On October 19, 1999, at 1615 hours Pacific daylight time, a Cessna 150H, N22795 collided with a ditch and nosed over during a precautionary landing following a rough running engine near El Monte, California. The airplane, operated by Tom King Aerial Enterprises under 14 CFR Part 91, sustained substantial damage. The commercial pilot sustained minor injuries. The purpose of the flight was to conduct banner towing near the Los Angeles Coliseum, Los Angeles, California. The airplane had departed from Chino, California, at 1540, and was en route to the tow area when the accident occurred. Visual meteorological conditions prevailed at the time of the accident and no flight plan was filed. The pilot stated that he was flying westbound over the 60 freeway, just west of the 605 freeway, when he began to have engine problems. He was at cruise flight when the engine began to shake "excessively." He increased the throttle, the vibration increased, and he noted that the engine had lost partial power. He began to review his airplane's emergency checklist in an attempt to isolate or identify the engine problem but he was unable to do so. He completed the items on the checklist, the engine began to run even rougher, and he felt the engine might quit all together. The pilot began to look for a suitable landing site to make an emergency landing. He selected the landfill site because he felt it would keep him away from populated areas. On his final approach to land, he touched down and began to apply the brakes, and saw that there was a small trench in his path. When the airplane hit the trench, it flipped over on its wings where it came to rest. The pilot told police officers that he did not see the trench prior to landing because of its contour and coloring, which made it difficult to see. Review of the maintenance records disclosed that a Lycoming O-360-A4M engine was installed in accordance with STC SA 4795SW. The engine had accumulated a total time of 3,683 hours, with 1,348 since major overhaul. A postaccident examination of the engine was conducted by a Safety Board investigator. The tachometer in the airplane read 1,438.84 hours. During recovery of the airplane, 14 gallons of fuel was found in the right main tank and 10 gallons found in the left main tank. External examination of the engine revealed no apparent damage except to the bottom spark plug hole of the number two cylinder. The case numbers matched. The crankshaft was rotated by hand with accessory gear and valve train continuity established. Thumb compression was developed in each cylinder. The bottom spark plugs were Champion REM 37BY and the gaps were measured at 0.022 inches, with ovaling of the center electrodes evident. The number 3 and 4 spark plugs were oil soaked. The number 1 spark plug had deposits of burned and dried oil on it. The right magneto was timed at 25 degrees before top dead center. The left magneto timing was 24 degrees. Rotation of the crankshaft produced a bright blue spark from each magneto's ignition lead in the correct cylinder firing order. The number 2 bottom spark plug was found out of the cylinder but was retained by the ignition lead. The helicoil insert remained around the threads of the spark plug. The corresponding bore in the cylinder head for the helicoil insert had a stripped appearance. The number 2 bottom plug electrode gap was about 0.022 inches and the color was a light gray. The top spark plugs were Champion REM 38E. All electrodes exhibited a light gray color and the gaps appeared to be about 0.022 inches. Close examination of the number 2 cylinder head revealed a crack from the bottom spark plug bore to the exhaust valve seat.