Magnetic Deviation

[liquid_rockface]

Hi guys, I need some help here!

I have been flying in Flight Simulator 2004 for some time now, and just recently, with the acquisition of FS Navigator, something has been brought to my attention: Magnetic deviation.

Now, I cannot say 100% for sure that what I am experiencing is that, therefore I am requesting some help here!

I was flying out of London Stansted, RWY 23, due north for Manchester. First you go to BKY VOR, then you turn left to BUZAD intersection, then you continue north to OLNEY and WELIN until you reach TNT VOR (Trent).

Now, I plotted the whole journey out in FSNav, and saw I would need to fly at 334 degrees to OLNEY and WELIN, and 335 to TNT. I thought that rather than be boring and fly autopilot, I'd dial in the numbers to my OBS and fly it manually. I was flying the Cessna Grand Caravan C208.

I like to double check sometimes, so I put the headings both into my OBS and my HSI. Now, the first thing that was weird is that they did not match up at all. I must add that I was also flying with FSPassengers, so it is possible that one of them had a "failure" but nothing like this was mentioned in my after-flight report...

The HSI showed it as being almost dead on, and the OBI rather badly off....I never worked out how that was.....

Anyway, so I tried flying it now without FS Passengers, and now they show the same thing, almost. There seems to be a degree or so difference between the two instruments. Furthermore, if I fly exactly at 334 degrees towards TNT, I am a good 2 degrees off my course! I checked in FSNavigator, and it stated that the magnetic deviation of TNT is 5 degrees, and that the NAV Deviation (what's the difference?) is 2 degrees.

Now, does that mean that I have to ACTUALLY fly 331 in order to be on course? This is what the computer flies if I let it do autopilot. I have never noticed this phenomenon before, but I don't normally fly in England, which is quite north, and surely has quite a pronounced magnetic deviation compared to more southerly countries...

My question basically is this: FS2004 doesn't provide you with the figures for magnetic deviation, to the best of my knowledge. So if you flew exactly how FS Flight Planner told you, I assume you'd be off course all the time. Secondly, am I correct that you have to compensate for magnetic deviation when dialling a course into your OBS, so if the heading should be 334 and there is 2 degrees magnetic deviation you should fly 332? And thirdly, why is it that my OBS and HSI always seem to be about 1 degree off from one another?

Thank you very much for any help! :bigeyes:

[Genet]

You don't mention whether you are aware of the fact that MSFS models/simulates drift between the magnetic compass and the HSI readings. If you are ignore this reply.

At regular periods during flight you need to cross check between your magnetic compass and your HSI reading and adjust your HSI to read the same as your magnetic compass reading.

This is especially true after manouevering (ie course adjusments aerobatics and only after you have allowed the magnetic compass time to stabilise following the manouevre particularly if they are rapid.

There are two ways of aligning the HSI reading with with the magnetic reading.

1. press the "D" key which is set by Microsoft as the default key to align them automatically

OR

2. As in real life turn the relevant HSI knob to the magnetic reading


Hope this helps and seasons greetings as well.

[liquid_rockface]

Yes I am aware, and I sometimes press D, but you should know that in the C208 Grand Caravan this is not relevant, as I think it auto-adjusts all the time. You cannot turn the HSI. It is not like in the Cessna Skyhwawk C172...

Anyway, according to my reckoning, if you adjusted the HSI to the magnetic heading, it would be out anyway because of the magnetic deviation....

[Cobra]

i didnt quite follow your post but could it be wind drift?

:confused:

[liquid_rockface]

Cobra, no. Basically, my VOR shows that I am dead on course, when I am actually about 2 or 3 degrees to the left of course (as seen on my FSNav moving map) and on GPS.

According to the FSNav, from the intersection BUZAD to the VOR TNT, should be 334 degrees. But I find that it really should be 332 in order to fly straight from BUZAD to TNT. If I really fly 334 degrees (and as I say, on the map and on the waypoint chart it APPEARS to be 334 degrees to the VOR) then I end up rather a lot to the left of the course.

