14 CFR § 135.158

Except as provided in paragraph (b) of this section, after April 12, 1981, no person may operate a transport category airplane equipped with a flight instrument pitot heating system unless the airplane is also equipped with an operable pitot heat indication system that complies with § 25.1326 of this chapter in effect on April 12, 1978.

A certificate holder may obtain an extension of the April 12, 1981, compliance date specified in paragraph (a) of this section, but not beyond April 12, 1983, from the Executive Director, Flight Standards Service if the certificate holder—

Shows that due to circumstances beyond its control it cannot comply by the specified compliance date; and

Submits by the specified compliance date a schedule for compliance, acceptable to the Executive Director, indicating that compliance will be achieved at the earliest practicable date.