Magnetic Variation variation

Discussion in 'Pilot Training' started by pilotod, Feb 15, 2012.

  1. pilotod

    pilotod Pre-takeoff checklist

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    On my sectional the variation is 9 degrees east. When I look at the AFD info an airport on my sectional the variation says "Magnetic Variation from 1980 is 12° East."

    Why would an FAA up-to-date sectional variation be different than an up-to-date AFD?

    And when I get a nav log done by the aopa the variation varies...it's not just 9 or 10, it's 12 sometimes. And my flight is all within 75nm.
     
  2. N801BH

    N801BH Touchdown! Greaser! Gone West

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    Large iron ore deposit ??:dunno::dunno::dunno:
     
  3. poadeleted20

    poadeleted20 Deleted

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    At least the A/FD warned you the mag var is 30 years old. OTOH, the sectional is up to date -- just compare them over time. So use the mag var from the sectional.
     
  4. N801BH

    N801BH Touchdown! Greaser! Gone West

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    Why would the A/FD not update their files???? I mean 30 years..:dunno:

    What kind excuse is that...:dunno::nono:
     
  5. bobmrg

    bobmrg En-Route

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    Once upon a time I was going to make this into an article but never got around to it. Variation is a very complex subject, as you will learn from this email. Forgive the formatting.

    * Jim.Grant@faa.gov * * Wed, Sep 3, 2008 at 6:19 AM *
    To: bobmrg@comcast.net
    Cc: Eva.K.Graham@faa.gov,
    Eric.Secretan@faa.gov

    Mr. Gardner: Please pardon our delay in responding to your inquiry. We
    wanted to verify the answers we have provided below. The questions that
    you raise are interesting, and not at all simple or straight forward.
    Allow me to illustrate in bullet form.

    · The Isogonic lines on Sectionals are very accurate for the date in which they were modeled by
    the National Geodetic Survey (NGS). However, this modeling by NGS is
    only done every 5 years (the "current" model is for 2005). The NGS model
    does include predicted annual change for reference points, which can be
    used to recompute the variation over time, but this is not currently
    done with the Sectionals (or any other FAA VFR chart). Because of the
    time delay in NGS creation of the model; the need to generate continuous
    isogonic line graphics for Sectional charts from the NGS model reference
    points; getting the charting graphics built; and chart print cycles; it
    is about one year before the "updated" isogonics are applied to the
    first Sectional charts after a new model is released. This means that
    the "current" (WMM-2005 model) Sectional isogonic lines will be on VFR
    charts from around 2006 through 2011. {As a note of interest: with the
    2010 NGS model, we intend to "forecast" it to 2013 using the annual
    rates of change before generating the Sectional isogonic graphics. This
    will mean that the 2013 isogonic lines will be on charts from 2011 to
    2016...which means that the isogonics will range from 0 to 3 years
    different than actual variation, rather than the current 1 to 6 years
    old}. Ref: National Geodetic Data Center (NGDC) Magnetic Field
    Calculator: _http://www.ngdc.noaa.gov/geomag/magfield.shtml _ Ref: The
    World Magnetic Model: _http://www.ngdc.noaa.gov/geomag/WMM/ _ ·

    Variation for NAVAIDs and airports are "assigned" and not the actual
    variation for that location (however the goal is to make it close).
    Variation for a NAVAID can be up to 3 degrees different than actual
    variation, and for an airport it can be up to 3 degrees different. This
    is why someone flying a 090 radial can have a 093 compass heading in
    zero wind conditions. Technically the variation in the A/FD is more
    current because it is updated every 56 days, but it is only changed if
    it is "reassigned." The reason "assigned variation" is used for airports
    and NAVAIDS is because there are infrastructure considerations with
    aligning NAVAIDs and airports to actual variation. For airports it means
    renumbering runways. For NAVAIDs, it means a physical "rotation" of the
    VOR radials (think fixed beams from a ground beacon) to correspond to
    the actual variation (which when it is done has impact on airways,
    airspace, etc.). Ref: FAA Order 8260.10D: 217. GUIDELINES. The
    identification and selection of navigational aids or airports as
    candidates for revision of MV of record require careful consideration
    and evaluation of a number of factors - as the impact of MV changes can
    be considerable. The applicable Air Traffic Service Area Office may have
    to initiate or revise published air traffic procedures; the Technical
    Operations Service (AJW-0) is directly involved in facility rotations
    and requires proper coordination. The Airports Division, or appropriate
    military authority, may have to arrange for repainting of runway
    designator numbers [see paragraph 858e(2)(e)]. Note: Guidelines
    pertaining to runway designation marking relative to magnetic changes
    can be found in AC 150/5340-1, Standards for Airport Markings, paragraph
    7d. ..

    a. MV versus Epoch Year Value. When the difference between the MV of
    Record and the nearest future Epoch Year value of any navigational aid
    or airport is 3 degrees or more, the MV of record must be changed to the
    nearest future Epoch Year value. When the difference is less than 3
    degrees, AJW-3 must consider implementing the nearest future Epoch Year
    value when workload permits. Factors to consider include whether the
    navigational aid is isolated or in close proximity to one or more other
    facilities, whether on airport or away from an airport, and the impact
    on instrument flight procedures.

    b. Facilities on Airports. At airports
    with localizer(s) or more than one navigational aid, the MV at the
    airport reference point (ARP) must be designated and assigned to all
    facilities at that airport, including all components of the ILS.

    c. MV versus OC Chart Value. Where the assigned MV of record differs from the
    MV shown on the Obstruction Chart (OC), the assigned MV of record must
    be used in the development of instrument flight procedures. d. Runway
    bearing must be assigned the same MV as the airport. Note: The actual
    runway bearing is published on airport diagrams to allow pilots to
    obtain a compass bearing check during runway line-up. This value may
    differ from the value computed using the assigned variation.

    e. At major airport terminal areas, the ARP MV of record at the designated
    controlling airport may be used in determining the MV applied to all
    navigational aids serving the terminal areas.

