91.181 - Course to be flown (in a non GPS aircraft)

mtpease87

Well-Known Member
A captain and I were having a debate earlier about basic IFR regulations, particularly 91.181 - course to be flown. We were debating whether or not you can legally fly between two VOR's, which are not part of a published airway if the VOR service volumes overlap, but do not encompass both VOR's.

I made the point that each VOR on a low altitude chart has a compass rose and you could easily measure a radial outbound from one VOR and inbound on the next - just like an airway. The captain was adamant that this was not legal, and was just an educated guess. He stated that this would not provide "positive course guidance" or something similar. He claimed that you cannot be sure you are actually on course unless it is a radial published on a chart. In my opinion, you would be legal and safe to navigate outbound on one VOR before intercepting a radial inbound to another.

Has anyone else thought about this? Keep in mind, this would be for a non GPS aircraft.

Here is 91.181:

Unless otherwise authorized by ATC, no person may operate an aircraft within controlled airspace under IFR except as follows:
(a) On an ATS route, along the centerline of that airway.
(b) On any other route, along the direct course between the navigational aids or fixes defining that route. However, this section does not prohibit maneuvering the aircraft to pass well clear of other air traffic or the maneuvering of the aircraft in VFR conditions to clear the intended flight path both before and during climb or descent.
 

Rocketman99

Frozen Guppy Manipulator
You can legally fly any radial from a VOR. It does not have to be published on a chart. Hence how you would sometimes get the clearance or instruction to "fly the XYY R115 to intercept V123 (or some other random point on an airway)." The argument that the only course you can fly from a VOR is one on a chart is nothing short of infantile. I bet said person also thinks the max DEMONSTRATED crosswind in a plane is also limiting?
 

MikeD

Administrator
Staff member
I made the point that each VOR on a low altitude chart has a compass rose and you could easily measure a radial outbound from one VOR and inbound on the next - just like an airway. The .
That's actually not correct. VOR/VORTACs only have a compass rose around them IF they already have an airway running to/from them.

As an example, IWA VORTAC at Phoenix-Mesa Gateway airport has no compass rose around it, neither on the Sectional nor on the TAC chart or the Low Altitude chart. Reason being is it's ONLY used for that airport's instrument approaches.....as well as a cross radial for backup identification of some intersections of other airways running to other NAVAIDS. It's not used for any enroute navigation to another VOR and has no airways running to/from it, therefore has no compass rose around it.
 

pwttogfk

Well-Known Member
Paragraph (b) of 91.181 lets you do exactly what you were describing. On one memorable flight in my old Cessna 150 going up to the MEA on a segment of V430 would've put me in icing, but by using terminal VORs and even a couple NDBs I was able to make it back using the much lower OROCA. I filed something like DLH V430 GPZ IDJ FSE CKN GFK and the only comment I got from the controller was that he hadn't seen someone file an NDB route in years.

Sent from my PC36100 using Tapatalk 2
 

MikeD

Administrator
Staff member
The only problem I'd see is in mountainous terrain, running into a situation where you may not know the MRA for the particular course you're chosen. With the OROCA, you can stay clear of terrain, but you don't necessarily have a method of guaranteeing Navaid reception; especially when you get those VORTACs that have notations like "not usable R175 SW through R280", or something like that. It can likely be estimated, but it would be just that, an estimation.

Insofar as whether a radial is useable for positive course giudance or not, aside from the restriction I noted above, you can navigate on any radial of a VOR/VORTAC.
 

ppragman

Direct BATTY
The only problem I'd see is in mountainous terrain, running into a situation where you may not know the MRA for the particular course you're chosen. With the OROCA, you can stay clear of terrain, but you don't necessarily have a method of guaranteeing Navaid reception; especially when you get those VORTACs that have notations like "not usable R175 SW through R280", or something like that. It can likely be estimated, but it would be just that, an estimation.

Insofar as whether a radial is useable for positive course giudance or not, aside from the restriction I noted above, you can navigate on any radial of a VOR/VORTAC.
You should be aware of these things before flying around in mountainous terrain. ;)
 

crazyjaydawg

Well-Known Member
In actuality, most RNAV equipped airplanes are only legal if they're receiving VOR signals to find their position. GPSs by themselves are not legal in a lot of 121 operations so anytime you get a direct to clearance from atc, your fancy FMS is just using all those VOR radials to triangulate a position. If that were to be legal according to your captain, then there would need to be an infinite number of airways so that you could fly across all of them when flying direct to a point.
 

MikeD

Administrator
Staff member
In actuality, most RNAV equipped airplanes are only legal if they're receiving VOR signals to find their position. GPSs by themselves are not legal in a lot of 121 operations so anytime you get a direct to clearance from atc, your fancy FMS is just using all those VOR radials to triangulate a position. If that were to be legal according to your captain, then there would need to be an infinite number of airways so that you could fly across all of them when flying direct to a point.
Which is what original RNAV was....a system that allowed you to create waypoints by using radial/distance from VORs.
 

Capt. Chaos

Well-Known Member
On ANC center one night:

ANC: Bonanza 123 radar contact lost.. are you picking up LVD yet?

Bonanza: yes

ANC: you are cleared direct LVD maintain 7000 and report the radial you are tracking when established.
 

Autothrust Blue

"Duuuuuude."
You can legally fly any radial from a VOR. It does not have to be published on a chart. Hence how you would sometimes get the clearance or instruction to "fly the XYY R115 to intercept V123 (or some other random point on an airway)." The argument that the only course you can fly from a VOR is one on a chart is nothing short of infantile. I bet said person also thinks the max DEMONSTRATED crosswind in a plane is also limiting?
Yes. Or even,

SMF SAC SAC*** ECA*** CEDES SFO

Incidentally, I think the AIM still states that random RNAV routes in flight plans must still be specified as radial-distance fixes from VORs within each enroute facility's airspace.
 

KHanson

Well-Known Member
Paragraph (b) of 91.181 lets you do exactly what you were describing. On one memorable flight in my old Cessna 150 going up to the MEA on a segment of V430 would've put me in icing, but by using terminal VORs and even a couple NDBs I was able to make it back using the much lower OROCA. I filed something like DLH V430 GPZ IDJ FSE CKN GFK and the only comment I got from the controller was that he hadn't seen someone file an NDB route in years.

Sent from my PC36100 using Tapatalk 2
That same controller, if put in southern Florida, would not be surprised that you filed a bearing airway...
 
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