Richard Santulli, the Wall Street mathematician who left Goldman Sachs to form NetJets in 1986, stepped down Tuesday as Chairman and CEO. Santulli is credited with inventing the fractional aircraft ownership business model. Fueled by the Dotcom boom of the early 1990s, the NetJets program soared to huge success in that decade, and the company was subsequently purchased by Warren Buffett's Berkshire Hathaway. Santulli remained in control of the company and the concept he founded. Subsequent expansion to Europe and a few down business cycles have challenged NetJets' profitability over the years. But Buffett has always stood firmly behind the company he knew first as a customer, then as owner. Santulli will remain as a consultant to NetJets for the next 12 months, advising interim CEO David Sokol. Santulli, an avid thoroughbred enthusiast among other interests, also said he plans to, "spend some more time with my young family and pursue other interests." In expressing his reluctance to accept Santulli's resignation, Buffett wrote in a prepared statement, "Richard Santulli is synonymous with the fractional jet ownership industry and his vision and energy has made NetJets the leader it is today."