the Twin Bonanza had trouble competing with the similarly capable but substantially lighter Cessna 310 and Piper PA-23, so Beechcraft used the basic Bonanza fuselage and many other Bonanza parts to create the twin-engined Travel Air, which was later developed into the Baron. Operational history The V-tail design gained a reputation as the "forked-tail doctor killer", due to crashes by overconfident wealthy amateur pilots, fatal accidents, and inflight breakups. "Doctor killer" has sometimes been used to describe the conventional-tailed version, as well. However, a detailed analysis by the Aircraft Owners and Pilots Association of accident records for common single-engine retractable-gear airplanes in the United States between 1982 and 1989 demonstrated that the Bonanza had a slightly lower accident rate than other types in the study. Pilot error was cited in 73% of V-tail crashes and 83% of conventional-tail crashes, with aircraft-related causes accounting for 15% and 11% of crashes respectively. However, the study noted that the aircraft had an unusually high incidence of gear-up landings and inadvertent landing gear retractions on the ground, which were attributed to a non-standard gear-retraction switch on early models that is easily confused with the switch that operates the flaps. Models starting in 1984 use a more distinctive relocated landing-gear switch, augmented by "squat switches" in the landing gear that prevent its operation while compressed by the aircraft's weight, and a throttle position switch that prevents gear retraction at low engine power settings. In the late 1980s, repeated V-tail structural failures prompted the United States Department of Transportation and Federal Aviation Administration (FAA) to conduct extensive wind tunnel and flight tests, which proved that the V-tail did not meet type certification standards under certain conditions; the effort culminated with the issuance of an airworthiness directive to strengthen the tail, which significantly reduced the incidence of in-flight breakups. Despite this, Beech has long contended that most V-tail failures involve operations well beyond the aircraft's intended flight envelope. Subsequent analysis of National Transportation Safety Board (NTSB) accident records between 1962 and 2007 revealed an average of three V-tail structural failures per year, while the conventional-tailed Bonanza 33 and 36 suffered only eleven such failures in total during the same 45 years. Most V-tail failures involved flight under visual flight rules into instrument meteorological conditions, flight into thunderstorms, or airframe icing. In addition to the structural issues, the Bonanza 35 has a relatively narrow center of gravity envelope,