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Asked by: 13289 views Airspace, FAA Regulations

The OROCA provides 1000 feet obstacle clearance in non-mountainous areas and 2000 feet clearance in mountainous areas. However, the 2012 FAA Instrument Procedures Handbook states (excerpt below) in a another sentence that OROCAs do not provide acceptable terrain clearance. My question: Why state that OROCAs provide 1000/2000 terrain clearance in one sentence yet in the next sentence terrain clearance is not acceptable? How do you plan safely? "OROCAs depicted on en route charts do not provide the pilot with an acceptable altitude for terrain and obstruction clearance for the purposes of off-route, random RNAV direct flights in either controlled or uncontrolled airspace."

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6 Answers

  1. John D. Collins on Jul 04, 2013


    The remaining portion of the paragraph explains why the OROCA is not intended for use to determine an off route, random RNAV route. In addition, flight at the OROCA doesn’t guarantee communications, radar reception, or navigation reception. An airway or any other route has a detailed survey and then test flown to verify the accuracy of the survey along with the suitability for communications and ground based navigation reception. This provides high confidence that obstacle protection on these routes. Follow up flight tests are conducted periodically on a schedule to assure that nothing has changed. It is impractical to survey and flight test every part of the US that is off route or to keep track of towers or other obstacles in this area. The information is intended for emergency use and for general situational awareness.

    When you plan a random RNAV route, I would suggest you don’t restrict yourself to just the IFR enroute charts. I also look at sectional charts. They also depict a Maximum Elevation Figure (MEF) which may be used for altitude planning, but its dimensions are 30 minutes of latitude and longitude whereas the OROCA altitude covers an area of 1 degree of latitude and longitude, so there are 4 times the detail. If you use the MEF, remember that you have to add 1000 feet or 2000 feet (mountainous) to get to an equivalent to the OROCA, as the MEF only has a buffer of at most a few hundred feet and it depicts the MSL altitude of the obstacle or terrain.

    You should also consider communication, radar, and navigation reception. I also consider the terrain, availability of airports along the route, roads and other possible landing sites. Just because I can go direct, I don’t always choose a direct route if a dog leg will provide me more alternatives in the event of an emergency. If you are crossing mountains such as we find here in the east, you will be assigned an altitude high enough to maintain radar surveillance.

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  2. Dan Chitty on Jul 04, 2013

    Thank you John for the excellent additional information.

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  3. Mark Kolber on Jul 05, 2013

    …it’s interesting, though, that the only FAA source to mention the OROCA obstruction limitation is the IPH. It does not appear in the AIM, for example (out of curiosity I searched my Summit database)

    Also, with reference to John’s comment about eastern mountains, that’s also true in the west. And if you look at an en route chart for, say, Colorado, you’ll see that, generally, the best routes over the Rockies for those who choose and have the capability to go IFR (another issue) are airways.

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  4. Ale on Jul 06, 2013

    1. The OROCA is not acceptable as a safe altitude for NORMAL IFR navigation,

    2. ” …OROCAs ARE intended primarily as a pilot tool FOR EMERGENCIES and situational awareness…” that is because “…OROCAs are not subject to the same scrutiny as other minimum IFR altitudes..” such as the MEA, MOCA, and MVA.

    In other words, the FAA don’t guarantee the 1000 & 2000 terrain clearance.

    In normal operation, OROCA best used for altitude situational awareness. Personally, I make use of OROCA when the ATC gives me a DIRECT route to a fix; just before I turn, I glance at the en-route chart and make sure that my present altitude is at least 1000 ft above OROCA; if not, I try to ask for higher to increase my safety margin.

    Reference: FAA-H-8261-1A (Instrument Procedures Handbook)

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  5. Dan Chitty on Jul 06, 2013

    Thank you all for the feedback. Very helpful.

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  6. Lion on Jul 08, 2013

    The following is stated in the air traffic control procedures; what does it means from a pilot viewpoint?

    When the prescribed minimum altitude for IFR operations is at or above 18,000 feet MSL and the atmospheric pressure is less than 29.92”, add the appropriate adjustment factor from TBL 4−5−3 to the flight level equivalent of the minimum altitude in feet to determine the adjusted minimum flight level.

    TBL 4−5−3
    Minimum FL Adjustment
    Altimeter Setting Adjustment Factor
    29.92” or higher None
    29.91” to 29.42” 500 feet
    29.41” to 28.92” 1,000 feet
    28.91” to 28.42” 1,500 feet
    28.41” to 27.92” 2,000 feet

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