Say I want to fly off-airways (both Victor airways and T routes) for my IFR flight, direct to my destination in the US. How do I know what my off-route altitude should be so I can file for that?

I would expect this would be an altitude above the OROCA and suitable for direction of flight, however I encountered this in the Instrument Procedures Handbook:

OROCAs are intended primarily as a pilot tool for emergencies and SA. 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. OROCAs are not subject to the same scrutiny as MEAs, minimum vectoring altitude (MVAs), MOCAs, and other minimum IFR altitudes. Since they do not undergo the same obstruction evaluation, airport airspace analysis procedures, or flight inspection, they cannot provide the same level of confidence as the other minimum IFR altitudes.

This suggests we cannot trust the OROCA. Then I suppose the 14 CFR 91.177 indicates I need to be at least 2000 ft above the highest obstacle within 4nm of my course in mountainous areas, or 1000 in non-mountainous areas. But how do I know what this is without using OROCAs? Do we check sectional charts, or is there something more practical? Checking a sectional seems reasonable on the ground for filing, but not so if diverting in the air and flying direct to some other airport on an off-airway course not planed on the ground.

Thanks for any help!


This article covers the topic quite nicely

To begin with, a direct route will only be approved in a radar environment. In many areas of the country this is not an issue. The pilot, however, must be aware that filing an altitude at or near the OROCA (Off-Route Obstruction Clearance Altitude) may not guarantee radar coverage. In that case, the pilot should expect to be assigned a different altitude or route.

Its also covered in this AOPA article a bit

If you plan a "direct" flight, point-to-point, follow the rules and enter the correct equipment. For example, if "/G" (or "slash Golf," meaning that your airplane is equipped with an IFR-approved GPS and a Mode C transponder) or "/I" (you have loran or VOR/DME and a Mode C transponder) are not entered for these kinds of flights, the air traffic control computer will reject your flight plan because that route can't be flown with less capable equipment. Direct routing requests also are approved only in a radar environment, so forget about filing direct, off-airways when radar is not available, or in areas of poor radar coverage.

So broadly speaking you can file the OROCA but ATC may issue a higher altitude for radar coverage reasons. I have also received direct routings en-route at previously filed altitudes if they meet the requirements. Where I fly its common for ATC to prefer an airway route to get you out of the terminal area but will generally clear you direct once in route.

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  • $\begingroup$ Thanks. If you lose comms and thus can't benefit from radar coverage, what do you do? Do you proceed direct still at your assigned altitude (don't have an MEA and MIA might change), and climb later if OROCA suggests it? $\endgroup$ – Cameron Jul 4 at 18:42
  • $\begingroup$ @Cameron that is a perfect question to ask on this site. Generally comments are not for extended discussion. Id be happy to answer/you will get a more verbose answer, if you post that. $\endgroup$ – Dave Jul 4 at 21:37

Dave gave a very succinct answer to your question. I might add that in reference to your additional question regarding diverting to another airport, using an altitude above the OROCA or MORA for each successive grid you encounter would be your starting point.

If you have two-way communication with ATC, they will direct you to an altitude that provides the best chance at radar contact and NavAid reception. If you are not in two-way communication with ATC, a climb to a higher altitude would be in order as long as you stay well above the OROCA or MORA.

The OROCA and MORA are based on the charted terrain and obstructions. They do not take into account a temporary obstruction or uncharted item. Just remember, as the citation you quoted says, you will have to be aware of any airspaces (including airways, procedure routes, SUAs, etc), radar dead zones, NavAid dead zones, etc. Even terrain and obstacles clearance is not guaranteed due to the route not being flight inspected.

Here are two interesting articles on such:

From Jeppesen

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