AHRS / Heading Indicator Gremlin

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Diamond_Dan
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Re: AHRS / Heading Indicator Gremlin

Post by Diamond_Dan »

SlowFlight wrote:When I was having issues with my HDG X-ing out, these were recommended:

1. Unplug charging accessories from the DC port (I had a poor quality charger for the iPad that created interference).
2. Turn off electronics (iPad, iPhone, etc.) or at least the device's radios (cell, wifi) to rule these out.
3. Recalibrate the magnetometer.
4. Ensure that brass screws instead of ferrous metal screws are securing the inspection port on the outboard wing on which the mag is mounted (right wing on my 2007 XL).
Can anyone give me a standard part number for this brass screw? If I am reading it right, Diamond IPC indicates "DIN7985-M4x12-Al" for a DA40-180 but I can't find this designation on any bolt ordering website. I am getting the HDG X'd out errors again after a year and the calibration did not do the trick this time. I believe my inspection cover screws were mistakenly replaced with mild steel so I'm going there next.
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ThomasD
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Re: AHRS / Heading Indicator Gremlin

Post by ThomasD »

DIN7985 is the base spec for the screw, M4x12 is width x length.
In addition to DIN7985 there are further designators following, like 'A2' or 'A1' or 'A4' - this is the material.
E.g. see
https://www.pts-uk.com/product_search.p ... erDIN=7985
(this site only has stainless steel so not the right material - I am guesssing that material 'A1' just might be brass?)
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Diamond_Dan
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Re: AHRS / Heading Indicator Gremlin

Post by Diamond_Dan »

DIN7985 is basically a pan-head Philips. A1 (Alpha one) is 303 Stainless. The IPC states Al (Alpha Lima) though I am wondering if it is a typo because I can't find this designation anywhere.

I pulled one of the screws yesterday and it has a very mild attraction to a magnet so I am pretty sure it is stainless of some kind, though I seem to remember it being brass on a previous annual.

I will give Diamond a call tomorrow.
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vontresc
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Re: AHRS / Heading Indicator Gremlin

Post by vontresc »

Ok a couple of things. We had magnetometer gremlins in our Da-40 for a while. Here's what we ended up finding.

The magnetometer is pretty sensitive. As one of the avionics shops pointed out the perfect location for it would be on a string about 30 feet behind the plane.

On startup it can be a bit flaky if near metal buildings and such. Usually it would clear up during taxi.

Check for ferrous screws near the magnetometer. We had an issue with this as well.

Recalibration seemed to help, but we kept having a few pilots that had constant heading failures. Turns out these pilots would fly with the NAV light always on. In our case switching to LED nav lights cured the issue. I presume in our case the lower current draw through the nav light wiring now creates less flux in the magnetometer.
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Re: AHRS / Heading Indicator Gremlin

Post by rwtucker »

Peter,

What kind of heading changes were you seeing with and without NAV lights?
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Re: AHRS / Heading Indicator Gremlin

Post by vontresc »

There was no heading change as far as I recall. It would basically just fail the heading mode with the big red X. Turning off the nav lights would result in the heading indication functioning normally again.

We ended up just replacing the nav bulbs with LED replacements from Aero-lites. So far so good.

This drove me nuts for the better part of 18 months.
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Re: AHRS / Heading Indicator Gremlin

Post by rwtucker »

That sound you hear is me knocking on wood. :D I have yet to be graced with the big red X but I have seen occasional heading peturbations that I did not resolve. We have a lot of secret military testing in this area so I figured that might be the cause. I'll have to think back to see if lights might have been a factor. I fly quite a bit in the evening and night.
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