NTSB Factual Report

On June 30, 2012, about 1300 Pacific daylight time, a Tecnam P2002 Sierra light sport airplane, N308TA, sustained substantial damage following a partial loss of engine power and forced landing about 1 mile south of the Watsonville Municipal Airport (WVI), Watsonville, California. The airplane was operated by Ocean Air Flight Services of Watsonville. The certified flight instructor (CFI) and rated private pilot receiving instruction were not injured. Visual meteorological conditions prevailed for the local instructional checkout flight, which was being conducted in accordance with 14 Code of Federal Regulations Part 91, and a flight plan was not filed. The flight was originating at the time of the accident.

In a written statement submitted to the National Transportation Safety Board investigator-in-charge (IIC), the certified flight instructor (CFI) reported that during the takeoff roll the engine was running 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 stated that he immediately took control of the airplane, lowered the nose to avoid stalling, and ensured that the ignition and fuel pump switches were in the correct position. The pilot further stated that he then tried to bring the power to idle, but he did not observe any indication of a reduction in power. The pilot reported that at this time the stall warning came on. As he lowered the nose he added full power, but there was no response from the engine. The pilot opined that with a row of trees in front of him and not able to clear them, he elected to fly between them, which he did, only clipping a few branches with both wings. The pilot subsequently landed in an open field and came to rest in an upright position. The airplane sustained substantial damage to both wings.

In a written statement submitted to the IIC, the student pilot reported that during the initial climb out the engine started to vibrate violently. When the throttle was pulled back to idle the vibration subsided, then when full power was applied there was not sufficient power to climb. The student stated that the CFI then took control of the airplane, and when he applied full power the engine again exhibited a strong vibration. The CFI subsequently landed in a small field and came to a quick stop.

At the request of the IIC, a Federal Aviation Administration (FAA) airworthiness inspector provided oversight as a certified airframe and powerplant mechanic performed an examination of the engine. The FAA inspector reported that the fuel selector valve rotated and had freedom of movement, control continuity was observed between the throttle and the dual carburetors, and that both carburetors were mechanically checked good. The fuel level in the carburetor float bowl was observed to be below the half full mark. The fuel filter showed no signs of any restriction to flow. No discrepancies were found with the engine driven fuel pump. The spark plugs were clean and showed no evidence of foul play or that would have been a contributing factor to the loss of power. The gascolator was full of fuel and showed no signs of water or sediments in the bowl.

During a subsequent inspection performed by a ROTAX engine technician and overseen by the FAA inspector, the inspector reported that there appeared to be a 30 percent to 50 percent blockage of the left main jet on the left carburetor. The inspector stated that while the source of the material could not be determined, he did note that the float inside the carburetor showed signs of flaking on the corner edges of the float.

The left carburetor float, left carburetor main jet, and the unknown material removed from the main jet were sent to the NTSB Materials Laboratory in Washington, D.C. for examination and analysis by an NTSB chemist. The chemist reported that the unknown material and sample from the carburetor float were examined using a Fourier Transform Infrared (FTIR) micro-spectrometer. The spectrometer was used to collect and process infrared wavelength absorbance spectra of each sample. The spectra obtained from the unknown sample suggested that the material was a straight chain, aliphatic hydrocarbon. A visual confirmation of the unknown sample spectrum compared to the spectrum of the known polyethylene carburetor float sample was made. The unknown material sample spectrum matched the known spectrum sample. The unknown solid material found in the carburetor jet was consistent with the known carburetor float material.

A test run of a ROTAX 912ULS engine was accomplished at the Rotech Flight Safety facility in Vernon, British Columbia by a Rotech technician, and overseen by the NTSB IIC.

To simulate the accident aircraft main jet blockage, the size of the main jet was reduced to about 50 percent of its original size and installed in the left carburetor test engine. The engine was started and idled at 2,000 revolutions per minute (rpm). After reaching operating temperatures the throttle was advanced to 3,000 rpm. At this time the engine was observed to shake and run extremely rough with poor throttle response. When the throttle was advanced to the full power setting, it would only reach a maximum of 4,500 rpm. It was also observed to shake and vibrate excessively. Full rpm for the ROTAX 912ULS engine with this propeller pitch is 5,800 rpm.

A second main jet was installed in the left carburetor, with the size of the jet reduced from its original size to simulate a blockage of 25 percent. After the throttle was advanced to the full power setting, the engine produced 5,800 rpm, with no shaking or vibrating noted.

As noted by the ROTAX technician during the examination, the engine logbook indicated that the 2/4 (left) carburetor floats were changed with new floats at 498.8 engine hours on November 1, 2011. The accident occurred about 9 months after the float change, at a total engine time of 804.8 hours, or 306 hours since the float change. ROTAX maintenance instructions state that the carburetors are to be removed and inspected every 200 hours of operation.

The most recent 100-hour annual/condition inspection occurred two days prior to the accident, June 28, 2012. A review of the ROTAX maintenance manual checklist used by the certified airframe and powerplant mechanic who performed the examination revealed 200-hour inspection relative to the removal/assembly of the two carburetors for carburetor inspection had been complied with.

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 current inspection (2 days prior to the accident), as deterioration of the material could be seen throughout.

NTSB Probable Cause

A partial loss of engine power during initial climb due to the blockage of the carburetor’s main jet by deteriorated float material. Contributing to the accident was an inadequate maintenance inspection by maintenance personnel.