Confused! FSX Cessna 172 - VOR - NAV Switch

[Arawn 0]

Hi everyone,

 

I am just exasperated with confusion over some things, but I'll keep this hopefully simple: I am trying to learn VOR navigation, and one of the things I'm trying to understand is how the autopilot works with this type of navigation. I am using the default Cessna 172SP. I create a flight plan using VFR and VOR to VOR. My flight will start from KBOK and go to the first VOR, OED.

 

Okay, so my NavLog tells me OED is at a heading of 051 with a frequency of 113.60. I load up the flight and dial this frequency into my active Nav1 radio and put a heading of 051 into my Nav1 gauge.

 

Now this is where I am confused: what does the 'Nav' switch on the Cessna autopilot do exactly? When comparing with my GPS, I am not really flying very close to the magenta line unless I zoom out a lot. Is this switch supposed to follow our flight path? I don't want to use the GPS switch. Am I supposed to hand fly to a certain point? This is very confusing! I'm really trying to understand how this all works. Thanks!

[N60JBravo 27]

The NAV/GPS switch controls the input source for the VOR1 head.

 

For VOR to VOR navigation leave it in the NAV position.

 

When the autopilot is set to NAV Hold it will follow the VOR1 regardless of it's source.

 

I have been doing this for years and very rarely do I end up right on the magenta line when flying VOR to VOR.

 

Only when the magenta line disappears completely do I worry about it.

 

regards,

Joe

[fs4fun 366]

Autopilot NAV mode causes the autopilot to follow your selected course, which is the GPS course if GPS is the selected source, or the VOR1 course if NAV source is selected.

 

Chances are the courses will not match up. VORs have their own fixed magnetic variation, and if FSX is correct, the OED VOR has 19E as its magnetic variation: http://www.pilotnav.com/navaid/faa-1992. The GPS will use the FSX magvar data and for OED lat/long it should be around 16.1E for year 2006. You can calculate magvar for a specific location/year here: http://www.ngdc.noaa.gov/geomag-web/#declination. If you want to update your navaids and magvar data, go here: http://www.aero.sors.fr/navaids.html.

[scott967 62]

Consider how the VOR works:  there's two types but the principle is the same.  It transmits two signals, one is omni-directional and the other is "rotated" at a precise rate.  The aircraft receiver compares the two signals and the difference is an indication of the aircraft's location relative to the VOR.   The "rotated" signal is adjusted so that it is aligned to magnetic north as determined periodically by the FAA or other aviation authority (an exception is in north-east Canada, where the VOR is aligned to true north).

 

Note that a magnetic compass will align to magnetic north, based on the actual magnetic variation (deviation -- both terms tend to be used), not necessarily the one assigned by FAA when they adjusted the VOR.  So the VOR and the compass readings may not match.  In FSX, the VOR "adjustment" is set as part of the navaid definition bgl file.  The compass reading is "adjusted" by a separate, global magnetic deviation bgl file (and updates for this file can be found online from various sources).  The FSX flight planner and GPS use this global magdec file for computing courses and headings, so might differ by a bit from what a VOR indicator or HSI is showing.  That's why if you want to follow a specific ground track using VOR you need to look at a chart, not just follow the FSX flight plan or GPS.  An IFR chart will show you the VOR setting for a defined route, while a sectional will provide a compass rose for the VOR that you can use to determine the VOR course.

 

Your VOR indicator / HSI is designed to compare the signal from the nav receiver with with a second signal created by rotating the knob (more advanced, entering a "course").  This signal is combined with the one from the receiver and the result is what drives the needle and sets the to/from flag.  The same signal is sent to the autopilot via the NAV/GPS selector switch, and when "NAV" is selected on the switch, using "NAV" or "APP" mode on the autopilot will use this same signal to create a roll command to the autopilot servos so the aircraft turns to the desired course (or radial, depending on how you want to look at it).

 

scott s.

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[Arawn 0]

Thanks for the responses guys. I feel a little better but I guess I have one more question to add:

 

So say I take off from Medford flying to OED... is the route shown in my GPS/flight planner not the route I should be following? Isn't this like my "highway"? So if I'm not following that line exactly am I not off of the flight path? Wouldn't that cause trouble if you were flying on VATSIM?

 

I guess basically getting from one VOR to VOR doesn't seem to be a problem, autopilot or no. But I feel like I'm doing something wrong not following the route/magenta line.

 

Now that I've got the flying part down, finding out where the hell I am going/what I'm supposed to be doing is a challenge.

 

Thanks again.

[N60JBravo 27]

I wouldn't worry too much about the magenta line.

 

As long as you are using the correct radial you are going to be close enough.

 

For me it boils down to either following the magenta line or use the VOR. AS I said earlier, they never match up perfectly.

 

Even if you are flying IFR you can be a couple miles off of the magenta line and still be "on course".

 

The system isn't perfect and many times I have seen the flight planner give me one course, as seen on the kneeboard, and the GPS give me another one, as seen on the GPS flight plan page.

 

regards,

Joe

[tttocs 301]

That magenta line is your GPS course, nothing more.  It is for a variety of reasons more accurate than the course you'll follow if using VOR navigation - for the reasons explained above and due to the fact that VOR signals themselves are less accurate the further you are from the station and can bend or "scallop" due to terrain and other issues.  My suggestion if you're looking to learn navigation by reference to ground stations is to go cold turkey and do it like we did in the old days before you even had GPS.  Don't put in a GPS flight plan, but instead really navigate using JUST the ground stations.

