ASPEN VS COMPASS... This isnt right...

ntbjounin

ntbjounin

Pre-takeoff checklist
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Jounin
So I thought I noticed this on my last flight, but then when I checked it was on the ground and I just assumed I was crazy.... However... Today I was flying North East for 3 hours and KNEW from ALL indications including looking out the window, and the WX from the Garmin 530, and my wx brief before the flight , that I had a wind from the NW. That means I should be heading to the left of my track....

Well, the HSI on the aspen read approximately 9-10degrees different from the compass. So when the compass read 045 (like it does every week), the Aspen showed 054. Not only did it indicate incorrectly... but those wind calculations and track are also relative to that info... So the wind calculator in the aspen was showing a wind from the E.... Well anyone with eyes can tell you that's not true...

The KFC200 AP tracked in GPSS mode just fine b.c obviously that just follows track, and heading doesnt matter.

Uhhh, so I guess my question is... Is the HSI compass resetable? or what? Can I do it in menu mode? I didnt try because I figured if it could be reset, it would be like the airspeeds, you have to be on the ground stopped. So anyway... Anyone have any input? It's not really a huge problem, but I'm curious...

OHHHH! I almost left out an important part... As with every flight, I take the runway, line up on Centerline and see 21 on the pavement, 210 on the compass, HSI, and backup DG, and guess what, both last flight, and today.... They all looked perfect (on the ground).

So.... Whats the deal? I'll be flying 3 hrs home on thurs if you have some tips I can try then.
 
Are you saying the Aspen HSI is out of sync with the Aspen HDG indicator? or both indications on the Aspen have a mismatch with the compass?

There is a method for calibrating the Aspen (I've had it done 3 times). The unit has to be in "Maintenance Mode" which requires a special button push combination known only to Aspen authorized service centers, and the aircraft is swung 360 degrees in two opposite rotations on the ground. Mine was done with a tech in the plane and another on the ramp with a calibrated compass directing the heading changes. Both the Aspen and the compass were adjusted as necessary. The result was a correlation within 4 degrees on all cardinal headings between the Aspen, compass, and TruTrak a/p (which as its own magnetometer) displays. Oddly enough, the TruTrak has a built-in routine that allows it to calibrate itself in flight.
 
heading indicator... you may have actually answered my question already, so thank you. but if anyone else has some ideas... throw them at me.
 
ntbjounin said:
heading indicator... you may have actually answered my question already, so thank you. but if anyone else has some ideas... throw them at me.
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Fly upside down? :D

What does your compass correction card say for 030 and 060 degrees?
 
The Aspen magnetic calibration can be reset and adjusted on the ground. The Aspen has to be put in the correct condition to do it which is something like go to the last menu page available and then hold down a couple or three soft buttons for awhile.

After it's in calibration mode the aircraft is to be driven in a circle and then the fine adjustments can be made.
 
Clark1961 said:
The Aspen magnetic calibration can be reset and adjusted on the ground. The Aspen has to be put in the correct condition to do it which is something like go to the last menu page available and then hold down a couple or three soft buttons for awhile.

After it's in calibration mode the aircraft is to be driven in a circle and then the fine adjustments can be made.
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Don't do it right next to the big steel hangar.
 
but actually.... it reads correctly on the ground.... so uhhh... yea... new thoughts?
 
ntbjounin said:
but actually.... it reads correctly on the ground.... so uhhh... yea... new thoughts?
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What does the 530 read? I didn't see that in there. I'd trust the 530 most of all, and after that I'd look at the compass and the Aspen. The Aspen may be getting fed bad information (which would make sense if your compass is off) and then following that you'd end up with bad numbers.

I've seen several planes where everything lines up great on the ground, but once you're in the air (of course once you've established your straight-and-level cruise flight) the differences in magnetic fields cause the compass to shift 10 degrees or so. Disclaimer: No experience with the Aspen unit, just thinking about what I've seen in other planes.
 
ntbjounin said:
but actually.... it reads correctly on the ground.... so uhhh... yea... new thoughts?
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Per your original post, it was off when the heading was 45 but OK when the heading was 210. That is an indication that you might need to run the compensation algorithm.

If it agrees at all headings on the ground as you swing in a slow circle, and is off on some headngs while flying, then you have a local magnetic field in your airplane that is changing depending on how hard the alternator is charging, or... That suggests that you may want to move some wires or the magnetic sensor.
 
I'd trust the 530 most of all, and after that I'd look at the compass and the Aspen
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Are we mixing up TRK and magnetic heading here? Does the Aspen show TRK?
 
the 530 is irrelevant b/c it doesnt show heading. only track. so obviously it was showing correctly since we were in GPSS mode on the AP and track is all that matters.

the aspen does show track but it shows it as a little blue diamond on the HSI display (much like the little pink diamond on the G1000)

b/c of the heading issue, it appeared that we were tracking upwind of our heading! haha yea right.
 
So did the Aspen track and 530 track match, just the Aspen heading and compass heading mismatch?
 
Ted DuPuis said:
So did the Aspen track and 530 track match, just the Aspen heading and compass heading mismatch?
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FWIW, the Aspen track information is fed directly from the GPS for display, no processing by the Aspen unit other than that required to convert the signal to an visual indication. The Aspen heading indication is driven internally by the unit's magnetometer.
 
Ted DuPuis said:
So did the Aspen track and 530 track match, just the Aspen heading and compass heading mismatch?
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The tracks would match because the Aspen gets its track information from the 530 (or in my case, the 430).

I had the same issue on my way to Nashville. On the ground, the Aspen showed correct heading information. In the air it was 17 degrees off. All of the wind vectors, and all of the true airspeed calculations are based on those figures, so all of them were off. Either the wind was way stronger than forecast, and from a different angle than forecast, or the Aspen was wrong. When I landed in the middle, it read proper on the ground again. When airborne again, it was back to being off. On the way home, everything matched exactly. My compass has always been accurate before, and I still see nothing to suggest it is all of a sudden off.

Darn, someone beat me to it. I take too long to type!
 
Well the thing to figure out is whether the compass or the Aspen is off - I'm trying to think of the best way to do that in flight. You could fly 8 headings (N, NE, E, SE, S...) on the Aspen and find out how the compass varies. If it's a constant difference, that may tell you something useful.
 
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