SYNOPSIS At dusk on Tuesday, August 24, 1993, at about 1720 edt, a Mooney M20B, N74580, owned and operated by Thomas N. Thompson of Lorton, Virginia, encountered a severe vibration after a portion of a propeller blade separated. This occurred while the airplane was on an ILS approach to Andrews AFB, Maryland. The pilot reduced power, then shut down the engine to reduce the vibration. He then made a forced landing on hilly terrain about 7 miles north of Andrews AFB at Landover, Maryland. The airplane was substantially damaged, but the pilot, who was the sole occupant, was not injured. Visual meteorological conditions prevailed and no flight plan was filed. The local flight originated at Hyde Field, Clinton, Maryland; it was conducted under 14 CFR 91. HISTORY OF FLIGHT The pilot reported that after taking off from Hyde Field on a local flight, he elected to practice instrument approach(es) at Andrews AFB. He obtained vectors for an ILS runway 19R approach. As the airplane was inbound to the outer marker, the pilot heard a noise, which was accompanied by a severe engine vibration. After shutting down the engine, he noticed the tip of one propeller blade was missing, although the propeller was windmilling. Also, he noted that the right aileron had become partially detached and that he was having a problem with roll control. He selected an open field for an emergency landing and retracted the landing gear to reduce drag, so the airplane would reach the field. After clearing obstructions, he made a wheels up landing in the field. PERSONNEL INFORMATION The pilot, Thomas N. Thompson, held a private pilot certificate with single engine land and instrument ratings. His last rating was obtained on October 28, 1992, which qualified as a biennial flight review. His latest medical certificate was issued with no waiver or limitation. AIRCRAFT INFORMATION The airplane, a Mooney M20B, N74580, was manufactured in 1961 and was properly certified in the standard (normal) airworthiness category. The owner, Mr. Thompson, had recently purchased the airplane; it had been registered to him on August 2, 1993. The airplane was being maintained under an annual inspection program and was last inspected on June 22, 1993. It had been flown about 26 hours since the last inspection and had accumulated a total flight time of about 2340 hours. The airplane was equipped with a Lycoming O 360 engine and a McCauley, Model 2D36C14 BE propeller. Serial number of the propeller hub was 611771. The engine had accumulated about 1269 hours, since major overhaul and it had been operated about 26 flight hours since the last annual inspection. WRECKAGE AND IMPACT INFORMATION After clearing trees, the pilot landed the airplane in a wheat field on hilly terrain. There was no apparent damage to the field as the wheat crop had been harvested and only wheat stubble and volunteer grass and weeds remained. Initial touchdown was in a south southwesterly direction on upsloping terrain with the gear and flaps retracted. There was evidence the airplane bounced during touchdown. It then slid to a stop and came to rest on a northwesterly heading. The airplane was substantially damaged during the occurrence. One propeller blade had failed about 15 inches outboard from the propeller hub. The tip of the failed propeller blade was not recovered during the investigation. The outboard portion of the other propeller blade was bent aft; there was evidence that this occurred during the wheels up landing. Examination of the airplane also revealed physical evidence of severe engine vibration. There was evidence that some of the engine components, including the alternator belt and an associated pulley were damaged by the vibration. Scrape marks were found where the engine cowling had contacted a flange behind the propeller hub, and the center and outboard attachment points on the right aileron had failed. ADDITIONAL INFORMATION With permission of the owner, a three inch portion was sawed off the end of the failed propeller blade and sent to the National Transportation Safety Board's metallurgical laboratory for an examination. Metallurgical examination of the fracture surface revealed that the propeller blade had failed from fatigue. A copy of the laboratory report is attached. Custody of the airplane was verbally released to the owner, Mr. Thompson, on August 25, 1993. Records that were retained were mailed to him on about September 26, 1993.