high" airfields in the event of an engine failure. The Metro and Metro II were limited to a maximum weight of in the US and countries using imperial units, and 5,700 kg in countries using SI units. When this restriction was lifted the Metro II was re-certified as the Metro IIA in 1980 with a maximum weight of and the Metro II's TPE331-3 engines replaced by -10 engines of increased power. The SA227-AC Metro III followed, also initially certified in 1980 for up to , this increasing to as engines and structures were upgraded. An option to go as high as was offered. Externally, improvements incorporated into the Metro III were a increase in wing span, four-bladed props, redesigned "quick-access" engine cowlings and numerous drag-reducing airframe modifications, including landing gear doors that closed after the gear was extended. Once again a corporate version was offered as the Merlin IVC (the model name was chosen to align with the contemporaneous short-fuselage Merlin IIIC). A version with strengthened floors and the high gross weight option was offered as a cargo aircraft known as the Expediter. Both the Expediter and the Merlin IVC were designated the SA227-AT. Finally, due to reliability problems with Garrett engines in the second half of the 1980s, the Metro IIIA was offered with two Pratt & Whitney Canada PT6A-45R turboprops in place of the Garrett units; however none were actually delivered. A special model was the SA227-BC Metro III built for Mexican airline AeroLitoral, which took delivery of 15 of the 18 of this model that were produced. Improvements beyond the Metro III provided better systems, more power and a further increase in takeoff weight. This design effort resulted in the SA227 CC (for Commuter Category) and SA227-DC models, initially called the Metro IV then renamed Metro 23, so named as they were designed for certification under FAR Part 23 (Amendment 34) standards. A Metro 23 EF with an external pod under the lower fuselage for greater baggage capacity was also offered as well as an Expediter 23 and Merlin 23. The SA227-CC was an interim model with TPE331-11U engines and only a handful were built. Further development In the 1960s, Swearingen Aircraft developed a prototype SA-28T eight-seat jet aircraft with a flapless delta wing. It shared the tail and cockpit with the Merlin/Metro. The two engines were to be Garrett TFE731 turbofans then in development; they were