Proper vertical separation at FL180

Discussion in 'Change to my Frequency...' started by labbadabba, Oct 26, 2016.

  1. labbadabba

    labbadabba Cleared for Takeoff

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    So with pilots at FL180 flying at 29.92 how do you account for pilots flying at 17,500ft with a baro of 30.42?
     
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  2. moonshine

    moonshine Line Up and Wait

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    Lower FLs are unavailable with certain alt settings below
     
  3. MarkZ

    MarkZ En-Route

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    In your scenario, the VFR aircraft flying at 17,500 with a local altimeter of 30.42 would be flying at a pressure altitude of 17,000.

    Conversely, had the same aircraft flown with a local altimeter of 29.42, the pressure altitude is then 18,000.

    Which is why FL180 is not usable when local altimeter settings dip below 29.92.


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  4. Cpt_Kirk

    Cpt_Kirk En-Route

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    Unusable altitudes are listed with their corresponding pressures in the FAR/AIM.

    I have made the mistake of hastily filing FL180 when the altimeter was below 29.92". ATC asked if I would like 16,000 or FL200 and I picked one.
     
  5. MarkZ

    MarkZ En-Route

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    It's not a mistake in my view. How can you remain certain the local altimeters won't drop below 29.92?


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  6. MAKG1

    MAKG1 Touchdown! Greaser!

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    Why do you have to?

    "Bugsmasher 123XY climb and maintain flight level 200."
     
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  7. Velocity173

    Velocity173 Final Approach

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    This isn't an issue at altimeter settings of 29.92 or greater. As Mark said, in your example, you've got an actual 1,000 ft difference in separation between the two. The higher baro actually helps in this case.

    With low settings of less than 29.92, the controller is compensating by issuing a higher usable FL to IFRs. In that case, the concern isn't about maintaining sep from a VFR at 17,500. It's about maintaining the 1,000 ft sep from IFRs at 17,000 ft.
     
    Last edited: Oct 26, 2016
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  8. luvflyin

    luvflyin Pattern Altitude

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    Yup. The other way to "picture" it is the plane at FL180 is at 18,500 when the altimeter is 30.42.
     
  9. labbadabba

    labbadabba Cleared for Takeoff

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    CRAP!!! Got it backwards. Thx.