Accident Summary
On February 23, 2011, at 1456 eastern standard time, a Cessna 172G, registration N3740L, was substantially damaged during a forced landing in Clarksville, Florida. The certificated commercial pilot and the passenger were not injured. Visual meteorological conditions prevailed. No flight plan had been filed for the aerial observation flight conducted under 14 CFR Part 91 between Tri-County Airport (1J0), Bonifay, Florida, and Sandy Creek Airpark (75FL), Panama City, Florida.
Pilot Account
According to the pilot, he first took off from 75FL with half-filled fuel tanks and landed about 30 minutes later at Bountstown Airport (F95), Bountstown, Florida, where he completely filled both tanks. He then took off again for a power line inspection flight, landing at 1J0 for a rest break. After taking off to continue the inspection, about 2 hours 45 minutes of flight time after refueling, the engine lost power. The pilot switched the fuel tank selector valve from "Both" to the left tank, then to the right tank, with no improvement. He applied carburetor heat, also with no improvement. He then pumped the primer knob and observed a couple of short bursts of power, but the airplane descended into trees, coming to rest nose-down in a near-vertical attitude. The pilot and passenger released their seat belts and dropped to the ground without injury.
The pilot reported that during the accident flight, the fuel gauges read differently: the left tank gauge indicated about 1/4 full, while the right tank gauge indicated off the full mark, perhaps around 3/4 full. He did not think much of it because the fuel selector was on Both, gauges normally showed greater burn from the left tank, he had seen fuel sending units go bad, and he knew by timing that he had enough fuel. He stated he did not run the airplane out of fuel. When he returned to the accident site with an FAA inspector the next day, there was a strong odor of fuel and fuel leaking from the right wing fuel cap vent.
The pilot stated that the owner informed him that an O-ring in the fuel selector had been replaced prior to the accident.
Post-Accident Examination
Weather recorded at an airport 20 nautical miles north at 1453 included clear skies, calm winds, temperature 26°C, dew point 8°C, and altimeter setting 30.12 inHg. A carburetor icing probability chart did not predict any probability of carburetor icing.
On August 5, 2011, the engine and fuel system were examined at a recovery facility under NTSB oversight. The airplane's wings and empennage had been removed for transport. The propeller and spinner remained attached to the engine, and the engine remained attached to the firewall with no observed damage to the engine mounts.
The right wing root fuel line opening was clear. The left wing root fuel line opening contained a small amount of organic material that appeared to be small, dried flower petals.
The gascolator bowl was dropped, and a small steel ball, approximately 3/16 inch in diameter, fell out. The bowl was otherwise clean. No damage was noted to the ball. The fuel strainer brass screen was clean except for a very small amount of fibrous, lint-like material on the outside.
Engine top spark plugs appeared normal in color and wear. The engine was prepared for a test run. After starting with external power, the engine ran at low power for about 50 seconds before being shut down due to no oil pressure indication. Subsequent turnover revealed positive oil pressure, consistent with an indication problem. The engine was run a second time for about 3 minutes up to about 2,000 rpm, with normal magneto checks.
The external fuel tank was then plumbed to the right wing root fuel line with the fuel selector in Right position. The engine started and ran at low power for about 40 seconds before losing power and quitting. Two additional attempts to start with the selector in Both were unsuccessful. A final attempt bypassed the fuel selector valve, plumbing the external tank directly to the gascolator inlet; the engine started immediately and ran smoothly for about 1 minute up to 2,400 rpm.
Fuel tank finger screens were removed with no evidence of blockage. No fuel tank contamination was observed using a flashlight.
Fuel Selector Valve
The fuel selector valve was disassembled. Debris was found inside the valve, and a small steel ball was missing from the right-tank side of the selector. No internal failures were evident that would have resulted in the ball liberating itself from the valve in flight. According to the airplane manufacturer's illustrated parts diagram, the fuel selector shaft included a cam that pressed against one or two small balls depending on the selection. Each small ball would push a spring-loaded larger ball away from an orifice to allow fuel flow. The smaller balls rested against the cam; if the cam was displaced upward, the smaller balls could fall into the main chamber and pass through the fuel feed line.
The airplane's owner reported that about 10 flight hours before the accident, he and a friend had fixed a fuel leak in the fuel selector by tightening the four retaining screws for the fuel valve cover/gasket. The cover was under significant pressure against the tightening. Neither the owner nor the friend was an FAA-certificated mechanic. When asked about differences in fuel burn after the fix, the owner stated he hadn't flown much and deferred to the accident pilot.
In a follow-up interview, the accident pilot stated he had smelled fuel in the airplane before the fix but not after. He flew the airplane several times after the fix for about 8 hours, and had filled the tanks before the day of the accident but could not recall if one tank took more fuel. He checked the fuel selector positions during preflight and noticed nothing abnormal after the fix.
According to maintenance records, new O-ring seals and a cover gasket were last installed in the fuel selector on July 16, 1992.