Aircraft equipment problem -- small aircraft engine runs rough as though approaching fuel starvation...

Attributes
ACN	307986
Time
Date	199506
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : 1o2
State Reference	CA
Altitude	msl bound lower : 8000 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zoa
Operator	general aviation : instructional
Make Model Name	Mentor (T-34)
Operating Under FAR Part	Part 91
Flight Phase	cruise other
Flight Plan	VFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : instrument pilot : private
Experience	flight time last 90 days : 28 flight time total : 305 flight time type : 45
ASRS Report	307986
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	other flight crewa other other : unspecified
Resolutory Action	flight crew : overcame equipment problem other other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Descending through 8000 ft returning from scott valley (ft jones) to san carlos. I noticed what seemed to be a rough engine over the north shore of clear lake. I immediately turned on the fuel pump and enriched the mixture, then checked the engine gauges. The fuel pump gauges indicated 10 gals in the right tank and less than 5 gals in the left. Before leaving scott valley the tanks were visually inspected and approximately half tanks were confirmed. I then called ZOA and requested an airport vector. They gave me a heading. I was 8 mi from clear lake, lampson field. I requested priority landing over the unicom frequency. Upon landing the propeller stopped and I had just enough energy to clear the runway. I had help pulling the airplane in and fueled it up. I then called the FAA at the mendocino FSS to make sure that I had landed and was alright. I didn't declare an emergency and thus a report was not required, as I was told by the FAA. I topped off the aircraft and was only able to fill 42 gals in the tanks which total 50 gals. The question is how the 8 gals of fuel never reached the engine. Prior to start-up I checked the fuel system and did a normal preflight and run-up. The aircraft operated normally in the run-up and sounded normal. The flight back to sql was uneventful and smooth. Prior to leaving sql I topped off again before heading back to mry. I reported this incident to the flying club manager and maintenance director and then grounded the airplane. They checked it out the next day and found nothing wrong with it. Callback conversation with reporter revealed the following information: the pilot later found that the flying climb mechanics had discovered that an insect nest had clogged the fuel tank vent. This probably caused the fuel to be less than free flowing since the engine pump would be working against a vacuum. The pilot also talked to several other members of the aeronautical club who had lesser, but similar experiences with the same beech mentor (T-34). Since the insect nest discovery no further fuel problems have occurred with the aircraft.

Original NASA ASRS Text

Title: ACFT EQUIP PROB -- SMA ENG RUNS ROUGH AS THOUGH APCHING FUEL STARVATION. LATER, AN INSECT NEST IS FOUND IN THE FUEL VENT SYS.

Narrative: DSNDING THROUGH 8000 FT RETURNING FROM SCOTT VALLEY (FT JONES) TO SAN CARLOS. I NOTICED WHAT SEEMED TO BE A ROUGH ENG OVER THE N SHORE OF CLEAR LAKE. I IMMEDIATELY TURNED ON THE FUEL PUMP AND ENRICHED THE MIXTURE, THEN CHKED THE ENG GAUGES. THE FUEL PUMP GAUGES INDICATED 10 GALS IN THE R TANK AND LESS THAN 5 GALS IN THE L. BEFORE LEAVING SCOTT VALLEY THE TANKS WERE VISUALLY INSPECTED AND APPROX HALF TANKS WERE CONFIRMED. I THEN CALLED ZOA AND REQUESTED AN ARPT VECTOR. THEY GAVE ME A HDG. I WAS 8 MI FROM CLEAR LAKE, LAMPSON FIELD. I REQUESTED PRIORITY LNDG OVER THE UNICOM FREQ. UPON LNDG THE PROP STOPPED AND I HAD JUST ENOUGH ENERGY TO CLR THE RWY. I HAD HELP PULLING THE AIRPLANE IN AND FUELED IT UP. I THEN CALLED THE FAA AT THE MENDOCINO FSS TO MAKE SURE THAT I HAD LANDED AND WAS ALRIGHT. I DIDN'T DECLARE AN EMER AND THUS A RPT WAS NOT REQUIRED, AS I WAS TOLD BY THE FAA. I TOPPED OFF THE ACFT AND WAS ONLY ABLE TO FILL 42 GALS IN THE TANKS WHICH TOTAL 50 GALS. THE QUESTION IS HOW THE 8 GALS OF FUEL NEVER REACHED THE ENG. PRIOR TO START-UP I CHKED THE FUEL SYS AND DID A NORMAL PREFLT AND RUN-UP. THE ACFT OPERATED NORMALLY IN THE RUN-UP AND SOUNDED NORMAL. THE FLT BACK TO SQL WAS UNEVENTFUL AND SMOOTH. PRIOR TO LEAVING SQL I TOPPED OFF AGAIN BEFORE HDG BACK TO MRY. I RPTED THIS INCIDENT TO THE FLYING CLUB MGR AND MAINT DIRECTOR AND THEN GNDED THE AIRPLANE. THEY CHKED IT OUT THE NEXT DAY AND FOUND NOTHING WRONG WITH IT. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING INFO: THE PLT LATER FOUND THAT THE FLYING CLB MECHS HAD DISCOVERED THAT AN INSECT NEST HAD CLOGGED THE FUEL TANK VENT. THIS PROBABLY CAUSED THE FUEL TO BE LESS THAN FREE FLOWING SINCE THE ENG PUMP WOULD BE WORKING AGAINST A VACUUM. THE PLT ALSO TALKED TO SEVERAL OTHER MEMBERS OF THE AERO CLUB WHO HAD LESSER, BUT SIMILAR EXPERIENCES WITH THE SAME BEECH MENTOR (T-34). SINCE THE INSECT NEST DISCOVERY NO FURTHER FUEL PROBS HAVE OCCURRED WITH THE ACFT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Aircraft equipment problem -- small aircraft engine runs rough as though approaching fuel starvation. Later, an insect nest is found in the fuel vent system.