Maybe the VOR is skew or something? LOL. I must try some other VORs....I would appreciate if somebody could try flying this course to understand better my question. It is by London Stansted airport.

[Cranium]

The magnetic variation for TNT is -3.4 degrees. Perhaps you're flying a magnetic course, but FSNav shows true headings. So in your example, if FSNav wants you to fly a TRUE course of 334 degrees, we need to subtract 3.4 degrees to get a magnetic heading = 330.6 degrees, close to 331 degrees no wind.

I'm not familiar with FSNav anymore - haven't flown MSFS for a long time, but is there a check box or something to ensure that headings are magnetic?

[Deckerd]

The magnetic variation for TNT is -3.4 degrees. Perhaps you're flying a magnetic course, but FSNav shows true headings. So in your example, if FSNav wants you to fly a TRUE course of 334 degrees, we need to subtract 3.4 degrees to get a magnetic heading = 330.6 degrees, close to 331 degrees no wind.

I'm not familiar with FSNav anymore - haven't flown MSFS for a long time, but is there a check box or something to ensure that headings are magnetic?

AFAIK radials are not corrected for magnetic deviation in FSNav. It's best to calculate the mag heading and place your bug on that course.

[liquid_rockface]

Ok great, thanks for the quick answers! So, does FS2004's flight planner give you the figure including the compensation when it calculates a route? Because I exported the route to FS2004 and it showed the same headings, but maybe it didn't calculate this, just took it blindly from FSNAV.

That's really annoying, so I have to examine every VOR to check the magnetic deviation!

Cranium, you mention wind, but as you know what direction your aeroplane is facing has no influence on the VOR....so wind compensation or not, the course you tune in should be regardless of wind...

[Cranium]

You are correct that wind doesn't affect the radial that you want to fly to/away from the NAVAID (TNT in this example, that you dial into the course window), but it does affect the magnetic heading you're flying to maintain your course on that radial.

[liquid_rockface]

Ok, I've done some more tests, this time further south (in Egypt) to check if that would make any difference. Again, I found that if I am on the course as shown in the GPS (that is, the course that the Flight Planner found) that the "real" VOR bearing is some 2-4 degrees off to the left.

I shouldn't imagine any of this has to do with magnetic variation anyway. VORs are on magnetic headings, and so is the HSI. All that should be required is to fly the heading to the VOR and that's that. So something's wrong, either the MS flight planner or the VOR.

If I want to fly up the 334 degree radial of the VOR, I should simply dial in 334 to the OBS and follow the dial. I don't see how magnetic deviation should have anything to do with this. So now I don't understand how this can be: I'm flying at 334 degrees, headed straight towards the VOR, I should be on the 334 radial inbound!! Not the 332! Forget wind, I turned wind off even.

Any more explanations? I'm going quite nuts now, I feel like I've probably been navigating wrong all the time (although managing quite well to get from A to B!) This is a new thing I've noticed, I'm sure that in the past I've double checked my GPS route with the VOR and it's all been perfect, so there must be some reason...

[Genet]

Having done some research out of idle curiosity it may well be that the bearing variations you are experiencing are distortions arising from the charts used. This is a theory and I do not claim my thinking is correct.:bigeyes:

I quote extracts from Air Navigation by Trevor THOM:-

"Representing the Spherical Earth on Flat Maps and Charts

....the process comprises scaling the earth down....
....then projecting the reduced surface onto a sheet.

This process ALWAYS leads to some distortion, either of areas, distances, angles or shapes. By using certain mathematical techniques when projecting the spherical earth onto a flat chart, the map maker can preseve some properties but not all....."

He goes on in detail but here is the interesting part that I think could make this your MSFS problem. To minimize distortion air navigation charts in the UK uses the "Transverse Mercator Cylindrical Projection". Using this method the cartographers select the 2 degree W meridian as the reference point as this meridian passes north south through the centre of the British Isles. This means no distortion occurs along this meridian and that the distortion either side of this line is minimal within the UK.

