HSI drift?



  • The HSI seems to have drifted by about 5 degrees, or perhaps it's misaligned from the whiskey compass. This may be related to previous issues with misalignments between the nav obs and hsi/di

    https://imgur.com/C6UdoVJ



  • DG/HSI drift is normal, and it happens in all real world aircraft... Pressing [D] will realign the DG with the Compass heading.



  • So in the real aircraft, where there is no D key, how do you realign it?



  • @vcapra1 That's a great question...! I never thought about that, and I don't know the answer...?



  • 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.



  • @RetiredMan93231 Isn't that what the gyro correction knob is for?



  • @pdd That's true for a DG, there isn't a knob for this on the HSI... Only a COURSE knob and a HEADING knob.



  • @RetiredMan93231 Thanks. A bit of a random question, but can you tell me how all this relates to magnetic variation, if at all?



  • @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.



  • @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.



  • @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



  • @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.



  • @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



  • @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.



  • @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.



  • @vcapra1 I'm using v0.7.1 and mine is 8 degrees out.

    HSI Error.jpg

    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?



  • @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.



  • 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.



  • @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.



  • @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...?