Finding Minimum Altitude when Off-Airway

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OverTQ

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OverTQ
Let's say I want to go from KABC to KXYZ direct (off-airway), and I'm trying to figure out the minimum IFR altitude.

91.177 says the minimum IFR en-route would be

(i) 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; or

(ii) In any other case, an altitude of 1,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown.

One could fly above the OROCA, but that's not necessarily the lowest altitude if you don't pass within 4NM of the highest obstacle in the rectangle.

One could pull out paper sectionals, draw an 8nm wide path, and manually search for obstacles along that entire route, but that's rather tedious.

Is there an easier way? Any online tools that can help with this?

I know that several flight planners will show a terrain profile, but I'm not sure exactly what the cross-section represents (does it show whats directly under the path? the highest within 1nm of the path? highest within 4nm?). Are the obstacles included?
 
Get above the highest minimum sector altitude for your route of flight, or get SVT or at least a MFD with terrain, a 750 alone would be good. All the new off airway stuff is designed with NextGen level equipment in mind.
 
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Is there an easier way? Yeah, pick something high enough where you aren't bothered with those questions.
 
Henning said:
Get above the highest minimum sector altitude for your route of flight
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The MSA from the approach plates? I believe that's 1,000 feet above, regardless of mountainous terrain designation, and doesn't do much good past ~25NM from the airport.

Even if I figure out this altitude, is there a good chance I won't be allowed to fly it due to being lower than the radar coverage?

Basically, I'm looking for a convenient way of figuring out the lowest altitude you'll likely be allowed to fly while IFR.

Cpt_Kirk said:
Is there an easier way? Yeah, pick something high enough where you aren't bothered with those questions.
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I appreciate you taking the time to respond.

Nistlerooy said:
In the ATC world there is something called an MVA, or minimum vectoring altitude. The FAA publishes these charts but I have no idea how you'd use them to plan. I still think OROCA is safest bet. But to be honest, no ATC will let you go direct the whole way. Plus they'll probably give you altitudes along the way. I know that doesn't help much.

https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/mva_mia/mva/
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Whoa! I thought I read in FAA materials that these were not publicly available. Interesting! Thanks.
 
OverTQ said:
The MSA from the approach plates?
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No, he was talking about the min. vectoring altitude for the sector.

If you are trying to find an altitude lower than the OROCA, probably the only way you are gonna figure that out is by surveying the route with a sectional like what you said in your original post, or by filing an altitude and having ATC give you their minimum vectoring altitude. Even by checking the area with a sectional, you are bound to miss a tower or two.

But in all honesty, if you are trying to find an altitude lower than the OROCA to fly, then flying IFR probably isn't the smartest option. The only reason you would need to be that low is if you were going a very short distance. Would you really want to be min running it in IMC? Altitude is your friend.
 
Thanks for the reply.

Gucci Pilot said:
Even by checking the area with a sectional, you are bound to miss a tower or two.
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Yea, that's a concern as well.

Gucci Pilot said:
But in all honesty, if you are trying to find an altitude lower than the OROCA to fly, then flying IFR probably isn't the smartest option.
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I think that's often true, but sometimes terrain can take the OROCAs far higher than is necessary to be safe. I've seen MOCAs that are 7,000 feet below the OROCA.
 
I use ForeFlight and the profile view on the map view. There is a setting that permits you to select the corridor width of the course. I set it to 8 NM, which is +/- 4 NM from the center line. Then I slide the altitude up until I am satisfied with the terrain and obstacle clearance.
 
OverTQ said:
The MSA from the approach plates? I believe that's 1,000 feet above, regardless of mountainous terrain designation, and doesn't do much good past ~25NM from the airport.

Even if I figure out this altitude, is there a good chance I won't be allowed to fly it due to being lower than the radar coverage?

Basically, I'm looking for a convenient way of figuring out the lowest altitude you'll likely be allowed to fly while IFR.



I appreciate you taking the time to respond.



Whoa! I thought I read in FAA materials that these were not publicly available. Interesting! Thanks.
Click to expand...

No, I was thinking a sectional. IFR charts don't give all the information required for direct flights, that's why you need advanced technology to take advantage of it.
 
OverTQ said:
………………….
One could pull out paper sectionals, draw an 8nm wide path, and manually search for obstacles along that entire route, but that's rather tedious.

