Accident Overview
On March 31, 2019, at approximately 1630 Pacific daylight time, a Cessna 172L (registration N7033Q) operated as a personal flight under 14 CFR Part 91 was substantially damaged during an accident near Placerville, California. The pilot reported no injuries.
Sequence of Events
The pilot stated that after departing the traffic pattern, the engine began to run rough. He checked throttle position, mixture control, fuel tank selection, and engine oil pressure. When he turned on carburetor heat, the roughness increased without a corresponding increase in engine speed. At an altitude of about 1,500 to 2,000 ft above ground level, the pilot decided to return to the airport. The airplane was unable to maintain altitude and landed in trees approximately 300 ft short of the approach end of runway 23.
Postaccident Examination
Examination after the accident revealed that both fuel tanks and the gascolator were full of fuel, and all fuel lines were secure and unobstructed. The engine air inlet filter was free of debris, and all internal passages of the filter box and carburetor heat ducting were clear. The engine contained 7 quarts of oil, and there was no evidence of catastrophic engine failure. The propeller blades, engine, and ancillary components showed no evidence of impact damage.
The inner surface of the exhaust pipe was coated in black soot, and similar sooting was present on the electrodes of all spark plugs.
The carburetor data tag was partially worn, preventing identification of the manufacturer, though it appeared to be a Marvel-Schebler type with part number 4SPA and serial number 10-3678-12. Examination revealed a fuel leak at the body-to-bowl join, and all four bowl securing screws were loose. The bowl was loose and could be moved by hand; disassembly showed fretting and polishing on the bowl and body join surfaces. The accelerator pump plunger assembly shaft was loose, and its packing and washer had detached and were floating freely on the shaft. The brass-type float had a float level of 3/32 inch, rather than the required 7/32 inch, though it did not leak when tested. Teflon-tape pipe sealing material was present on the threads of the carburetor inlet fuel fitting.
Maintenance History
Maintenance records indicated that the engine was last overhauled in January 2000, about 1,587.8 flight hours prior to the accident. The records did not indicate that the carburetor had been overhauled at that time, nor were there entries for any carburetor work over the subsequent 19 years. The last documented maintenance event was an annual inspection on November 13, 2018, 37.8 flight hours before the accident.
Relevant Service Bulletins
The 4SPA carburetor was subject to several service bulletins. Precision Airmotive Corporation's MSA-3 (1991) established a 10-year overhaul period. Marvel-Schebler Aircraft Carburetors SB-17 (August 12, 2010) directed inspection of the carburetor every 100 hours for body-to-bowl fuel leaks. Lycoming mandatory SB 366C (June 2, 2016) directed similar inspections every 50 hours.
Pilot Observations
The pilot had about 1 hour of flight experience in the accident airplane. He noted that when operating other Cessna 172s, engine speed typically dropped 100–200 rpm during carburetor heat checks, but the accident airplane dropped about 400 rpm. The airplane's owner was aware of this larger drop; pilots had reported it previously, but examinations found no anomalies. The owner recommended leaning the fuel/air mixture when necessary. The before-takeoff checklist in the Cessna 172L Owner's Manual includes a carburetor heat check but specifies only to "check operation," without an acceptable rpm drop range.