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Per Part 95 of Title 14 of the CFRs the Minimum Reception Altitide ("MRA") is defined as "the lowest altitude at which the intersection can be determined using the ground-based navigation aids." Sounds pretty straight forward, but sometimes doesn't make sense on the charts.

For example, notice the intersection BISOP on V257 between GCN and DRK VORs. Both legs of the airway to the north and the south of BISOP have an MEA of 10000. The MEA is defined in the Glossary of the Instrument Procedures Handbook as "the lowest published altitude between radio fixes that assures acceptable navigational signal coverage ...." BISOP is an intersection between DRK r-349 & GCN r-196.

If 10,000 feet guarantees naviational signal at BISOP what's the point of the 11,000 foot MRA? Clearly it must be to receive Peach Springs VOR which is 55nm away, but what is the point? Any aircraft at 10,000ft can receive DRK and GCN as guaranteed by the MEA, and BISOP can be easily identified by the intersection between DRK r-349 & GCN r-196 with nothing more than a VOR receiver. Why would they need a 3rd VOR that requires an altitude higher than MEA in this case?

It makes sense to have an MRA at higher altitude than MEA when its a long straight Victor airway to help aircraft that don't have DME to identify the fix with a cross radial, but BISOP can be identified both by DME and as the intersection of the two radials mentioned above.

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    $\begingroup$ Bisop is 53 NM DME from DRK on the 349R and the 196R from GCN. If you don't have a DME receiver you need to use the 196R from GCN and the 077R from PGS. Could be the way it was flight checked (requiring DRK DME) and not the 349R. The angle between DRK R349 and GCN 196R is only 27 degrees. Not much of an intersection angle. I don't know what the official intersection design criteria requires for a minimum intersection angle. $\endgroup$
    – user22445
    Commented Feb 12, 2022 at 3:35
  • $\begingroup$ @757toga I fleshed your comment out into an answer, but you hit the key point first. $\endgroup$
    – Ralph J ♦
    Commented Feb 12, 2022 at 5:13
  • $\begingroup$ @RalphJ: Also if you are flying north on the DRK 349R you can identify BISOP at 53 DME and remain at the MEA of 10,000, no need to go to 11,000. $\endgroup$
    – user22445
    Commented Feb 12, 2022 at 15:49
  • $\begingroup$ @757toga Agree. DME makes things much easier than doing everything VOR-VOR only. Just for fun, I wonder how many aircraft fly IFR these days with neither DME nor an IFR-certified GPS... kinda doubt it's very many, but I could be wrong. $\endgroup$
    – Ralph J ♦
    Commented Feb 12, 2022 at 16:34

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You can't identify the intersection precisely using the DRK and GCN radials because they're too nearly reciprocals of one another. With thin lines on paper, that works well, but with the actual accuracy of VOR receivers, you wouldn't have a good sense at all of when you reached BISOP - you'd just know that you're somewhere roughly about there. On the other hand, the PGS radial is at almost a right angle to the DRK and GCN radials, so you'd have a clear "centered" indication on your VOR receiver set up for PGS 077. And that is the time you reached BISOP, and start your "turn, time, twist, throttle, talk, track" for the new leg. Much more precise that way.

Thus, you have to be able to receive PGS along with whichever of DRK or GCN you're tracking to or from. And while both of the latter can be received at BISOP at 10,000, you have to be at 11,000' there to receive PGS. Thus the higher MRA.

Hat tip to @757toga who identified the key point in his comment.

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  • $\begingroup$ thanks for the answer. I understand that a cross radial might be marginally more accurate, but I don't buy it. Victor airways are 8 miles wide, plus many victor airways are composed of radial/reciprocals that are 2 or 3 degrees off and that doesn't seem to matter, so 27 degrees is better than 2 or 3 degrees that some radial/reciprocal airways use. So, I don't think that is why. Is it more precise using PGS? Yes. Do we need that level of precision for an 8 NM wide victor airway? I am not aware of such requirement. I guess I'll have to dive into the TERPS. :) $\endgroup$
    – Devil07
    Commented Mar 1, 2022 at 19:28
  • $\begingroup$ @Devil07 In that 2-3 degree case, you aren't identifying a midpoint fix that you turn on, you're just swapping from the FROM of one VOR to the TO on the other as guidance. At BISOP, you have to execute a turn, so it's important to be able to identify it precisely. Centering up a 90 degree cross-radial will do that more precisely (especially with VOR's only good to +/- 2 degrees) than centering up the DRK and GCN radials. $\endgroup$
    – Ralph J ♦
    Commented Mar 1, 2022 at 23:35
  • $\begingroup$ I do apprediate your point, but I don't see why there needs to be any certain level of precision for making a 27 degree turn in the middle of no where. I've flown that victor airway many times and when the CDI for GCN R196 centers you turn right and when you finish turning you're within 1 degree of centerline. When I'm heading back to KPRC, I turn left on DRK radial 349 and given its distance i'm it remains centered. I'm just not aware of any safety reason for having some arbitrary super level of precision in this situation. Its got to be some TERPS technical requirement. $\endgroup$
    – Devil07
    Commented Mar 4, 2022 at 3:36

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