At approximately 0614 Central time, the airplane was destroyed after impacting terrain following loss of control while maneuvering near Rachel, Texas. The pilot, sole occupant of the airplane was fatally injured. Instrument conditions prevailed for the Part 135 cargo flight, which originated in San Antonio, Texas, and was destined for McAllen, Texas. Preliminary radar data show the airplane cruising at 9000 feet MSL at 0537 when the pilot requested to descend to 7000 feet. Almost 20 minutes later, the pilot reported losing his vacuum system and then requested a descent to 5000 feet. Radar data depicted the airplane passing through 6600 feet at 0606. At 0613, the pilot requested to divert to Brooks County Airport (BKS), near Falfurrias, Texas. Just prior to the request, radar depicted the airplane in a right turn from a heading of 180 degrees toward 130 degrees. At 0613:59, radar depicted the airplane passing through 4600 feet. The next 12-second radar sweep depicted the airplane passing through 4100 feet. The next radar sweep depicted the airplane passing through 300 feet. At 0625, the automated weather observing system at BKS reported wind from 330 degrees at 4 knots, visibility of 10 statute miles, sky condition overcast at 4200 feet AGL, temperature 72 degrees Fahrenheit, dew point 64 degrees Fahrenheit, and an altimeter setting of 29.95 inches of Mercury.