Is there an easier way?
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Not that I'm aware of.
Keep in mind that ATC will have a Minimum Instrument Altitude that's the lowest they can assign for that area which will usually be higher than the altitude prescribed by that method. 91.177 is a good place to start for planning purposes but you'll probably need a cruise clearance to actually use it, so yeah …draw your course line on a sectional, survey the chart for the highest obstacle within 4 nm of the line, round up to the nearest thousand, add a thousand (or two, depending) and file that. But don't be too surprised if ATC assigns something higher.
 
OverTQ said:
Let's say I want to go from KABC to KXYZ direct (off-airway), and I'm trying to figure out the minimum IFR altitude.

91.177 says the minimum IFR en-route would be

(i) 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; or

(ii) In any other case, an altitude of 1,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown.

One could fly above the OROCA, but that's not necessarily the lowest altitude if you don't pass within 4NM of the highest obstacle in the rectangle.

One could pull out paper sectionals, draw an 8nm wide path, and manually search for obstacles along that entire route, but that's rather tedious.

Is there an easier way? Any online tools that can help with this?

I know that several flight planners will show a terrain profile, but I'm not sure exactly what the cross-section represents (does it show whats directly under the path? the highest within 1nm of the path? highest within 4nm?). Are the obstacles included?
Click to expand...

Here is an online flight planner where you can specify the width of the "corridor" that it uses for the vertical profile. I use that for general preflight planning, as well as the MOCAs and OROCAs on the charts. Then ATC often override my filed altitude with their own, but given my preparation I have perspective to decide whether I want it or not. Of course if the routing is different, the original vertical profile may become useless, which is where onboard tools may come into play, like the GTN-750 in my case which shows enroute "safe altitude" for a given route. In a pinch, you always have the OROCAs as a fallback.
 
Nistlerooy said:
In the ATC world there is something called an MVA, or minimum vectoring altitude. The FAA publishes these charts but I have no idea how you'd use them to plan. I still think OROCA is safest bet. But to be honest, no ATC will let you go direct the whole way. Plus they'll probably give you altitudes along the way. I know that doesn't help much.

https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/mva_mia/mva/
Click to expand...

Interesting link for MVAs. But ATC clears me direct all the time, the longest so far being HTW-SAV. The controllers at HTS (4nm from HTW) had no idea what or where SAV is.
 
Just another reason for carrying full sectional chart coverage even on IFR flights so you can determine the highest obstruction within 4nm of your course. But if you don't have anything else, you can always use the big brown number OROCA off your L-chart for an altitude which guarantees 1000 feet (2000 mountainous) obstruction clearance anywhere in that lat/long box. And remember that since ATC won't let you go off-airways unless you're above their MVA (even if your sectional-based minimum obstruction clearance altitude is lower), they'll back you up on this.
 
Ron Levy said:
Just another reason for carrying full sectional chart coverage even on IFR flights so you can determine the highest obstruction within 4nm of your course. But if you don't have anything else, you can always use the big brown number OROCA off your L-chart for an altitude which guarantees 1000 feet (2000 mountainous) obstruction clearance anywhere in that lat/long box. And remember that since ATC won't let you go off-airways unless you're above their MVA (even if your sectional-based minimum obstruction clearance altitude is lower), they'll back you up on this.
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Sectionals are visual navigation charts. They have neither the scale nor the fidelity to satisfy the exacting requirements of IFR off-route operations. The cartographers that design and maintain Sectional charts (and TACs for that matter) are permitted to not chart all terrain high spots for charting clarity. The are also permitted to move such terrain points for clarity.

I learned that several years ago when TRACONS were still designing MVA charts on Sectionals. We found a whole lot of errors using a 1:24,000 GIS programs. MVAs have long since been designed by a computer tool that does the job correctly.
 
OverTQ said:
Is there an easier way? Any online tools that can help with this?
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Yes, there are easier ways, but none I'm aware of that are readily available to the general public. TARGETS worked well for this, SDAT was capable as well but I found that to be more cumbersome.

You don't say why you want to figure out the MIA. Is it because you want to fly at that altitude all the way from KABC to KXYZ?

In practice, unless it's a pretty short flight, the lowest altitude ATC will assign for enroute purposes is the highest MIA/MVA along the route, rounded up to the next cardinal altitude, with possibly 1000 feet added so as not to shut down other airports along the way, and perhaps another 1000 feet for direction of flight.
 
aterpster said:
Sectionals are visual navigation charts. They have neither the scale nor the fidelity to satisfy the exacting requirements of IFR off-route operations. The cartographers that design and maintain Sectional charts (and TACs for that matter) are permitted to not chart all terrain high spots for charting clarity. The are also permitted to move such terrain points for clarity.

