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« on: January 26, 2017, 07:54:29 am »
I think I managed to outwit the ModeC/S + filter last night. I was night flying in VMC on top of an overcast layer, and I am sure there was not a soul about, futher East Midlands Radar were giving me a traffic service and they were quiet, and nothing seen.
I started a descent from 3500 to 3000, at around 500fpm, and within a few seconds got a ModeC alert, red ring @ -100ft. I swiftly hit the altitude hold and as quickly as it appeared it then disappeared. I had a good look out and was so convinced it was my own ModeC transmission I continued descent. On doing so the same thing happened but within around 2 seconds the warning vanished again.
What I think is happening is that the cabin pressure is lagging a few hPa behind the actual barometric pressure as the autopilot starts the descent, then equalises again. So the transponder is reporting a slightly higher pressure.
I found I can sometimes get the same short spurious -100 below by opening my DV window!!
Not sure if this helps in development. On the whole though, the filter is very helpful.