Author Topic: Magnetic instrumentation  (Read 2756 times)

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Offline Adrian Chitan

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Magnetic instrumentation
« on: May 17, 2015, 02:54:27 AM »
Hi guys,

in VFR flights, most of the navigation is done using or relying to some extent on a magnetic instrument. But let's see how reliable these things are and what are some of the instruments that need magnetic information.

The magnetic compass


The civil aviation aerial magnetic compass is a very sensitive instrument that basically is a magnet that aligns to or follows the Earth's magnetic flux. It is composed of a sensitive magnet linked to a heading disk sunk into a kerosene like liquid for stability and movement dampening. The good thing about the magnetic compass is that it does not need electricity, vacuum, nor static or dynamic pressure sensors. It works 24/7 even when the plane is in the hangar getting a face-lift. This ensures that in any circumstance, with no other means of navigating you will be able to conduct the most basic navigation - take a heading.

Magnetic compass errors

Declination (variation)

Variation of a magnetic compass has to do with the fact that magnetic North is not at true North.


Because of this difference, if the pilot would use the displayed heading of the compass he/she would not be flying as opposed to true North as he/she is expected, but as opposed to a point in Canada or Russia. So every point on the Earth has a certain declination from true North  and if you draw a line through all points that have the same declination you would get a line called a isogonic line. The image bellow shows the isogonics going through the US. The line with 0 degrees declination is called the agonic line.


So, if you were to fly around Chicago on that particular year (the magnetic poles move continuously so new isogonic maps have to be constructed every year) you could directly read your heading from the magnetic compass, while if you were to fly through Southern Washington state you would have to correct the heading by 20 degrees towards East. So how does a pilot get information about the declination? Through his/her charts, of course. Sectional charts contain this information given on a magenta dashed line as can be seen bellow (like a class D airspace but this is straight and has a number of degrees on it).


Now, I'm happy to say that FSX/FS9/P3D and X-Plane use magnetic declination (though I saw no pilot correcting for this, EVER, in the virtual world). The only problem is that I don't know if the correction is built in the virtual compasses or not. Anyway, they all are/were lacking precision. In the case of FSX/FS9/P3D the values are the ones inserted there by Microsoft when the simulators were created (yes, Lockheed Martin didn't update anything concerning actual flying in P3D). In the case of X-Plane, up until update 10.20 the values evolved through time but they didn't resemble the real ones. After update 10.20 the developers may or may not have put in a fix to this (they responded that they knew about the problem and that it will be fixed in the next updates) - don't know whether it was fixed or how. There is also good news for FSX/FS9 flyers. There is a free app made by Herve Sors which reads the real isogonics and inserts them in the sim cfg/dat files.

How to spot a declination difference in your sim? In X-Plane you can get this data out in real time and go online and check with official surveys to see if they are equal for the actual place. In FSX/FS9/P3D there are cfg/dat files that store this information. Also, for both sims one can compare the intercept course for an ILS in a newly acquired AIRAC and what he actually intercepts when flying.

Deviation

Deviation is an error that is dependent on the setting of the magnetic compass. Metal and magnets around the magnetic compass add movement to the magnet. The body of the aircraft deviates the compass from showing a perfect reading of the actual heading. Also electrical and electronic equipment create electromagnets that also influence the compass. Usually the radio stack adds the largest error to a cockpit compass.

That is why, periodically, an aircraft owner will take his plane to be magnetically checked. This consists of putting the plane in different headings and seeing what the compass shows with all electrical equipment on board ON. The difference between expected and obtained values are written on a card just bellow the magnetic compass.

Again, I am happy to say that in all sims I've seen this card with values written on it, though I've again seen no virtual pilots using it. Furthermore I've also seen a plate that had different values for radios ON and radios OFF, which is quite encouraging :D.


Dip error

Angle of dip error has to do with the fact that the lines of magnetic flux go into the ground at the poles. So as you get closer to the magnetic poles, there is a dipping angle that pulls one of the magnet ends towards the ground. This is not an error in itself because the magnetic compasses are constructed so that the pivot point is upward the center of gravity of the magnet-disk ensemble which ensures that the maximum tilt angle of 18o (most compasses) will not be exceeded in normal latitudes flying (far enough from the actual magnetic poles). As you get closer to the poles, the dip becomes so great that the compass is useless.


Acceleration errors

Because of the dip in the compass ensemble, other errors appear as side effects. One of them shows up when accelerating in an easterly or westerly direction. Because during acceleration the center of gravity will lag behind the pivot, the ensemble tilts forward. Add to that the dip towards the pole and you will get a false turn towards the closest magnetic pole (so towards North in the Northern hemisphere and South in the Southern one).

Turn errors

The other side effect of the dip in magnetic compasses is turning through a northerly heading towards East or West. The turn will add its tilt to the compass and along with the magnetic dip, the compass will lag behind the actual heading of the aircraft in the turn. The lag will diminish as the plane gets to East or West and be 0 at these headings. If the plane was turning through South, the pilot will see the opposite effect in the turn: the compass will lead ahead of the actual heading of the plane.

The amount of lead or lag is somewhat dependent on the latitude of the flight and the rule of thumb is that if you're flying at 30o latitude you should recover 30o before your wanted heading on northerly turns and 30o after your wanted heading on southerly ones.

At this chapter, FSX is slightly better (or I think it is) for it has accurate modeling of all these effects (well...without the added jerking of a real compass) so virtual pilots can actually practice turning by the compass as in a real airplane. Though I am not sure about how X-Plane simulates this, I am aware that X-Plane is just now making up ground on the system's simulation chapter - inheritly concentrating on the flying part. Having said that, some of the payware aircraft add-ons that I fly almost hide the magnetic compass, or draw it in a very low resolution that is almost unreadable. That is because the policy is to read the HI or HSI and not care about the old compass. Nevertheless, there are some X-Plane aircraft that have very high resolution and in-your-face magnetic compasses, but I didn't take the time to test them, yet :(. I've also sent a question in the official forum so that the developers can answer it and put this issue to rest :D, though I still do think FSX is better at this.

Magnetic compass slaves

There are two instruments that are slaved to some degree to the magnetic compass and eliminate some of the errors that the magnetic compass has.

Heading Indicator (HI)


The heading indicator (also called a Directional Gyro - DG) is the first of the two magnetic compass slaves. It is a gyroscopic instrument that eliminates all the above compass errors. The only thing that it needs is that the pilot realign it to the compass heading (with or without corrections for declination and deviation, whatever the pilot wants) every once in a while (once every 15 minutes of flying). This actually means just turning a knob under the HI to rotate the heading plate.

Usually you will see the word "VACUUM" written on this instrument because the gyroscopes are turned by blowing air onto little buckets. This air stream is created by a vacuum pump driven by the engine.

Horizontal Situation Indicator (HSI)


The HSI is the evolution of the HI, still being in the gyro instrumentation class. Unlike the HI, the HSI is automatically and continuously realigned to the compass. But not to a cockpit compass but to a more precise tail magnetic compass (away from all avionics and metals that can induce errors). The signal from this compass is amplified and used to continuously realign the HSI. That is why there is no realignment knob on the HSI.

To top it all off, the HSI includes a Course Deviation Indicator (CDI) to combine two instruments into one (HI + radio navigation) giving the pilot a good situational awareness concerning navigation.

Cheers,
and if you have time, start experimenting with the magnetic compass of your simulator and let me know if you see the expected behavior :D.

 

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