14 C.F.R. § 121.342
Pitot heat indication systems
No person may operate a transport category airplane or, after December 20, 1999, a nontransport category airplane type certificated after December 31, 1964, that is equipped with a flight instrument pitot heating system unless the airplane is also equipped with an operable pitot heat indication system that complies § 25.1326 of this chapter in effect on April 12, 1978.