At 1830 Eastern time, the business jet received substantial damage when the left engine cowl separated and impacted the horizontal stabilizer during cruise at flight level 250 near Stilesville, Ind. Visual conditions prevailed; the pilot and copilot were uninjured. The flight originated from Abraham Lincoln Capital Airport (SPI), Springfield, Ill., at 1700 Central time and was en route to Dayton, Ohio. The flight diverted to SPI where it landed without incident.