To me this implies that highly accurate charts can only be done on a regional basis.

So why is it a problem in MSFS and FS Navigator. There could be two reasons or a combination of both:-

1. As I understand it when MSFS was first designed the MSFS virtual world was based on these flat charts (ie the world was not modelled as a sphere). That would mean the origin on these charts most likely would be the US (I do not know this to be fact) which could explain why you experience a variation between the VOR radial and your aircraft instruments in the UK and Egypt. The error is likely compounded by the fact that VOR's NDB's Airfields are placed as per their supposed accurate longitude and latitude. I suppose the way to test my theory would be to try a similar flight in the US starting by making a BIG ASSUMPTION that the centre of the universe for MSFS may be SEATTLE.:red:

2. FS Navigator and Microsoft made their worlds based on charts drawn by different makers their biase being their countries of origin. (ie FS Navigator is German/European based and MSFS is US based).

I believe this is will not exist with FSX whose virtual world is now a spherical representation of earth... I seem to recall reading this somewhere but can't find where I read or gleaned this snippet.

Any way it will be interesting to see my theory get shot down in flames by the experts and hopefully we will become wiser for it.:red:

[liquid_rockface]

That sounds good...I'll be trying that one! LOL @ Seattle being the centre of the MSFS universe!!

[liquid_rockface]

Ok, here's a report on a flight from KSEA Seattle.

I took off from KSEA, headed out to KPAE Snohomish, to PAE VOR. Both FSNav and FS2004 reported the heading as 342 degrees.

I was flying the Cessna Grand Caravan as usual (I have tried other planes, it didn't change anything).

I dialled in the frequency and heading into the OBS. Checked that the HSI matched the magnetic compass heading, which it did.

First I flew the autopilot on GPS, and saw that the VOR track was this time, wait for it, on the RIGHT of the GPS track (in England it was on the left, something to do with which hemisphere you are in?) I might add that magnetic deviation for PAE was some 20 degrees, yet I was still only about 2 degrees off, like in Britain, just on the other side.

I then flew the VOR, and saw that I was off to the right of the GPS track, as expected. I then made the autopilot fly the GPS, and surprisingly it stuck just a little bit to the right of the GPS track, even though there was no wind, that was surprising, I've never seen that before.

Another observation is that throughout, the HSI's VOR needle was more to the left than the NAV2 VOR OBS (just about half a degree, but still, it should be exactly the same!!!)

Has anybody else experimented yet? I am dying to hear some responses from other users (maybe also people with and without FSNav, I could swear my VORs were all perfect before I installed FSNav...but I could be wrong)...

[Cranium]

Here's another thing to remember about navaids. They differ in their precision and sometimes, the tracks they fly across the ground. Subtile differences will be apparent when you're flying a VOR-to-VOR airway, versus a GPS track. You're still within the protected airspace of the airway regardless, sometimes the courses even differ marginally (I can't think of any off the top of my head, right now, but I remember some GPS approaches with different inbound courses than the VORTAC course - even with the VORTAC located along the centerline of the runway). But, you're still on the same airway.

[liquid_rockface]

Right. I've kindof come to the conclusion that I cannot expect 100% accuracy from anything (Strange of MS to programme anything as sophisticated as a natural inaccuracy!!). As long as I arrive at the right place, and I'm not too far off course, it shouldn't make much difference.

Sometimes the VOR and GPS track is identical, but mostly it's not, and I even noticed that the airplane rarely flies exactly on the GPS track line, if you zoom in to 1NM. But from 15NM it looks like it's perfectly on track. That might explain something else too....but what explains that the HSI VOR is slightly different from the regular VOR...hmmm

[Genet]

I would surmise that if the needle deviation is to the left (west) in the UK and to the right (east) at KSEA/KPAE area of Washington state that the longitudinal meridian chosen as the origin for MSFS is in central USA. The obvious longitude between these two points, likely to be THE ORIGIN is 90 degrees W, which incidentally happens to run through Illinois. This state has Chicago and the early MSFS's had Meigs field as the default airfield which makes me believe that it is very likely that the 90 degree W meridian is the origin.

