On April 15, 1999, at 1530 hours Pacific daylight time, a Piper PA-31-310, N57AS, sustained substantial damage to the horizontal stabilizer when the right engine upper cowling separated in flight near La Verne, California. The aircraft was owned and operated by Air Desert Pacific Corporation of La Verne, and was on a postmaintenance test flight under 14 CFR Part 91 of the Federal Aviation Regulations. Visual meteorological conditions prevailed and no flight plan was filed for the local area flight. The commercial pilot, the sole occupant, was not injured. The flight originated from the Bracket Airport in La Verne at 1525. The operator did not report the accident to either the Safety Board or the Federal Aviation Administration (FAA). FAA inspectors from the Los Angeles Flight Standards District Office discovered the damaged aircraft during an airport visit. The inspectors interviewed the pilot and the operator's maintenance personnel. According to the FAA inspector's report, the aircraft had been in maintenance for an adjustment to the right engine manifold pressure differential controller. The procedure required the uncowling of the engine. A certificated airframe and powerplant mechanic removed the cowling and made the required adjustments, then left for the day. A noncertificated mechanic replaced the cowling and the pilot then proceeded to perform the postmaintenance test flight. During climbout, the right engine upper cowling separated and wrapped around the right horizontal stabilizer. The pilot experienced difficulty in controlling the aircraft until the airspeed was reduced, then he returned to the airport and landed. Inspection of the cowling disclosed that the inboard latches on the upper cowling were not damaged. In his written statement, the pilot notes that his failure to detect the unlatched inboard cowling fasteners resulted from "poor preflight inspection habits and a failure to follow the checklist."