HSI drift?
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One more thing to keep in mind is the overall accuracy of the magnetic compass... In the real world, it is not really a very precise instrument, and is subject to errors that can be introduced from many different sources including the aircraft structure and electrical systems... For example the POH says that just turning on the Pitot Heat or Air Conditioner can cause compass errors of up to 10 degrees. A magnetic compass Deviation Chart, which shows the difference in the actual vs. magnetic compass heading, is normally created for each specific aircraft and carried in the aircraft at all times. The heading errors shown on this chart can vary by 5 degrees or more depending on your heading. Then, there are local magnetic disturbances that can come from the actual geology of the terrain, or man made objects you are flying over... These are often shown by notations on aeronautical charts.
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@RetiredMan93231 I understand what you're saying about causes of compass error or deviation. But on the basis that the HSI is supposedly slaved to the compass, doesn't that mean that the HSI should match the compass even if the compass is subject to an error?
I don't fully understand how the slaving of the HSI to the compass works, so if what I'm thinking is not the case it would be interesting to have an explanation of why.
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@Sender46 , The HSI (or a DG) is much more precise than the Compass... If you make a 90 degree turn using the HSI as your reference, it will be precisely 90 degrees, while the Compass may show a heading change of 85 or 95 degrees depending on the Deviation. Also, turning electrical systems like the Pitot Heat on or off will not affect the HSI reading. I too don't understand exactly how the HSI is "slaved" to the Compass to eliminate the drift and recalibration...?
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@RetiredMan93231 Are you saying that the differences we are seeing between the HSI and compass (8 degrees in my case) are normal and representative of a real aircraft?
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@Sender46, I'm saying that you should not expect exact agreement between the Compass and HSI at all times... This is true in the real world, but I don't know how true it is in MSFS. It depends on how true to life the magnetic compass and HSI are modeled in MSFS... But, for the purposes of real world pilot training you should be fully aware of these differences in instrument behavior and accuracy.
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@RetiredMan93231 LOL, are you a retired politician? ;-) Thanks for the carefully worded answer ..... which leaves me still not knowing whether 8 degrees difference is realistic or not.
While Googling trying to educate myself on how the HSI works and why it is more accurate than the whiskey compass, I have found that the HSI indicator is slaved to a gyro compass (or flux gate compass). This link https://www.aircraftsystemstech.com/2017/05/direction-indicating-instruments.html provides a good explanation of this and how it achieves the HSI's accuracy. See the Remote Indicating Slaved Gyro Compass (Flux Gate Compass) section in particular.
Having now understood how this works, I get that some difference between the HSI and whiskey compass should be expected but I'm still left questioning whether 8 degrees difference (at EGHP Popham BTW) is realistic or not ......
EDIT
Thinking about this further, magnetic variation cannot be a factor in causing this difference because the slaved gyro compass and the whiskey compass will both be at the same location and therefore subject to exactly the same magnetic variation. Local magnetic disturbances that can come from the actual geology of the terrain, or man made objects you are flying over will also not be a factor in causing this difference because the slaved gyro compass and the whiskey compass will both be subject to exactly the same magnetic disturbances. -
@Sender46 said in HSI drift?:
Thinking about this further, magnetic variation cannot be a factor in causing this difference because the slaved gyro compass and the whiskey compass will both be the same location and therefore subject to exactly the same magnetic variation. Local magnetic disturbances that can come from the actual geology of the terrain, or man made objects you are flying will also not be a factor in causing this difference because the slaved gyro compass and the whiskey compass will both be subject to exactly the same magnetic disturbances.
Of course you are absolutely right about this. However at this point we are not certain that we are looking at a sim bug, or that we are looking at some awesome compass error modeling :-) For example, the compass could be based on magnetic heading, while the HSI could be, erroneously, based on the true heading. I've taken the trouble to check this by starting at an airfield at the Southern most tip of New Zealand, which has a magnetic variation of more than 20o. The difference between the HSI and compass was about 2o, so we can pretty much rule out a confusion between true and magnetic heading in the sim.
This still leaves the discrepancy unexplained though. However it is comforting to know that the HSI exactly corresponds to the magnetic heading, so at least navigating on the HSI will get you where you want to go.
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@corvus5624 said in HSI drift?:
The difference between the HSI and compass was about 2o, so we can pretty much rule out a confusion between true and magnetic heading in the sim.
So, you only saw a 2 degree diference... I wonder why @Sender46 is getting an 8 degree difference? It would appear that the HSI/Compass comparisons are much different at different locations.
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Not sure what it depends on exactly. Please find an example below. This is on 70R at KNPA. The HSI reads 70o, while the compass reads 73o - 74o.
Note that KNPA is near the agonic line (where the magnetic variation is zero), so magnetic variation errors are ruled out.
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@corvus5624 Look closely at the HSI in your last post. While it's aligned with 70 at the top, it's not aligned with 160 on the right or 340 on the left. It's slightly off. This error gets even more dramatic as you point in different directions (for me, it's worst pointing east or west). This also happens with the NAV gauges, and the course arrow on the HSI. This is sometimes annoying when trying to quickly calculate back course heading or something.
I think what we're seeing here is at least partially accounted for by textures which aren't exactly centered properly. Even with the fixes from before which aligned the heading at the top properly, everything else was still ever so slightly off. This is probably a symptom of trying to represent a circular object with square pixels, but this is the only aircraft I've seen this issue in.
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@vcapra1 The small discrepencies at 160 and 340 look like parralax error to me, from looking down on the gauge as shown by the angle of the knobs.
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Perhaps the best way to deal with the magnetic compass errors is to do what they do in the real world... Simply create a custom Deviation Chart for the aircraft, using one of the many forms that are available to download, and reference it when you fly, as you would in the real aircraft... This would actually make the flying experience even more realistic.
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@RetiredMan93231 said in HSI drift?:
@vcapra1 The small discrepencies at 160 and 340 look like parralax error to me, from looking down on the gauge as shown by the angle of the knobs.
There is some parallax in this screenshot. I've tried to remove it by positioning the camera directly in front of the HSI. This aligned the 340o marks almost perfectly, but the 160o mark was off a 1 degree. So there are some textures imperfections, but not enough to explain a 4o deviation, let alone 8o. But boy, I have never taken such a close look at the HSI and/or the compass :rolling_on_the_floor_laughing:
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@RetiredMan93231 Now that you've mentioned parallax error, I've realised there is parallax error on the whiskey compass. Sliding over directly in front of it (rather than looking at it at an angle from the pilot's position) changes my reading by 3 degrees. So the difference I'm seeing between the HSI and whiskey compass (at EGHP Popham) is actually 5 degrees, not 8 degrees. Can any real world pilots comment on whether that could be considered normal and realistic or not?
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Well, here's my two penn'orth, FWIW. I think you'll find that in the real world, hardly anyone even bothers to look at the wretched thing, except perhaps for a gross error check on the first flight of the day, or when adjusting the DG for gyro drift after an hour or two's flying.
It's notoriously inaccurate and in anything but the smoothest air, is not easy to read, swinging all over the place. About the only time it's used in earnest is during check rides when the evil Instructor/Examiner simulates a vacuum or electrical failure and you lose your HSI or DG. Even then it's just an exercise to ensure you have some idea of how to read it in an emergency.
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@tb10driver Thanks for your input. Getting your perspective makes me more inclined to think that maybe what I'm seeing is actually realistic (whether the difference is intentional by Just Flight or not).
