Going back to the original question in the thread concerning B1900s and Metroliners use as commuter aircraft. The Feds required the aircraft demonstrated performance data for an engine failure at takeoff. The B1900D is certified in the commuter category and has demonstrated performance data for the segmented climb. As of Dec 20, 2010 that data is required for all part 121 aircraft. The DHC-6 twin otter and the Metroliners do not have the required performance data. The B1900C is also in this category, but a supplement to the AFM can be purchased that contains the appropriate performance data and charts.
Reference to 121 regulations
§121.173 General.
(a) Except as provided in paragraph (c) of this section, each certificate holder operating a reciprocating-engine-powered airplane shall comply with §§121.175 through 121.187.
(b) Except as provided in paragraph (c) of this section, each certificate holder operating a turbine-engine-powered airplane shall comply with the applicable provisions of §§121.189 through 121.197, except that when it operates—
(2) Until December 20, 2010, a turbo-propeller-powered airplane described in §121.157(f), the certificate holder may comply with the applicable performance requirements of appendix K of this part.
121.157(f)
(f) Other nontransport category airplanes. No certificate holder may operate under this part a nontransport category airplane type certificated after December 31, 1964, unless it meets the applicable requirements of §121.173 (a), (b), (d), and (e), was manufactured before March 20, 1997, and meets one of the following:
(1) Until December 20, 2010:
(i) The airplane was type certificated in the normal category before July 1, 1970, and meets special conditions issued by the Administrator for airplanes intended for use in operations under part 135 of this chapter.
(ii) The airplane was type certificated in the normal category before July 19, 1970, and meets the additional airworthiness standards in SFAR No. 23, 14 CFR part 23.
(iii) The airplane was type certificated in the normal category and meets the additional airworthiness standards in appendix A of part 135 of this chapter.
(iv) The airplane was type certificated in the normal category and complies with either section 1.(a) or 1.(b) of SFAR No. 41 of 14 CFR part 21.
(2) The airplane was type certificated in the normal category, meets the additional requirements described in paragraphs (f)(1)(i) through (f)(1)(iv) of this section, and meets the performance requirements in appendix K of this part.