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I have a couple of questions regarding the differences between Offroute Obstacle Clearance Altitudes on low altitude charts vs Maximum Elevation Figures on sectionals. I was looking at a flight from KLEX to K22 in Kentucky. The OROCA in the area is 6,100 feet on the low altitude chart. The MEF on the sectional is 2,700 feet. First question is why such a variation between the two? I know the OROCA adds 1,000 in non-mountainous and 2000 feet in mountainous but that doesn't account for the difference. I also understand that OROCA takes into account an additional 4nm surrounding the OROCA so I'm wondering if this is what causes the value to be at 6,100?

I'm also curious about filing altitude for an IFR GPS direct flight. I've made this flight before and filed and received an altitude below the OROCA. Was this possible because the Minimum Vectoring Altitude was below the OROCA? Sometimes the winds aloft favor a lower altitude in this area particularly on the return trip headed west.



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1 Answers

  1. Bob Watson on Dec 19, 2012

    Thanks for the tough question!

    (I had to look this one up. What I found was…)

    The OROCA is based on the area within the bounding rectangle AND 4 nm outside of it. Just to the south east of the area you are talking about are some 4100′ mountains so I’m guessing that’s the elevation contributes to the 6100′ OROCA you’re talking about on that route.

    +7 Votes Thumb up 7 Votes Thumb down 0 Votes

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