If it is installed, it is supposed to be "on" according to FAR 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 §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.

 

Jeff

 

My biggest complaint is (flying the L3 since 2002) frequently intruders don't have their transponder on. When I can get them frequently on unicom and ask they say it is not necessary out side of the 30 mile Vail.