Accident Overview
On June 30, 2012, at about 1300 Pacific daylight time, a Tecnam P2002 Sierra light sport airplane, registration N308TA, sustained substantial damage following a partial loss of engine power and forced landing approximately 1 mile south of Watsonville Municipal Airport (WVI), Watsonville, California. The aircraft was operated by Ocean Air Flight Services of Watsonville. The certified flight instructor (CFI) and a private pilot receiving instruction were not injured. Visual meteorological conditions prevailed for the local instructional checkout flight conducted under 14 Code of Federal Regulations Part 91, and no flight plan was filed. The flight was originating at the time of the accident.
Pilot Reports
The CFI reported that during the takeoff roll the engine ran smoothly, the student rotated at 60 knots, and they began climbing. Suddenly the engine sputtered and started vibrating violently, followed by a significant loss of power. The CFI immediately took control, lowered the nose to avoid stalling, and checked that the ignition and fuel pump switches were in the correct position. He then attempted to bring the power to idle but did not observe any reduction in power. The stall warning activated. As he lowered the nose, he added full power with no engine response. Facing a row of trees, he chose to fly between them, clipping a few branches with both wings, and subsequently landed in an open field, coming to rest upright. The airplane sustained substantial damage to both wings.
The student pilot reported that during initial climb out the engine started vibrating violently. When the throttle was pulled back to idle, the vibration subsided; when full power was applied, insufficient power was available to climb. The CFI then took control and upon applying full power the engine again exhibited strong vibration. The CFI landed in a small field and came to a quick stop.
Engine Examination
At the request of the NTSB investigator-in-charge (IIC), a Federal Aviation Administration (FAA) airworthiness inspector oversaw an examination of the engine by a certified airframe and powerplant mechanic. The fuel selector valve rotated freely, control continuity was observed between the throttle and dual carburetors, and both carburetors were mechanically checked good. The fuel level in the carburetor float bowl was below half full. The fuel filter showed no restriction. No discrepancies were found with the engine-driven fuel pump. Spark plugs were clean with no evidence of contributing to the power loss. The gascolator was full of fuel with no water or sediment.
During a subsequent inspection by a ROTAX engine technician, a 30 to 50 percent blockage of the left main jet in the left carburetor was found. The technician noted the float inside the carburetor showed signs of flaking on the corner edges. The left carburetor float, left main jet, and unknown material removed from the jet were sent to the NTSB Materials Laboratory.
Materials Analysis
An NTSB chemist examined the unknown material and a sample from the carburetor float using a Fourier Transform Infrared (FTIR) micro-spectrometer. The spectra from the unknown sample indicated a straight chain, aliphatic hydrocarbon. Visual comparison of the unknown sample spectrum to the known polyethylene carburetor float sample spectrum showed a match. The unknown solid material found in the carburetor jet was consistent with the known carburetor float material.
Engine Test
A ROTAX 912ULS engine was test-run at the Rotech Flight Safety facility in Vernon, British Columbia. To simulate the blockage, the main jet size was reduced to about 50 percent of original and installed in the left carburetor. The engine idled at 2,000 rpm, then when throttle was advanced to 3,000 rpm, the engine shook and ran extremely rough with poor throttle response. At full power, maximum was 4,500 rpm (full rpm with that propeller pitch is 5,800 rpm), with excessive shaking and vibration. A second test with 25 percent blockage allowed the engine to reach 5,800 rpm with no shaking or vibration.
Maintenance History
The ROTAX technician noted that the engine logbook showed the left carburetor floats were changed with new floats at 498.8 engine hours on November 1, 2011. The accident occurred about 9 months later at 804.8 total hours, or 306 hours since float change. ROTAX maintenance instructions require carburetor removal and inspection every 200 hours. The most recent 100-hour annual/condition inspection occurred two days prior to the accident, on June 28, 2012. The ROTAX maintenance manual checklist used during that inspection indicated compliance with the 200-hour inspection relative to removal/assembly of the two carburetors for carburetor inspection.
The ROTAX Investigation Report concluded that, consistent with ROTAX Service Instruction 912-021, the carburetor float in question should have been rejected during the most recent inspection (two days prior), as deterioration of the material could be seen throughout.