I learned that several years ago when TRACONS were still designing MVA charts on Sectionals. We found a whole lot of errors using a 1:24,000 GIS programs. MVAs have long since been designed by a computer tool that does the job correctly.
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It might be permitted, but in my experience it isn't done much. I have always been impressed with the rendering detail when identifying individual lakes, mountains, and ridges on sectional. In the days before GPS and metadata, I used to have to correlate photos and video I shot with sectional charts. The only problem with sectionals for IFR is there is too much information on them to make them effective to incorporate into an instrument scan.
 
Henning said:
It might be permitted, but in my experience it isn't done much. I have always been impressed with the rendering detail when identifying individual lakes, mountains, and ridges on sectional. In the days before GPS and metadata, I used to have to correlate photos and video I shot with sectional charts. The only problem with sectionals for IFR is there is too much information on them to make them effective to incorporate into an instrument scan.
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IME it's done quite a bit. In my last seven years in ATC I served as an Airspace and Procedures Specialist as well as a controller. Using some of the extremely accurate software tools available today the inaccuracies on sectional charts jump out.
 
Hank S said:
Interesting link for MVAs. But ATC clears me direct all the time, the longest so far being HTW-SAV. The controllers at HTS (4nm from HTW) had no idea what or where SAV is.
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Yes, it was very interesting. I notice he's deleted his post/wonder why?
Anyway, I studied with much interest Salt Lake Centers MIA chart altho' I doubt that it'd be very useful for flight planning without some reference points. ATC must overlay these charts on the radar screen?
 
peerlesscowboy said:
Yes, it was very interesting. I notice he's deleted his post/wonder why?
Anyway, I studied with much interest Salt Lake Centers MIA chart altho' I doubt that it'd be very useful for flight planning without some reference points. ATC must overlay these charts on the radar screen?
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I can speak to MVAs, but not MIAs.

MVAs for use by the FAA are made into geo-referenced files by a specialized branch in Silver Springs, MD, that are then used to make the video map symbol generator for the controller's radar display.
 
Henning said:
It might be permitted, but in my experience it isn't done much. I have always been impressed with the rendering detail when identifying individual lakes, mountains, and ridges on sectional. In the days before GPS and metadata, I used to have to correlate photos and video I shot with sectional charts. The only problem with sectionals for IFR is there is too much information on them to make them effective to incorporate into an instrument scan.
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It's done often. The contour levels are also more artistic than accurate. The aren't readable in any case. Lakes and rivers generally are not moved because they are considered visual navigation features.
 
peerlesscowboy said:
Yes, it was very interesting. I notice he's deleted his post/wonder why?
Anyway, I studied with much interest Salt Lake Centers MIA chart altho' I doubt that it'd be very useful for flight planning without some reference points. ATC must overlay these charts on the radar screen?
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Possibly, if the facility so chooses. When I was at ZAU the MIAs were hand drawn on single-sided sectional charts and displayed on back-lit frames above the radar consoles. Topography in that area of the country is such that MIA areas tend to be pretty large and don't differ a lot in altitude between them so there wasn't any real need to have them on the scope.
 
roncachamp said:
IME it's done quite a bit. In my last seven years in ATC I served as an Airspace and Procedures Specialist as well as a controller. Using some of the extremely accurate software tools available today the inaccuracies on sectional charts jump out.
Click to expand...

Differences so great it causes a hit or miss question when trying to determine a safe altitude to overfly?
 
aterpster said:
I can speak to MVAs, but not MIAs.

MVAs for use by the FAA are made into geo-referenced files by a specialized branch in Silver Springs, MD, that are then used to make the video map symbol generator for the controller's radar display.
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They have to be approved by the folks in Silver Springs but not necessarily made by them. IIRC the only quibble they'd ever had with any of my MVA charts was an allowance for a tower that had been removed. Permanent physical removal of a tower does not mean it will be removed from the obstacle database. Who knew?
 
Henning said:
Differences so great it causes a hit or miss question when trying to determine a safe altitude to overfly?
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Difference so great a pilot cannot be sure he's at least four miles from the highest obstacle within 1000/2000 feet when he's trying to determine the subject of this discussion.
 
Well, what's a prudent pilot able to do? Use a sectional, I'd wager.

dtuuri
 
My general strategy is to ask the controller what the minimum he can give me. Unless you're on a published route, he's not going to give you less than his MVA in practice anyhow.
 
dtuuri said:
The OP asked the question in the context of flight planning.
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Got it. It's unlikely you'd know ahead of time what the MVA is on an off-airways routing, so plan accordingly but be prepared for ATC to send you higher.
 
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