JP256 wrote:..... Apparently, there is an FAR that states that if you have a transponder, it is to be turned on at all times. And since all of the 1090ES ADS-B devices are ON and active whenever the transponder is turned on, it would not be "legal" to turn off the transponder in flight.....

Not sure what has been added or revised in the FAR's since the ADS-B mandate, but I just checked my older FAR-AIM and it looks like FAR 91.215 is applicable here.

http://rgl.faa.gov/Regulatory_and_Guida ... kup/91.215

here's the key portion ( 91.215[c] ):

"(c) Transponder-on operation. While in the airspace as specified in paragraph (b) of this section or in all controlled airspace, each person operating an aircraft equipped with an operable ATC transponder maintained in accordance with Sec. 91.413 of this part shall operate the transponder, including Mode C equipment if installed, and shall reply on the appropriate code or as assigned by ATC."

So maybe the whole works CAN be turned off, as long as you are not operating in airspace(s) which require txp-on ops, all of which is spelled out in 91.215(b)