degrees of a turn that, if exceeded, would make a particular approach undoable. But if you were cleared direct to the fix where this notation would be located and you were not exactly on the Victor Airway specified, could you still fly the approach? What is the "magic" that applies this term to some approaches? See the following as examples:

RNAV (GPS) RWY 13 KASX

RNAV (GPS) RWY 27 KARR

Yet, if I were flying Eastbound to Northbrook (OBK) VOR on the 269 degree radial and intending to fly the VOR RWY 26 into 3CK, apparently I could turn more than 180 degrees to get back onto the inbound course of 269 degrees to fly this approach as there is no note telling me the the Procedure is N/A when arriving at OBK on the 269 degree radial.

I would appreciate it if anyone has a definate answer to this question and can support their response with references that I can eventually quote when asked this question again.

Thanks

Dave Hooper

5 Answers

  1. Best Answer


    Nathan Parker on Dec 29, 2011

    I recall from TERPS that 120 degrees is the magic number.  TERPS criteria were not designed with circumventing airways in mind, so all bets are off when you’re cleared direct to a fix.  For slow aircraft, you probably have plenty of room to maneuver safely to intercept the airway, since you have 4 nm of prmary protected airspace on each side of the airway, and 2 nm of secondary.  If you were at airliner airspeeds, probably not.  The recommendation I make to students, which I’d be very surprised if they managed to remember, is that you should slow down to Procedure Turn airspeeds before making such a turn.

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  2. Nathan Parker on Dec 29, 2011

    ” this approach as there is no note telling me the the Procedure is N/A when arriving at OBK on the 089 degree radial.”
     
    You would actually be on the 269 radial and the turn is forbidden by the planview note.  You have greatly reduced clearances in the approach phase of flight.

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  3. Nathan Parker on Dec 29, 2011

    Sorry, my math skills are weak.  That radial is not included in the planview note.  Unless the 269 radial is an airway, it’s not a published part of the procedure and TERPS doesn’t consider any turns from that direction, so it’s not an implicit admission that they are ok.

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  4. John D. Collins on Dec 30, 2011

    From the TERPS, paragraph 1510:

    1510. EN ROUTE OBSTACLE CLEARANCEAREAS.  

     

    En route obstacle clearance areas are identified as primary and secondary. These designations apply to straight and turning segment obstacle clearance areas. The required angle of turn connecting en route segments to other en route, feeder, or initial approach segments shall not exceed 120°.

     

    Nathan has pretty much covered the topic, other than the specific cite I added. The current IFR system is still based on Airways for enroute navigation.  This means that there must be routes that transition from airways to approaches. These are generally called feeder routes. With modern GPS RNAV capability, it is common to fly direct routes that are not part of the airway structure. These type of routes are called random RNAV routes. One of the criteria for flying a random RNAV route is that ATC must provide radar surveillance.  When ATC is vectoring an aircraft, they have a minimum vectoring altitude that allows them to provide for terrain clearance. As Nathan pointed out, the notes in the approach charts only address arriving via airways and not by random RNAV routes. That is why the note reads “Procedure NA for arrival on OBK VOR/DME AIRWAY RADIALS CW 299 thru 327. 

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  5. Matthew Waugh on Jan 01, 2012

    It’s both a turn and altitude issue. I’ve also seen “Procedure N/A” when the required altitude loss from the airway would have required a near vertical descent.
     
    Also keep in mind – “cleared direct OBK” is not the same as “cleared via V-123 OBK” – even if you cover the same track. It doesn’t make the procedure any easier, but it might make it more legal and, as noted above, it implies radar surveillance which can change the rules.

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