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Encoder and Magnatometer question

Postby Rich_Klarich » Wed Nov 05, 2014 9:04 pm

2 issues with a Sport 200SX.

Magnatometer flags as failed 1/2 the time after an engine start (Alternator off). IF I turn off the SX200 via the main master battery switch, then restart by repowering the master, which I've done straight to alternator since the engine is running (3 position master switch, down being off, center is battery, up is alternator), all is fine. Ideas?

Second- Encoded readout is about 300' LOW. Transponder is a King 76C with the mode C readout on it's faceplate.

The aircraft altimeter is well calibrated to over 10,000' msl.

I can't tell from reading the manual and I didn't build the panel, so I don't know how the encoded altimeter gets to the 76C, whether it is grey code or serial and what if any difference there is.

It must be correct, as it seems to function, but I can't for the life of me figure out why the actual EFIS altimeter isn't what is sent out from the aircraft- OR- is it going out at 29.92 and ATC's computer's correct it for local pressure? This might be the case as it was lower than normal indication on the 76C when there was a 30.40 high pressure altimeter setting last week.

I didn't think to ask ATC what they were showing.
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Re: Encoder and Magnatometer question

Postby katieb » Wed Nov 05, 2014 10:13 pm

In the EFIS installation manual, we recommend wiring the EFIS to a separate Avionics master switch to isolate it from the variations in voltage associated with engine start. The manual states that backlight life can be shortened by these fluctuations. It should also say that the AHRS can have trouble with alignment if the engine is started with the EFIS on and connected to the master circuit. Because you don't have an avionics master, your standard procedure should be to restart the EFIS system with the master switch after engine start. If you have the AHRS on a pullable circuit breaker, pull it out before engine start and push it in after the starter deactivates. I've heard of other panels without an avionics master built by a professional panel builder, and wondered why anyone would wire it this way. It's just good form to keep sensitive electronics isolated from the starter circuit.

The AHRS restart suggestion and a stronger push for an avionics master are both listed as something to add to the next revision of the installation manuals.

I can't answer your transponder question. Hopefully someone with experience with it can chime in.
Katie Bosman
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Jabiru USA Sport Aircraft
GRT Tech Support/Marketing 2012-2014
GRT Pilot Since 2008
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Re: Encoder and Magnatometer question

Postby Bobturner » Wed Nov 05, 2014 10:53 pm

Mode C and S transponders squawk pressure altitude. This is because many aircraft have blind encoders which cannot be adjusted. The ATC computer adjusts the pressure altitude for local altimeter settings.
Set the EFIS altimeter setting to 29.92 and see if the transponder now reports correctly (you many notice the transponder readout is usually labeled "FL", e.g, Flight level, which is always pressure altitude.)
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Re: Encoder and Magnatometer question

Postby Rich_Klarich » Wed Nov 05, 2014 11:52 pm

Bob and Katie,
Thank you.

I agree about the EFIS and thanks for the 2992 check on the encoded FL.

I do have a pullable breaker for the EFIS, separate from the EIS4000 breaker, so I can pull the EFIS breaker at start until I fit an avionics master or EFIS master.

The starter is a normal prestolite and good Concorde 25RG battery.

Hopefully a good data point to baby the EFIS magnatometer at engine start. I do deselect the alternator before shutdown and have a zeftronics voltage regulator with the overvoltage relay, a pretty straightforward setup. 35amp denso alternator.
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Re: Encoder and Magnatometer question

Postby Rich_Klarich » Mon May 11, 2015 8:46 pm

You probably get this a lot. Though the Sport passes all the magnatometer ground checks for control movement, I was losing winds and more with excessive high and low inflight readings on the status page. At condition inspection I brought an old aircraft compass and went around the tail (magnatometer in front of vertical Stab, builder said it did not work in the wings for him, there are fuel tanks and pumps out there).

Found 2 feet of the left rudder cable strongly magnetized near the tail- it would swing the compass 180 degrees from 4 inches. Near the magnatometer, guessing a magnetic pickup tool was in contact with it at some point in its past.

Solution- drop the rudder cable at the rudder control bar and undo 10 feet of it at the fairleads/passthroughs.

Take a $12 bath exhaust fan apart for the shaded pole motor. Test far away from aircraft and magnatometer and compass without a watch on- practicing demagnetizing objects. The nice thing is the cable ends fit though the gap as it is nice and large compared to most permanent magnet demagnetizing tools.

2 passes over the affected area of the cable and it passes the compass check.

There was no rhyme or reason as to the inflight working and failing before- the rudders moving would not obviously show up as a problem on the magnatometer fields.

This is a 120v item not used for it's actual design, so I take no responsibility for any accidents!

Hope this helps someone. It's nice being able to demag or mag tools, too.
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