Authorities say the pilot of a single-engine, experimental aircraft was killed when his plane crashed near Elk City, Okla.  The Federal Aviation Administration says the Dahlman-Dower Tailwind W-8 left Sayre about 11:15 a.m. Thursday and disappeared from radar around noon.      Beckham County Sheriff Scott Jay says the FAA called and requested that his office go to the Sayre airport to see if the plane had landed there, but nothing was found. Jay says a rancher who lives near Elk City notified authorities about 1:15 p.m. that he had found a debris field and the crash site near the border between Beckham and Roger Mills counties.  Jay says the pilot was a male but he had no other details.      The FAA's website shows the plane is registered to a Hailey, Idaho, company.