HSI drift?
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Yes, DGs drift and require regular pilot adjustment to match the whiskey compass. But my understanding is that in an HSI, drift is usually compensated for by the use of a slaved gyro system that constantly corrects the heading indication. In which case, if this is working correctly the HSI should not drift and there should not be any need for a means of pilot adjustment.
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@RetiredMan93231 Isn't that what the gyro correction knob is for?
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@RetiredMan93231 Thanks. A bit of a random question, but can you tell me how all this relates to magnetic variation, if at all?
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@pdd, Magnetic Variation is the angular difference between True North and Magnetic North, which varies with your location and is marked on your aeronautical chart. The aircraft Compass and DG will both be pointing to Magnetic North, which is used as the basis for determining your Magnetic Heading and Course.
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@vcapra1 Judging from the screenshot the plane seems to be in a bank, however the turn and bank indicator indicates level flight. What is the suction gauge saying? Is there enough suction to keep the gyros going?
Anyway, in a turn I would expect the compass to wobble around, so compass readings might be inaccurate.
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@corvus5624 The plane wasn't in a bank, or at most in a very slight bank. The attitude indicator and turn coordinator both show this. Suction was fine. Anyway, I tested this on level ground and had the same issue
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@vcapra1 I take your word for it, it is a bit hard to judge from the screenshot, but I thought I could discern a horizon that whas at angle wrt the plane.
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@corvus5624 Ah, yeah the thing in view out the window is an island (Fire Island on long island, i think), the horizon isn't in view
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@vcapra1, Another thing to keep in mind is the parralax error you will get when viewing the whiskey compass from a side angle... Try using the arrow keys to move the cockpit camera viewpoint to the center of the cabin so you can see the compass head on. Also, remember that when the compass lubber line is to the RIGHT of 21 (less than 210 degrees), it will be to the LEFT of 21 (less than 210 degrees) on the HSI, since they rotate in opposite directions.
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@RetiredMan93231 Unfortunately that wasn't the issue, even when looking straight on at each one at a time, there's still about a 5 degree difference.
Just to make sure there was no actual drift, I hit the [D] key and nothing happened. So I think it's a texture misalignment.
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@vcapra1 I'm using v0.7.1 and mine is 8 degrees out.
Hitting the [D] key makes no difference (not that I would expect it to with the HSI). I thought Just Flight had sorted out the instrument texture errors but it appears maybe they haven't.
Have you submitted a ticket?
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@Sender46 @vcapra1 I tried to reproduce this behavior, and managed to do so. However the error itself does not seem to be consistent. It is almost as if the compass starts with a random error. I've seen deviations of 5o between HSI and compass, but also close to zero. I've checked with LittleNavMap, which displays the magnetic heading of your aircraft using SimConnect, and the HSI always corresponds to the SimConnect magnetic heading. So it seems that the compass is off, not the HSI.
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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.