 

If you're flying on airways and are concerned with the things like obstacle clearance and MEAs, keep in mind that clearances on those airways are designed with the navaids being used in mind.  That is, they have "slop space" and you'll be fine using VORs as charted.  I'd also suggest you grab some enroute charts which, among other things, will tell you when you should be switching over from the VOR you're flying from to the VOR you're flying to.  Again, the airways are designed with these switchover points in mind, and the points are not always where you'd expect them to be, especially when in rugged terrain.

 

When using the AP, I'd also recommend you get used to switching from NAV to HDG mode as you get close to each ground station.  You do not want the AP coupled to the VOR as you approach and pass over the station, so use your heading bug or hand fly when near.

 

Enjoy and feel liberated from the tyranny of the magenta line.  :-)

 

Scott

[Arawn 0]

Thanks everyone. I have been doing a good amount of reading and I feel better about this topic BUT I do have one more question!

 

So I've used a generated route from FSX, flying low altitude airways from KSEA to KPDX using a Cessna 172SP. So The plan has me go direct from KSEA to the SEA VOR, and then it says to fly to numerous waypoints on airway V495 (heading 169). So I take fof from KSEA and pass the SEA VOR (heading 199) and then turn to get on course for the airway. Now I should be flying a course of 169, right? How can I know without GPS that I am staying on course when flying on this airway? I guess because the next VOR, BTG, also has a heading of 169. So this means once I leave the SEA VOR I should tune to the BTG VOR which should therefore also align me with the airway?

 

So my first waypoint on this flight path is CIDUG. It is 25.8 NM away from the SEA VOR. So how do I know when I'm passing over this waypoint? Is this done by calculation? I.e. distance between SEA VOR and CIDUG is 25.8 NM, at estimated 115 knots GS, with ETE being 0:13. So this means that if I'm flying at 115 true airspeed at a heading (course? still trying to wrap my head around some things) of 169 and thirteen minutes have elapsed then I should be passing over this waypoint, right? Also, the SEA VOR has an altitude of 635 feet at heading 199 at estimated ground speed of 115 knots. This means that I will most certainly be climbing out while passing this VOR. So therefore my true airspeed will most certainly be lower than 115 knots, right? So I need to factor this into my calculations?

 

Sorry for all of the questions. I know you guys don't get paid to answer them!

[Tube and Rag 3]

Go to www.skyvector.com - at the top of the page, click on the globe labeled "charts" - next click on "Enroute Lo" - next click on the box over Seattle, this is Enroute Low chart L-1.

 

This chart actually provides many of the answers you seek, but it will take you some time to learn to interpret it.

 

CIDUG is located by the intersection of the SEA168R and the TCM134R.  So you would put TCM (109.6) in VOR2, and dial the second OBS to 134 degrees.  As you're flying along V495 (with the needle centered, of course), when the VOR2 needle centers, you're at CIDUG.

 

The next intersection on V495, ADLER, can be located as the SEA168R at 37 DME, or alternately by crossing the OLM095R.

 

The next thing on V495 is a changeover point (COP), located 82 miles from SEA and 20 miles from BTG.  This is the point you'll stop navigating outbound on the SEA168R, and start navigating inbound on the BTG345R.  COPs aren't typical.  Usually on a straight airway, you changeover navigation halfway between VORs.

 

If you're keeping a navlog, you are hopelessly hooked!  Yes, you have to factor some climbing time into your planning.  You could compute this down to something quite precise, or maybe just flightplan the first leg at 75 knots instead of 115.

[Arawn 0]

Love it! Thanks! Two questions: What does SEA168R and TCM134R mean? I am looking at the SkyVector charts and I see where you get those numbers from. So it looks like the route takes you on a heading of 168 departing KSEA. What does the "R" mean? Is that for the runway? Like departing runway 16R (which has a heading of 163, so I assume you adjust sometime during climbout to get on the exact heading?)

 

And then the other thing is about the NavLog. I have FSX compute the NavLog and I see you can create a flight plan with SkyVector as well. They also ask that you put in a ground speed. So is this ground speed for cruise altitude since for climbout and descent it can't possibly be that?

 

Okay, I swear that's all I have to ask. lol

[Tube and Rag 3]

R is for radial, as in the Seattle VOR 168 degree radial,

 

Yes, when you take off, you will have to initially fly a heading that takes you to the radial.  So if you did take off on runway 16R (R for right, in this case), the VOR is to your left, so you could just pick up a 160 degree heading until the OBS centered on the 168R. 

 

When the needle does center, turn to heading 168 degrees.  For now, your course (the 168R), and your heading (168 degrees) are the same.  In the easy FSX weather schemes, there is a wind aloft from the west.  This will slowly move the airplane left of course, and the needle will deflect to the right.  When you notice it doing so, turn 5 degrees towards the needle (173 degree heading).  When the needle centers again, take out some--but not all--of that 5-degree correction;  try turning left 3 degrees to a heading of 170.  Now your course is 168, but your heading (170) is corrected for wind.  Repeat this process as often as necessary to keep the needle centered.  You'll quickly learn how to "bracket" wind corrections.

 

Ask all the questions you need, most of the folks here are happy to help,

 

If you use the Skyvector flightplanner, use your cruising speed.