If I am right there will be little or no deviation along this line of longitutude.

It would also be interesting to see what happens in the opposite hemisphere between 0 degrees E longitude and 180 degrees E longitude and see if 90 degrees E longitude also acts as an origin.

[liquid_rockface]

I don't understand this whole thing about longitudinal meridian. Surely if you fly from point A to point B, your heading between those two points (assuming that you fly in a straight line and arrive exactly at the right place) will be the same, no matter where in the world you are. Furthermore, the VOR just checks whether you're heading towards it at a certain angle. Never mind where you are.....so I don't entirely understand how that can be so. However maybe it's a bug in the game, I'll fly around Meigs now :)

[Cranium]

Here's some more information for you. Magnetic deviation is based on deviation from true north. If the the true north pole (the axis around which the Earth rotates) was colocated with the magnetic north pole (the thing that the N needle in your compass points to, then there wouldnt be any deviation.

Look at the first picture below:
http://www.fighterops.com/forum/attachment.php?attachmentid=554&d=1167074454

This is a picture of where the magnetic north pole is and the lines of magnetic deviation caused by the variations in magnetic field. As you go from Vancouver to Quebec (not my neck of the woods, so you Canucks, feel free to correct me :smile: ) the lines of magnetic variation cause deviations in where your compass/HSI will point. This requires a change in heading while traveling your great circle route between the cities.

The second picture below shows the diagram on most maps that show the declination and reference where true and magnetic north are in relation to map center (I believe).
http://www.fighterops.com/forum/attachment.php?attachmentid=555&stc=1&d=1167074732

So, to recap: You'll never fly a constant heading when flying directly between two points that are separated by a large distance. The variations in the location of magnetic north and the magnetic field generated by the Earth, cause compass deviations picked up by the flux valve that automatically slews the HSI to magnetic north.

Questions?

[Genet]

What I think is that if Microsoft based their earth model on these flat charts they used a chart which has its origin along a line of longitude. The line of longitude chosen is arbitary and decided at the point the cartographer draws the map. All I was doing was suggesting a process of ellimination to determine which line of longitude was used.

It therefore follows that based on the fact that the further you move from the point of origin the greater the errors in shape, size, line and angle become albeit small with mathmatical correction. Now the errors you were reporting seemed to indicate that they were at or near the extremes from the point of origin.

So when you said you had a 2 degree left deviation flying north in the UK and a 1 to 2 degree right deviance flying north on the west coast of the US the errors indicated that the origin should be somewhere between those two points and in the central US because of the direction error your instruments were displaying (The errors point toward their origin). The deduction that it might be 90 degrees W is a best guess as I would think a cartographer would naturally want to choose 0, 90W, 180, 90E. When I noticed that 90W was near Meigs it seemed to fit. So I believe that the line of longitude chosen by the maker of the map used by Microsoft is likely to be 90W subject to you confirming that the navaids bearings match your instrument bearings (ie there is negligible or no error displayed by your aircraft navigational instruments).

If there is little or no deviance between the two at 90W then it would seem my deductions are correct.

The next point is having created a world which is based on small errors that cannot be erased, Microsoft and 3rd party developers then placed objects on the surface of this world based on their accurate positions referenced by lines of latitude and longitude. If they are being placed where they actually should be on a world created from a map that has errors the objects in this case NAVAIDS could but not necessarily always make the error worse, but furthermore the only place they are likely to match the map perfectly is at the origin of the map.

[liquid_rockface]

Wow Genet, looks like you may be right! I flew around Meigs, and there was almost no error at all, then I found 90W, which is in China, again almost no error. The middle of those two is around Ireland, and again no error. I'll have to find something now around 45W/E to check if that's maximum error!

Strange though, that in Ireland I have almost no error, but in London I have....London is on the 0 line too...