On May 7, 2000, about 1840 Alaska daylight time, a wheel equipped Cessna 170B airplane, N992EZ, sustained substantial damage after colliding with tree-covered terrain, about 6 miles north of Delta Junction, Alaska, at latitude 63 degrees, 59 minutes north, and longitude 145 degrees, 30 minutes west. The airplane was being operated as a visual flight rules (VFR) personal flight under Title 14, CFR Part 91, when the accident occurred. The solo certificated private pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated at the Valdez Airport, Valdez, Alaska, about 1645. During a telephone conversation with the National Transportation Safety Board investigator-in-charge on May 8, the pilot reported that while in cruise flight, about 600 feet above the ground, the engine RPM suddenly dropped to idle. He said that emergency engine procedures did not restore full engine power, and he selected a forced landing area that contained trees. The airplane collided with the trees, and sustained substantial damage to the wings, fuselage, and empennage. A Federal Aviation Administration (FAA) airworthiness inspector from the Fairbanks Flight Standards District Office (FSDO), traveled to the accident scene on May 8. He reported that the airplane came to rest on the lower portion of the engine cowling. He said that after the airplane was lifted off of the engine cowling, he was able to gain access to the outlet port of the exhaust muffler. He said that when he inserted a probe into the muffler, he noted "there was something loose within the muffler." The muffler assembly was subsequently removed from the airplane, and shipped to the NTSB investigator-in-charge. On May 15, the muffler assembly was cut open in the presence of the NTSB investigator-in-charge. An internal inspection revealed extensive thermal destruction of the internal baffling material. The baffling material that remained in the muffler measured about 4 inches by 3 inches. By placing the remaining material in various position over the exhaust port, the NTSB investigator was unable to position it in such a way that a blockage would occur.