Block Altitude vs. OROCA

A

AA5Bman

Line Up and Wait
Joined
Jun 14, 2009
Messages
792
Display Name
Display name:
He who ironically no longer flies an AA5B
Recently, I filed IFR to depart an unfamiliar mountain airport due to smoke. It was marginal VMC at worst, I’d say, but since the smoke was bad and I didn’t know how much worse it was going to get along my route, and because it is often worse at altitude (or really, since you’re farther away from the ground the reduction in visibility means it appears worse), I went ahead and filed.

The flight was basically up and over the mountains, direct (off airway). It was a very windy day with lots of up and down drafts, so I requested and got a “maintain block 14,000-16,000”. I set the oxygen and the autopilot, went up and down in the drafts, and made my way home battling a big headwind.

I didn’t think about it at the time, but in retrospect, some of the sectors I flew through had an OROCA of 15,000. But since I was cleared for my 14-16k’ block, it really didn’t occur to me to stay above 15,000’ and I let the plane wander within the block so I didn’t have to constantly fight it. But should I have? I should note here that this portion of the flight was all VMC and I had ample terrain clearance.

Interestingly, for a solid 45 minutes I lost radar and radio contact. Perhaps they would have revised my block higher due to the OROCA in that sector? There were two times they relayed info to me via passing firefighting aircraft, so I assume if they wanted me higher they certainly had the ability to do so.

So - thoughts on a block altitude clearance when it conflicts with a published OROCA?
 
Last edited:
Am surprised. Around my area in VFR we always get asked "If we're familiar with terrain and can maintain clearance" even on FF ...
 
ATC is assigning your black based on their MIA/MVA. Could very well be lower than the OROCA.
 
@danhagan Hah I would feel patronized if VMC. "Center, Cessna 12345, affirmative, I'll do my very best not to run into anything!"

@Velocity173 Copy, but which takes precedence? I think the block aspect of this makes it a little ambiguous. If they assigned you "climb and maintain 12,000'" and the OROCA for that sector is 15,000', would you have some obligation to request higher?
 
AA5Bman said:
@danhagan Hah I would feel patronized if VMC. "Center, Cessna 12345, affirmative, I'll do my very best not to run into anything!"

@Velocity173 Copy, but which takes precedence? I think the block aspect of this makes it a little ambiguous. If they assigned you "climb and maintain 12,000'" and the OROCA for that sector is 15,000', would you have some obligation to request higher?
Click to expand...

They’re not going to give you a block that doesn’t comply with their MVA/MIA on your route. Just like they’re not going to approve a block with other IFR aircraft mixed in between on a hard altitude.

(video)
https://pilotworkshop.com/ifr-focus/issue9/
 
Last edited:
@Velocity173 I read the article (and watched the video) that you linked to, and I think what you're saying is that the OROCA is essentially an advisory number for the pilot, and it's up to the pilot to maintain obstacle clearance. This has real-world significance when, for instance, climbing out of an airport with no ODP (where you're naturally below the OROCA yet also potentially IFR). Further, if ATC has assigned an altitude, they should be doing so with obstruction clearance in mind but they have much more detailed charts (MVA or MIA charts) that help them know what obstruction clearance is along your actual route of flight and accordingly, might assign an altitude that is lower than the published OROCA for your location. Finally, in that situation, you wouldn't be required to climb or request a climb to an altitude higher than the OROCA, even if you are, in fact, off airway.

Let me know if I have that straight.
 
AA5Bman said:
So - thoughts on a block altitude clearance when it conflicts with a published OROCA?
Click to expand...

OROCA is not a regulatory minimum altitude defined in 91.177. They are charted as a guide only, primarily for emergency use, or when you don't want to or need to do more in-depth evaluation. Remember, they cover a big area, most of which you're not flying through.

Instead, the minimum altitude for non-airway routes in mountainous areas is 2000 ft above the highest obstacle within 4 nm either side of the route being flown.

How can you determine the minimum altitude for your off-airway route? Not using an IFR enroute chart. Rather, by using a VFR chart. Obviously this would be ideally a preflight planning exercise. Or, you can ask for their minimum altitude.
 
AA5Bman said:
@Velocity173 I read the article (and watched the video) that you linked to, and I think what you're saying is that the OROCA is essentially an advisory number for the pilot, and it's up to the pilot to maintain obstacle clearance. This has real-world significance when, for instance, climbing out of an airport with no ODP (where you're naturally below the OROCA yet also potentially IFR). Further, if ATC has assigned an altitude, they should be doing so with obstruction clearance in mind but they have much more detailed charts (MVA or MIA charts) that help them know what obstruction clearance is along your actual route of flight and accordingly, might assign an altitude that is lower than the published OROCA for your location. Finally, in that situation, you wouldn't be required to climb or request a climb to an altitude higher than the OROCA, even if you are, in fact, off airway.

Let me know if I have that straight.
Click to expand...

