On September 7, 1995, about 1600 Alaska daylight time, a wheel equipped Cessna 172, N737SP, crashed during a forced landing, about 30 miles southwest of Glennallen, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country personal flight to Gulkana, Alaska, when the accident occurred. The airplane, operated by Alaska Flying Network, Kenai, Alaska, sustained substantial damage. The certificated airline transport pilot received serious injuries. The sole passenger was not injured. Visual meteorological conditions prevailed. No flight plan was filed. The flight originated at the Kenai airport about 1230. The pilot rented the accident airplane from the operator and indicated that the flight was to Girdwood, Alaska, and then to Merrill Field, Anchorage, Alaska. In a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), the pilot reported that he planned to fly to Gulkana to pick up a friend. The pilot did not inform the operator of the change in destination. The airplane's fuel capacity is 43 gallons with 3 gallons as unusable fuel. The airplane had been flown 1.2 hours prior to the accident flight. The pilot indicated that prior to his departure, he did not fuel the airplane but measured the fuel in each fuel tank and departed on the accident flight with 30 to 32 gallons of fuel. According to operators's airplane records and comparison to the airplane's hobbs meter, the accident flight was 2.3 hours. The total flight time since the airplane was fueled was 3.5 hours. The pilot reported that he landed at the Sheep Mountain airstrip (60 miles southwest of Glennallen, Alaska) for lunch after poor weather conditions in the area forced a delay of the flight. About 1530, the pilot again departed for Gulkana. While in cruise flight, about 4,200 feet mean sea level, the pilot indicated that the engine power decreased to idle, but he did not think that the engine quit. Emergency procedures, including activating the engine primer, did not restore engine power. The pilot performed an emergency landing in a remote area. During the landing, the airplane collided with several trees and descended to the ground. The pilot reported that the airplane came to rest inverted. The airplane was reported overdue by the operator and an alert notice (ALNOT) was issued at 2221 by the Federal Aviation Administration (FAA). The operator reported checking weather conditions at Sheep Mountain that were 2,000 feet overcast, with wind conditions of 030 degrees at 10 knots, gusts of 16 to 18 knots. On September 8, 1995, about 0930, the pilot manually activated the airplane's emergency locator transmitter (ELT). Search personnel located the accident site about 1500. Under Supplemental Type Certificate (STC) SA703GL, the airplane's original Lycoming O-320-E2D engine that produced 150 horsepower, was replaced with a Lycoming O-360-A4M engine that produced 180 horsepower. The STC covered O-360-A4 series engines and Sensenich 76EM8S5 and 76EM8SPY propellers. The STC also required a supplement change to the pilot's operating handbook (POH) and a placard on the instrument panel alerting the pilot of the larger engine installation. On June 23, 1995, a Lycoming O-360-A1A engine that also produced 180 horsepower, was installed in the accident airplane. The STC did not cover O-360-A1 series engines. The installation was approved by an FAA, Anchorage Flight Standards District Office (FSDO) inspector. Under airworthiness directive (AD) 69-09-03, Sensenich propellers type 76EM8, that are installed on Lycoming O-360 engines (except A4A and A4G engines) require marking of the engine tachometer with a red arc from 2150 to 2350 RPM to prevent propeller blade tip failures. The accident airplane did not have the required tachometer marking. According to the engine manufacturer, at the minimum allowable fuel flow, a O-360 engine has a fuel consumption rate of slightly less than 6 gallons per hour at 45 percent of rated power to slightly less than 15 gallons per hour at 100 percent power. Operators of similar powered airplanes reported that the engine usually consumes 8.8 to 8.9 gallons per hour in a cruise configuration. On September 13, 1995, the airplane was inspected at the accident site by a representative of the operator's insurance company to assess the retrieval of the airplane. He was accompanied by an FAA certified mechanic. During the inspection of the airplane, about 1 tablespoon of fuel was recovered from the fuel line between the gascolator and the carburetor. The airplane was recovered by representatives of the operator on September 21, 1995. Recovery personnel indicated that no fuel was present in the wing fuel tanks. On October 10, 1995, the airplane was examined by the NTSB IIC and a FAA inspector. At the time of the accident, the engine had accrued 159.4 hours of operation. The examination revealed that the carburetor heat control was in the "ON" position. The mixture and throttle controls were full forward. The fuel selector was positioned on the left fuel tank. The required 180 horsepower engine installation placard was attached to the instrument panel. The carburetor heat, mixture, and throttle control cables were attached to the carburetor. Engine gear and valve train continuity and thumb compression was established by hand rotation. Engine spark was produced by both magnetos. On October 16, 1995, the carburetor was disassembled by an FAA certified mechanic. He reported that no mechanical malfunction was found.