On September 16, 2006, about 1600 Alaska daylight time, a float-equipped Piper PA-12 airplane, N593V, sustained substantial damage when it collided with trees during a forced landing on a small lake, at Soldotna, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country flight under Title 14, CFR Part 91, when the accident occurred. The airplane was operated by the pilot. The commercial certificated pilot and the sole passenger were not injured. Visual meteorological conditions prevailed. The flight originated from Border Lake, near Clam Gulch, Alaska, about 1545, and was en route to Shadura Lake, near Kenai, Alaska. No flight plan was filed for the personal flight. A Federal Aviation Administration (FAA) inspector, Anchorage Flight Standards District Office (FSDO), Anchorage, Alaska, examined the airplane after it was recovered. The inspector reported that the airplane received structural damage to the leading edge of the wings, wing rib damage, and damage to a wing lift strut. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on September 26, the pilot reported that he was in cruise flight about 800 feet above the ground, when the engine began to run rough and lose power. The pilot indicated that he observed 1/4 full on the right wing fuel tank sight gauge. He applied carburetor heat and moved the fuel mixture to rich. The engine power momentarily improved, but then decreased to zero. The pilot selected a small lake for a forced landing spot, but during the landing approach, he had to maneuver the airplane beneath power lines to reach the lake. Just before touchdown on the lake, the airplane collided with several spruce trees. After landing, the pilot secured the airplane and returned the following day to begin recovery. He said he intended to drain fuel from the wing fuel tanks, but he found that the left wing tank was empty, as well as the right wing fuel tank. He also discovered that the right wing fuel indicator, which is a vertically oriented sight gauge tube, was still indicating 1/4 full. Removal of the wings and draining of the fuel revealed about two gallons of fuel remained in the fuel system. In the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) submitted by the pilot, the pilot indicated that the amount of fuel on board at takeoff was 10 gallons. The pilot said that he purchased the accident airplane in September, 2005. The left wing fuel tank quantity indicator was an original float-type rod. The right wing tank quantity indicator was installed in the airplane in May, 2005, under Supplemental Type Certificate (STC) SA02115AK, for use in Piper Aircraft equipped with an F. Atlee Dodge 30.5 gallon fuel tanks. The STC utilizes a "U" shaped, clear segment of plastic tubing installed vertically through a trim panel on the inside of the cockpit, at the inboard end of the wing root. The upper and lower ends of the tubing have a 90 degree bend at each end to match the distance between the fuel tank's upper and lower outlets. The clear tubing functions as a visual column indicator (sight gauge) of the tank's fuel level. The installation instructions for the sight gauge include the installation of a length of synthetic fuel hose between the ends of the sight gauge, and the fuel tank outlets. The length of the hose is not specified in the STC, only stating, "as required." During the examination of the airplane by the FAA inspector, the pilot and a mechanic noted that they found an excessivly long length of fuel hose between the lower sight gauge attach point, and the lower fuel tank outlet. The length of hose was arched upward between the two points of attachment. The calibration instructions for the sight gauge include establishing fuel quantity level marks for the airplane when it rests at a three-point attitude, and another set of fuel quantity marks for the airplane when it is at a level attitude. These marks are shown as being placed along the surface of the trim panel, on either side of the clear sight gauge tube. The instructions do not have any requirement for marking the surface of the sight gauge, however, the sight gauge in the accident airplane had horizontal lines drawn at several points along its length.