MADDOG
Similar to the first generation of the DC-9 family, the second generation uses second-digit notation, with zero for variant names (Series 80) and non-zero for subvariant or version names (Series 81 through Series 88). Because there was only one variant within the second generation, the Series 80 became the family name and the Series 81 through Series 88 became variant or version names. The first Series 80, DC-9 line number 909, made its first flight on October 18, 1979, as the Super 80, which then became the preferred designation for the newly developed aircraft family. Although two aircraft were substantially damaged in accidents, flight testing was completed on August 25, 1980, when the first variant and respectively production model, the JT8D-209-powered Series 81, was certified under an amendment to the FAA type certificate for the DC-9. The flight-testing leading up to certification had involved three aircraft accumulating a total of 1,085 flying hours on 795 flights.
After production of the first generation ended in late 1982, a new designation with McDonnell Douglas initials, MD-80, was proposed as the type designation for the second generation and in July 1983, McDonnell Douglas decided that the Super 80 would be officially designated the MD-80. However, the type designation according to the type certificate (TC) is still the original (DC-9 prefix) to save on certification costs, but could also be provided with the new (MD prefix) written in parentheses, e.g. DC-9-81 (MD-81), DC-9-82 (MD-82), DC-9-83 (MD-83) and DC-9-87 (MD-87). Only the last variant, the MD-88, was officially certified under the MD designation.
The MD-87 and MD-88 made their first flight on December 4, 1986, and August 15, 1987, respectively. Although not certified until October 21, 1987, McDonnell Douglas had already applied for models DC-9-87 and DC-9-87F on February 14, 1985. The third derivative was similarly officially designated DC-9-87 (MD-87), although no nameplates were stamped DC-9-87.
For the MD-88, an application for a type certificate model amendment was made after the earlier changes, so there was not a DC-9-88, which was certified on December 8, 1987. The FAA’s online aircraft registry database shows the DC-9-88 and DC-9-80 designations in existence but unused.
[source: WIKIPEDIA]