14 CFR § 135.149 - Equipment requirements: General.
(a) A sensitive altimeter that is adjustable for barometric pressure;
(b) Heating or deicing equipment for each carburetor or, for a pressure carburetor, an alternate air source;
(c) For turbojet airplanes, in addition to two gyroscopic bank-and-pitch indicators (artificial horizons) for use at the pilot stations, a third indicator that is installed in accordance with the instrument requirements prescribed in § 121.305(j) of this chapter.
The following state regulations pages link to this page.