Ah, somewhat true but think of an OROCA as a rough number for safety and fine tuned like an MIA/MVA or even an MEA on an airway. That’s why it’s really only used in an emergency. Pretty much what @RussR stated above.

Here’s an example in airspace I worked approach (NBC). Theoretically you could be cruising down V437 SW bound at 4,000 and request a lower block. “Cessna 345, maintain block 4,000 through 2,000.” Obviously 2,000 is below the OROCA of 2,400 but still complies with the MEA on V437. You could take that even further if you were off route (V437) because NBC’s MVA is as low as 1,500 in most areas. Three different altitudes and different criteria on how those altitudes are calculated.

E1063D88-86B3-4CEE-AB1B-25FFB3F88F9A.jpeg
 
Okay got it. I think the bottom line is that OROCAs are primarily advisory, not “regulatory”. The more I think about it, I can think of a number of places where the MEA on an airway is lower than the OROCA of the sector, so maybe I was just overthinking things!
 
RussR said:
OROCA is not a regulatory minimum altitude defined in 91.177. They are charted as a guide only, primarily for emergency use, or when you don't want to or need to do more in-depth evaluation. Remember, they cover a big area, most of which you're not flying through.

Instead, the minimum altitude for non-airway routes in mountainous areas is 2000 ft above the highest obstacle within 4 nm either side of the route being flown.

How can you determine the minimum altitude for your off-airway route? Not using an IFR enroute chart. Rather, by using a VFR chart. Obviously this would be ideally a preflight planning exercise. Or, you can ask for their minimum altitude.
Click to expand...
Yes indeed. And, sectional charts aren't up to the task if you don't want to add some "pad." Also, 91.177 2 (i) states: "In the case of operations over an area designated as a mountainous area in part 95 of this chapter, an altitude of 2,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown"

That means not less than 4 nautical miles in all directions from your present position.
 
AA5Bman said:
Recently, I filed IFR to depart an unfamiliar mountain airport due to smoke. It was marginal VMC at worst, I’d say, but since the smoke was bad and I didn’t know how much worse it was going to get along my route, and because it is often worse at altitude (or really, since you’re farther away from the ground the reduction in visibility means it appears worse), I went ahead and filed.

The flight was basically up and over the mountains, direct (off airway). It was a very windy day with lots of up and down drafts, so I requested and got a “maintain block 14,000-16,000”. I set the oxygen and the autopilot, went up and down in the drafts, and made my way home battling a big headwind.

I didn’t think about it at the time, but in retrospect, some of the sectors I flew through had an OROCA of 15,000. But since I was cleared for my 14-16k’ block, it really didn’t occur to me to stay above 15,000’ and I let the plane wander within the block so I didn’t have to constantly fight it. But should I have? I should note here that this portion of the flight was all VMC and I had ample terrain clearance.

Interestingly, for a solid 45 minutes I lost radar and radio contact. Perhaps they would have revised my block higher due to the OROCA in that sector? There were two times they relayed info to me via passing firefighting aircraft, so I assume if they wanted me higher they certainly had the ability to do so.

So - thoughts on a block altitude clearance when it conflicts with a published OROCA?
Click to expand...
How do you know you were out of Radar Contact for the 45 minutes? It’s possible while you were out of direct Radio with ATC, I’m assuming Center, that they had Radar for some, maybe most of the time. Anyway, read this about OROCA.
https://www.faa.gov/air_traffic/flight_info/aeronav/acf/media/RDs/17-02-316-Improving-OROCA.pdf
 
It should not be surprising that MEAs are frequently lower than the OROCA for the sectors they pass through. Look at the area covered by an OROCA vs an MEA. If memory serves, there isn't a reference to the OROCA in Part 91 with regards to minimum IFR altitudes either. I was going to bring up the fact that OROCAs are getting renewed attention, but it was covered above already.
 
AA5Bman said:
“radar contact lost”
Click to expand...
Gotcha. Just sayin that you might have been reacquired sometime during the 45 minutes you were out of Radio Comm with them. Ain't no thang really. But I'm still wondering. When you got comm with them again, did they re identify you, or just start talking to you again.
 
AA5Bman said:
Okay got it. I think the bottom line is that OROCAs are primarily advisory, not “regulatory”. The more I think about it, I can think of a number of places where the MEA on an airway is lower than the OROCA of the sector, so maybe I was just overthinking things!
Click to expand...

Refresh your knowledge of 91.177 and 91.181. The MEAs for airways can be lower than OROCA because the route is not random. The altitudes are only valid for a surveyed corridor which you are required to fly in the center.

Terrain clearance is always my responsibility. While ATC should keep me out of trouble, I don’t bet my life on it. Communications fail, humans make errors, I would have not made a block request lower than OROCA and if assigned an altitude lower than the OROCA I would have clarified the clearance with the controller unless it was vectors.
 
You must log in or register to reply here.
Back
Top