Gyro Drift or HSI Settings

[Jimm 437]

I know I am asking a really dumb question here, but here's the issue.  As I typically fly one to two hour short hops from airport to airport, I will set up my aircraft, including a flight plan and as I follow the route, I find that my HSI doesn't maintain the same heading indication, compared to the gps.  I have to compare the two and make adjustments, via the course knob on the HSI (which I found to be the easiest fix).  Is this discrepancy known as gyro drift or is it something else.  Since I'm flying the default C172SP, it's all about the guages, but the GPS tends to help a lot, especially when course correcting with the HSI.  Last night, during a flight from KLAX to KSAN, I was merely doing a test flight, so no filed flight plan, just straight navigation via the GPS.

 

So I am here to get the straight talk, what are the proper procedures for setting the HSI correctly prior to flight and if my course corrections, via the HSI course knob is ok, or if I need to be doing something else.  I haven't progressed to tuning radios yet, but if this is also necessary, I'll add that to my list of things to take care of as well.  I'd been flying in this fashion for a while now, but jsut recently noticed how far off the HSI can get, compared to what the GPS shows.

 

Any assistance is appreciated.

[skelsey 765]

You are correct -- the heading indicator is a useful instrument because it is not subject to the dip, turning and acceleration errors asociated with the magnetic compass, but it drifts because of gyroscopic precession. This means that you have to periodically align it with the magnetic compass.

 

You have to do this roughly every ten to fifteen minutes and you must do so in level flight (because if you are turning, climbing, descending, accelerating or decelerating the magnetic compass reading will be affected).

 

Some aircraft have an instrument which is automatically slaved to the magnetic compass. These are however very expensive, hence why you don't tend to see them in light singles!

[Jimm 437]

Well thank you very much for that info...quite helpful.  So you're basically saying that what I am doing is perfectly fine, correct?  What I'd been doing to help "align" the instrument to the GPS is to navigate to a hard bearing (i.e. 90, 180, 270) and then check it against what the GPS says and then adjust the HSI to match.  It all seems legit so far, and with respect to the C172SP, as compared to other aircraft, this is a little more manual work to be done, which is fine.  I think I really started to see this issue come up when flying a flight plan and the message would come up to make a turn, and of course the heading I set differed from what the GPS was telling me.

[sd_flyer 6,041]

HSI suppose to be slave to you compass. Not much adjustments are necessary. You are probably referring to heading indicator or directional gyro. 

[skelsey 765]

Yep, spot on. You don't really need to turn to a round figure, but I can see how it makes it easier! Incidentally, I'd also cross-check it against the wet (magnetic) compass every so often as well -- the GPS is easier to use, but the magnetic compass despite its limitations is usually the most indestructible source of heading information on the aeroplane.

 

As sd_flyer says, strictly speaking the correct name for the instrument is "heading indicator" -- an HSI (Horizontal Situation Indicator) is slightly different and usually if not always slaved automatically to the magnetic compass.

 

In days gone by even a gyro heading indicator was a luxury -- many aircraft built in the 30s to the 50s were originally fitted with "P-type" compasses which were quite substantial pieces of engineering mounted horizontally in front of the pilot. The compass needle was free to rotate and, obviously, always pointed at magnetic north. The bezel was marked with headings and had to be manually rotated by the pilot to align N with the compass needle, thereby indicating the current magnetic heading at the top.