ATL88MA053

While en route at FL 370, the plts declared an emerg, rprtd a fire in the rgt eng & reqd vectors to the nearest suitablearpt. They were vectored twd the Lexington bluegrass arpt & clrd to dscnd. Shortly after declaring an emerg, the plts rprtd they secured the rgt eng & extinguished the fire lgt. As the acft was on final apch to lnd, the local ctlr (using binoculars) confirmed that both the gear & flaps were extended. However, as the acft contd onto a short final apch, an increase in its pitch attitude was noted. It then dropped below the lvl of the rwy threshold & impacted in a pasture short of the rwy. After initial impact, the acft went thru a stone fence, hit 2 utility poles, crossed a hiway, came to rest at the perimeter fence & burned. An auto was dmgd by debris; 2 occupants recd minor injuries. An inv revealed the acft impacted with the gear extd, but the flaps & flap handle were fnd in the up psn. An exam of the rgt eng revealed its outer exhaust cone was cracked in the vcnty of a fire detection loop. No otr preexisting mech problems were evident.