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I flew with some friends today and they showed up on my 740 with uAvionix Echo on the ground. After we took off, I noticed there was traffic on the screen but my friends did not show up. In fact they never showed up during the flight. The last time we flew, they showed up the entire flight. Both flights were in the same geographic area. I haven't changed a thing, so I don't know what to make of this. If anything, it would seem the closest traffic would be the most important to display.
You have not provided enough information to take even a reasonable guess.
For starters: What ADSB-out devices do your friends have / what frequencies do they use? What frequencies do your ADSB-in and ADSB-out devices use, and how are they set up? How close were your friends flying (a mile apart? wingtip-to-wingtip?)? What altitude were you flying one the trip you received them as targets, and what altitude were you flying when you didn't? Where were you flying?
ADSB Traffic will likely never be exhaustive, lots of variables in play so hard to diagnose an individual missing aircraft. EG, does other traffic have ADSB out? Does FAA have them in their radar system? Is FAA receiving your ADSB out message? Are you receiving the ADSB ground broadcast? Is the FAA network having a glitch? (I'm not asking for answers here...just listing different reasons traffic will be missing on ocassion)
For anyone else reading this thread, George called and we may have figured out what was going on. They were formation flying most of their flight, and it's possible the flight characteristics of all the aircraft were very close (altitude, position, speed, heading), they may have been considered "ghost traffic", and removed from the display. To test this theory, turn off Ghost Traffic filter (Menu -> Setup -> Alerts & Warning -> Traffic Alerts -> Filter Ghost Traffic
We often fly in formation and have planes disappear off adsb, but since we are primarily using eyesight to maintain the formation, it's not a big deal to us.
I will try the "ghost filter" technique and see how that works. Good idea.