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Hi Peter,
As far as I know the Socata TB20 is metal but has a composite cabin, cowl and tail. Ground Radar is one source and when you fly through a powerful long range beam that can really upset electronics. These are hard to discern because it can happen tens of miles from the site and their antennas turn slowly. But the software should recover. I don't know about aerospace electronics but in medical we must demonstrate that our systems come back to normal within seconds after a defibrillator hit. If they remain in la la land instead of recovering we would not get the agency blessing. Anyway, there is another noise source but this one could only be correlated if you'd record the NAV or GPS data the instant the AP quits. There could be a high powered AM station on the ground. Also, some VHF and UHF TV transmitters use highly directional antennas so you might get hit with the full brunt well after passing a mast. They also concentrate the beam to a very narrow vertical range of just a few degrees, mostly to save energy costs. Therefore, the magnitude of the EMI effect depends on the altitude when you fly through their antenna pattern. Last but not least there are satellite feeder stations for TV and communications which work with a beam width of just a few degrees and point upward. Due to the narrow beam width the field strength can be tremendous. Again, these can often be identified as a cause if the location where the AP fails happens to correlate. Then there is always the chance that a certain data pattern the AP sees upsets the software. But that would be a very bad sign. There is a way to test for at least some of the EMI behavior but it would have to happen in a shielded environment and that can be expensive or hard to find. You can blast the unit with variable frequencies. It is a test that all system have to go through after completing a design. What I do for pre-compliance is a trick that can pinpoint vulnerable spots: I use an EMCO near field probe kit (little loop and point antennas on a stick with a BNC at the end) or just a 2" loop soldered to a coax if I don't have the kit with me. Then I send a few watts into the probe and go over the unit under test in a dousing rod fashion. It is tedious but usually finds the culprit. The oil pressure EMI issue is a bit scary. Does Socata know about that? They should really fix this. Protecting an input from EMI isn't rocket science. If it is legal you could use ferrite toroids and have these affixed on the cable bundle right before the gauge or its electronics box if it has a separate one. 43 material (Amidon) works pretty good at VHF. Even Radio Shack has some but in aircraft I'd stay away from the snap-on cores because they can come off when you hit rough air. Regards, Joerg http://www.analogconsultants.com |
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