VOR Accuracy for Instrument Training and Checkride

[somorris]

Lots of good information in this thread! Thanks everybody!

 

[aterpster]

Correct me if I’m wrong, but depending on the geometry…
GPS uses great circle headings. VOR no. I’ve seen 6+ degree discrepancies, but I’m not sure if that applies to your situation.

All radio navigation is great circles. That's the nature of RF. It's course, not heading.

 

[Kritchlow]

VOR radials are along a great circle route. All a great circle route means is it is the shortest course between two points on the surface of a sphere. The VOR radio signals propagate along a straight line of sight between the VOR station and the VOR receiver (excepting reflections). When plotted on a chart, they appear curved, although for short distances, the curve is not easily discernable.

All radio navigation is great circles. That's the nature of RF. It's course, not heading.

Indeed that was my error.

 

[flyingron]

VOR radials are along a great circle route. All a great circle route means is it is the shortest course between two points on the surface of a sphere. The VOR radio signals propagate along a straight line of sight between the VOR station and the VOR receiver (excepting reflections). When plotted on a chart, they appear curved, although for short distances, the curve is not easily discernable.

Correct. It's a straight line in the bearing direction from the station. You'll find its bearing to (magnetic) north changes continually along it (except in certain degenerate conditions).

 

[dtuuri]

...make sure you clearly use the VORs as your primary course guidance for the VOR approach. If, as an examiner, I saw you fly the approach with the GPS centered and the VOR off course, we’d have an interesting discussion.

Primary course guidance? Why can't he simply "monitor" the VOR while he flies the GPS course he knows is more accurate? Here's the pertinent words in the AIM after I took out the garbage, my emphasis in red font:

1−2−3. Use of Suitable Area Navigation
(RNAV) Systems on Conventional
Procedures and Routes
a. ...
1. ...
2. Use of a suitable RNAV system as an
Alternate Means of Navigation ... For example, if equipped with
a suitable RNAV system, a pilot may fly a procedure
or route based on operational VOR using that RNAV
system without monitoring the VOR.

b. ...
c. Uses of Suitable RNAV Systems. Subject to
the operating requirements, operators may use a
suitable RNAV system in the following ways.
1. ...
2. ...
3. ...
4. ...
NOTE−
1. ...
2. ...
3. ...
4. Pilots may not substitute for the NAVAID (for example,
a VOR or NDB) providing lateral guidance for the final
approach segment. ...
5. Use of a suitable RNAV system as a means to navigate
on the final approach segment of an instrument approach
procedure based on a VOR, TACAN or NDB signal, is allowable.
The underlying NAVAID must be operational and the NAVAID monitored
for final segment course alignment.
​

As long as the VOR is not OTS it looks to me like using GPS for final approach course guidance is approved as long as the VOR is monitored (un-monitored for other procedures). To me, that means that the CDI doesn't go more than two dots (4°) from centered. Not so?

​

 

[Sluggo63]

Primary course guidance? Why can't he simply "monitor" the VOR while he flies the GPS course he knows is more accurate? Here's the pertinent words in the AIM after I took out the garbage, my emphasis in red font:

1−2−3. Use of Suitable Area Navigation
(RNAV) Systems on Conventional
Procedures and Routes
a. ...
1. ...
2. Use of a suitable RNAV system as an
Alternate Means of Navigation ... For example, if equipped with
a suitable RNAV system, a pilot may fly a procedure
or route based on operational VOR using that RNAV
system without monitoring the VOR.

b. ...
c. Uses of Suitable RNAV Systems. Subject to
the operating requirements, operators may use a
suitable RNAV system in the following ways.
1. ...
2. ...
3. ...
4. ...
NOTE−
1. ...
2. ...
3. ...
4. Pilots may not substitute for the NAVAID (for example,
a VOR or NDB) providing lateral guidance for the final
approach segment. ...
5. Use of a suitable RNAV system as a means to navigate
on the final approach segment of an instrument approach
procedure based on a VOR, TACAN or NDB signal, is allowable.
The underlying NAVAID must be operational and the NAVAID monitored
for final segment course alignment.
​

As long as the VOR is not OTS it looks to me like using GPS for final approach course guidance is approved as long as the VOR is monitored (un-monitored for other procedures). To me, that means that the CDI doesn't go more than two dots (4°) from centered. Not so?

​

You’re right. And that’s how we fly them at work. Pull the VOR approach from the database and fly it with the GPS, but we have to have the raw data displayed somewhere.

His question was about the checkride, and I wonder if a DPE would want them to fly solely using the VOR as course guidance.

After the checkride, that’s the way I would absolutely fly it in the real world if I had to actually do a VOR approach, or a (gasp) NDB.

 

[MauleSkinner]

As long as the VOR is not OTS it looks to me like using GPS for final approach course guidance is approved as long as the VOR is monitored (un-monitored for other procedures). To me, that means that the CDI doesn't go more than two dots (4°) from centered. Not so?

Agreed. But the premise at the time of my post was that there was a large discrepancy between the VOR course indication and the GPS course indication, and that the OP believed that the VOR was potentially out of tolerance by itself, so potentially indicating more than 4 degrees from centered.

 

[dtuuri]

...or a (gasp) NDB.

LOL. Gave me good chuckle there. I've been practicing an NDB approach on my old TD2 simulator with a GNS 530W. I find the GPS so much more complicated and workload intensive than what I'm used to in my day that I completely forget to look at the relative bearing indicator (no RMI). I have to watch the GPS like a hawk to make sure it flips the course when it's supposed to after the procedure turn, which it never does, and then manually jump legs in the flight plan menu. It sure is accurate, but what a pain to use.

 

[IK04]

I like NDB approaches. Done gazillions of 'em.