    · Commercial GPS and other software variation models: There are many different ways that
    various systems handle variation. Some have a fixed 5-year model like is
    depicted on the Sectional charts. Some have a yearly variation model.
    And some have a dynamic model that uses the 5-year model and applies the
    appropriate variation change rate to determine actual (up to date)
    variation. But it is still even a little more complicated than that,
    because different systems may have more or less variation reference
    points included for modeling because of data size limitations. Ref:
    Aeronautical Information Manual: *l. Conventional Versus GPS Navigation
    Data* There may be slight differences between the course information
    portrayed on navigational charts and a GPS navigation display when
    flying authorized GPS instrument procedures or along an airway. All
    magnetic tracks defined by any conventional navigation aids are
    determined by the application of the station magnetic variation. In
    contrast, GPS RNAV systems may use an algorithm, which applies the local
    magnetic variation and may produce small differences in the displayed
    course. However, both methods of navigation should produce the same
    desired ground track when using approved, IFR navigation system. Should
    significant differences between the approach chart and the GPS avionics'
    application of the navigation database arise, the published approach
    chart, supplemented by NOTAMs, holds precedence. Due to the GPS
    avionics' computation of great circle courses, and the variations in
    magnetic variation, the bearing to the next waypoint and the course from
    the last waypoint (if available) may not be exactly 180 degrees apart
    when long distances are involved. Variations in distances will occur
    since GPS distance-to-waypoint values are along-track distances (ATD)
    computed to the next waypoint and the DME values published on underlying
    procedures are slant-range distances measured to the station. This
    difference increases with aircraft altitude and proximity to the NAVAID.
    We hope this information provides you with some of the answers you were
    looking for.

    Best Regards, James L. Grant Staff Cartographer IT Planning
    & Product Development Team Aviation System Standards Federal Aviation
    Administration 301-713-2631 ext. 198
     
  6. FlyWithScott

    FlyWithScott Filing Flight Plan

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    Wow. Thanks for sharing that.


    Sent from my iPad using Tapatalk HD
     
  7. dbman5

    dbman5 Pre-Flight

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    Actually, although they try to keep things to 3 degrees, VORs can be off up to 6 degrees according to (a) one of the guys in Oklahoma and (b) what I've found on the charts. When I told them I found one that was off 10 degrees they were surprised but said it was possible due to backlogs.

    This is why I've said before that the stuff you read in all the training manuals (Jep, Gleim, etc.) about using the VOR to determine true north is wrong. Some of the on-line flight planners do (or at least 'did') the same thing - although at least some are starting to use the NGS modeling program. FWIW, a 6 degree error is a 1 mile error for every 10 miles travelled. That's not major but wind variations will create enough problems on their own so why build in an error if you don't have to.

    With VORs gradually disappearing and GPS taking over, I doubt the manuals will ever get changed to even explain that "VOR true north" isn't really true north - just kinda close.

    Also, why would they change an airport when it is off by 3 degrees when the runway numbers are only to the closest 10 degrees? (That's a rhetorical question - I don't expect or need an answer. The way it was worded it looks like it was a typo anyway.)
     
    Last edited: Feb 16, 2012
  8. Graueradler

    Graueradler Pattern Altitude

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    If runway 7 is actually already at 74 degrees, 3 degrees change could take it to 77 degrees making it runway 8.
     
  9. jasc15

    jasc15 Pre-takeoff checklist

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    Interesting. So the Magellan GPS I have from ~1999 (in the manual's FAQ, there is a question about Y2K compliance:rofl:) will likely not have reliable magnetic variation built in. I'll have to make sure everything is referencing true north.
     
  10. pilotod

    pilotod Pre-takeoff checklist

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    If it's not influential people/organizations affecting government it's also magnetic influence. Interesting to know and I feel a bit smarter knowing that things in aviation aren't exact even though it's printed/painted.

    I'm curious, are GPS IFR approaches more accurate than VOR's?
     
  11. pericynthion

    pericynthion Pre-takeoff checklist

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    Very informative, thank you.
     
  12. John Collins

    John Collins Pattern Altitude

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    Yes. Yes. A single Yes is too short to post.
     
  13. pilotod

    pilotod Pre-takeoff checklist

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    I found an explanation why the magnetic variation might be different in a services flight plan calculations. Skyvector said in an forum post that I read that they use the NFD (National Flight Database) and the installation values that are the magnetic deviations....even if the database hasn't been updated for each installation. Apparently it might have something to do with the latest survey of that airport.

    Here's a link to that discussion.

    http://skyvector.com/content/magnetic-course-numbers-few-degrees

    It at least answers my original question. I was wrong to state that the A/FD gave the magnetic variation too. I looked at skyvectors information again and noticed that it wasn't the A/FD where I saw the MV.