On June 4, 2006, at 1256 Pacific daylight time, a Piper PA-28R-200, N4438T, lost oil pressure during flight and the certified flight instructor (CFI) made a precautionary landing in a field in Alpine, California. During the landing, the landing gear punctured through the wings and resulted in substantial damage. Anglo American Aviation International was operating the airplane under the provisions of 14 CFR Part 91. The CFI and private pilot undergoing instruction were not injured. Visual meteorological conditions prevailed and no flight plan had been filed for the local area instructional flight. The airplane departed from Gillespie Field, El Cajon, California, at 1200. In an interview with the aviation maintenance technician (AMT) immediately following the accident, he stated that the oil sump quick drain valve was replaced on June 4. The AMT flew the airplane to Gillespie Field from Borrego Springs and noted a 2-quart loss of oil. The AMT replaced the oil and retorqued the newly installed valve. The CFI test flew the airplane and did not note any loss of oil. The CFI reported that following the maintenance test flight, he departed with the student on the accident flight and approximately 50 minutes into the flight, the oil pressure decreased and the revolutions per minute were fluctuating. The CFI shut down the engine and performed a forced landing in a field. In an interview with the operator, he reported that the installed quick drain was a Safe-Air P5000. Examination of the quick drain showed that this was the installed drain. The Federal Aviation Administration issued airworthiness directive (AD) 81-11-02 R1 on October 2, 1981. The AD requires that within the next 50 hours time in service or at the next oil change, install two warning placards, one on each side, on the lower engine mount beneath the intake and the exhaust tubes which reads: CAUTION Unapproved oil drain valves can cause inadvertent draining when the gear is retracted- use only Piper approved part. The AD also notes that Piper part number 492-172V is the approved part for this installation. In the closed position, the valve extension is 0.21-inch. The Safe-Air P5000 quick drain valve extension is 1.9 inches.