On July 17, 1997, at 2330 eastern daylight time, a Schweizer 269C helicopter, N6129Q, was destroyed during a forced landing to the intersection of Lochraven Boulevard and The Alameda in Baltimore, Maryland. The certificated commercial pilot received minor injuries and the police observer passenger was seriously injured. Visual meteorological conditions prevailed for the local police observation flight that originated at the Martin State Airport (MTN), Baltimore, Maryland, at 2210. A company flight plan had been filed for the flight conducted under 14 CFR Part 91. The helicopter was operated by the Baltimore City Police Helicopter Unit located at MTN. In a telephone interview, the pilot reported that he had responded to a request for assistance on an "alarm call" on Crestlyn Road in Baltimore. The purpose of the flight was to check the roof of the building at that address for the requesting officer. The pilot reported that he was performing right-hand orbits above the building at approximately 500 feet above mean sea level (MSL). The pilot stated that after completing the second orbit: "...there was no warning, no hesitation. All of a sudden there was a loud 'clunk', the aircraft gave a 'jerk', then it was just quiet. I went from making the turn to looking for a place to land. I let the collective down a little bit, not all the way, then I saw the street and I gave it a little [collective] to clear the trees...there was nothing left at the bottom, it was like we fell through." In a written statement, an Officer of the Baltimore City Police Department identified N6129Q with the call sign "Fox." The officer reported that while observing the helicopter: "The motor stopped running in Fox...Fox went east and fell to the ground." The pilot reported that there were no mechanical deficiencies with the helicopter prior to the accident. The pilot stated, "The aircraft was fine. Everything was in the green." The helicopter collided with the trees and impacted the ground, coming to rest on the helicopter's right side. A preliminary examination of the wreckage revealed a connecting rod protruding through a hole in the top of the engine crankcase. RESEARCH AND TESTING On July 22, 1997, an examination of the helicopter's engine was conducted by a representative of the Textron-Lycoming Company at the Martin State Airport, Baltimore, Maryland. The examination was supervised by an Aviation Safety Inspector (Airworthiness) of the Federal Aviation Administration. The Inspector reviewed the Accident Investigation Engine Disassembly Report forwarded by Textron-Lycoming and concurred with the findings. The engine was an HIO-360-D1A that had accumulated approximately 500 hours since its last major overhaul. The overhaul was completed October 18, 1996. According to the disassembly report, the #4 connecting rod was protruding through the engine case. During disassembly of the engine, the #3 connecting rod bolts were removed intact, and measured after removal. The bolt opposite the number position side, was stretched .0005 inches over the maximum allowed. The #3 and #4 hydraulic tappets displayed a part number not designated by the parts book (PC 406-C) for this engine. According to the summary section of the report: "Review of the...overhaul records indicated the following: a. On the page identified as "Parts Replaced", connecting rod bolt part number LW-12596 is listed. The quantity which appears to be (2) is crossed out and a (8) is written in. b. On the page identified "Magnaflux and Zyglo", the connecting rod bolts and nuts are checked (ok). c. On the page identified Magnaflux and Zyglo, the Counterweight and [Engine Mount Legs] are checked (ok). The HIO-360-D1A engine does not use counterweights and does not have separate [Engine Mount Legs]. d. On the Preliminary Inspections Records page, as above, Counterweights, [Engine Mount Legs] and Connecting Rod Bolts checked off as "ok". e. There are two pages that refer to compliance of Service Bulletins, Service Instructions, Service Letters, and AD Notes. Textron Lycoming Service Bulletin 240, which is the "Mandatory Replacement of Parts at Normal Overhaul", is not listed." The #4 connecting rod, 4 pieces of bearing shells, and a nut and bolt assembly from the #4 cylinder were examined in the NTSB Materials Laboratory on April 7, 1998. According to NTSB Report No. 98-77: "Both fracture faces on the connecting rod arm were subjected to severe post-separation mechanical damage. However, an examination with the aid of a low power binocular microscope revealed crack arrest positions and ratchet marks, indicative of fatigue crack progression." Figure 5 of the report uses arrows to denote the galling spots on the fatigue origin areas.