Narrative:

I departed at XA09. My sister was aboard as passenger and between us had about 40 pounds of baggage. I had taken a full fuel load at the self service pump; just over 20 gals. Fuel seeped out of the fuel vent under the left wing during taxi to run-up; this is normal with full fuel. 2 tanks with 19.5 each 37 gals usable. My plane has a lycoming 0320-A2B. At XD10 approximately 6900 ft; and 20+ mi out from ZZZ I noticed some sputtering. I had throttled backed the engine to 2200 RPM and now it was wavering around 1700 ft and 1900 ft. I checked the primer; locked. Looked at temperature and pressure both fine. I ran the throttle back and it stabilized for awhile at 1700 RPM. Then more sputtering; I had been monitoring ATC and with my sister aboard it was a no brainer; I called a mayday and squawked 7700; reported my position and quick glanced at the fuel and estimated 15 gals (normal for this leg of the journey) when asked. Cycling the throttle got more response but sporadic I pulled back to 20% and set for best glide 60 IAS. My only thinking was fly the plane to the ground for precautionary landing. Again I looked at oil pressure and temperature; both low but fine. I appeared to have some reserve power when needed. I flew hang gliders for a couple of yrs and just used that site picture to locate a suitable site. Local knowledge told me this was far better than XXX or YYY. I saw an excellent opportunity nne of a lake. I stated I was going to put it down in a field with an oak tree in the center. I had reserved some power and with a mental snapshot of the field I swung downwind and used some power briefly to pull me through a 160 degree turn. Pulling the throttle all the way back I feathered in for a landing. I had to use some left rudder to turn away from an approaching ag road while on rollout and came to a stop facing 90 degree left of my original approach. I pulled the mixture to idle cutoff and shut off the magnetos and strobe leaving the radio and transponder on. I radioed my latitude longitude; cell number and no damage to plane persons or property to approach. I shut off the main breaker at XD17 I hopped out to look at the engine; no smoke; no oil splatters; no wires dangling; no holes in the engine. I had 6.5 quarts oil brownish and clear; sumped fuel under the engine; clear and blue. Standing back both fuel caps were on. I removed the right wing gas cap; and couldn't feel any gas but heard some slosh in the trailing edge. I walked over to the left wing standing on the tier 1 tab of the cap was above the rim of the fuel neck the other underneath. Secure but askew. The poh states the caps are non-vented type and must be properly secured or fuel will siphon out and feed unevenly during flight. After clearing it with the sheriffs; fire department and an FAA/NTSB person on a cell I took 10 gals of fuel from the land owner and did a full preflight. Minus my sister and bags I ran up the engine to temperature; checked magnetos; carburetor heat and went through my short field takeoff checklist. Took off climbed and circled to approximately 5500 ft. The engine was running fine and flew to ZZZ. I believe my failure to correctly secure the gas cap led to improper gas gauge reading and positive and negative pressures leading to fuel surges in the wings.

Related reports:

Google
 

Original NASA ASRS Text

Title: CH7A PLT CONDUCTED A FORCED LNDG WHEN ENG BEGAN TO SPUTTER FROM FUEL STARVATION.

Narrative: I DEPARTED AT XA09. MY SISTER WAS ABOARD AS PAX AND BTWN US HAD ABOUT 40 LBS OF BAGGAGE. I HAD TAKEN A FULL FUEL LOAD AT THE SELF SVC PUMP; JUST OVER 20 GALS. FUEL SEEPED OUT OF THE FUEL VENT UNDER THE L WING DURING TAXI TO RUN-UP; THIS IS NORMAL WITH FULL FUEL. 2 TANKS WITH 19.5 EACH 37 GALS USABLE. MY PLANE HAS A LYCOMING 0320-A2B. AT XD10 APPROX 6900 FT; AND 20+ MI OUT FROM ZZZ I NOTICED SOME SPUTTERING. I HAD THROTTLED BACKED THE ENG TO 2200 RPM AND NOW IT WAS WAVERING AROUND 1700 FT AND 1900 FT. I CHKED THE PRIMER; LOCKED. LOOKED AT TEMP AND PRESSURE BOTH FINE. I RAN THE THROTTLE BACK AND IT STABILIZED FOR AWHILE AT 1700 RPM. THEN MORE SPUTTERING; I HAD BEEN MONITORING ATC AND WITH MY SISTER ABOARD IT WAS A NO BRAINER; I CALLED A MAYDAY AND SQUAWKED 7700; RPTED MY POS AND QUICK GLANCED AT THE FUEL AND ESTIMATED 15 GALS (NORMAL FOR THIS LEG OF THE JOURNEY) WHEN ASKED. CYCLING THE THROTTLE GOT MORE RESPONSE BUT SPORADIC I PULLED BACK TO 20% AND SET FOR BEST GLIDE 60 IAS. MY ONLY THINKING WAS FLY THE PLANE TO THE GND FOR PRECAUTIONARY LNDG. AGAIN I LOOKED AT OIL PRESSURE AND TEMP; BOTH LOW BUT FINE. I APPEARED TO HAVE SOME RESERVE PWR WHEN NEEDED. I FLEW HANG GLIDERS FOR A COUPLE OF YRS AND JUST USED THAT SITE PICTURE TO LOCATE A SUITABLE SITE. LCL KNOWLEDGE TOLD ME THIS WAS FAR BETTER THAN XXX OR YYY. I SAW AN EXCELLENT OPPORTUNITY NNE OF A LAKE. I STATED I WAS GOING TO PUT IT DOWN IN A FIELD WITH AN OAK TREE IN THE CTR. I HAD RESERVED SOME PWR AND WITH A MENTAL SNAPSHOT OF THE FIELD I SWUNG DOWNWIND AND USED SOME PWR BRIEFLY TO PULL ME THROUGH A 160 DEG TURN. PULLING THE THROTTLE ALL THE WAY BACK I FEATHERED IN FOR A LNDG. I HAD TO USE SOME L RUDDER TO TURN AWAY FROM AN APCHING AG ROAD WHILE ON ROLLOUT AND CAME TO A STOP FACING 90 DEG L OF MY ORIGINAL APCH. I PULLED THE MIXTURE TO IDLE CUTOFF AND SHUT OFF THE MAGNETOS AND STROBE LEAVING THE RADIO AND XPONDER ON. I RADIOED MY LATITUDE LONGITUDE; CELL NUMBER AND NO DAMAGE TO PLANE PERSONS OR PROPERTY TO APCH. I SHUT OFF THE MAIN BREAKER AT XD17 I HOPPED OUT TO LOOK AT THE ENG; NO SMOKE; NO OIL SPLATTERS; NO WIRES DANGLING; NO HOLES IN THE ENG. I HAD 6.5 QUARTS OIL BROWNISH AND CLR; SUMPED FUEL UNDER THE ENG; CLR AND BLUE. STANDING BACK BOTH FUEL CAPS WERE ON. I REMOVED THE R WING GAS CAP; AND COULDN'T FEEL ANY GAS BUT HEARD SOME SLOSH IN THE TRAILING EDGE. I WALKED OVER TO THE L WING STANDING ON THE TIER 1 TAB OF THE CAP WAS ABOVE THE RIM OF THE FUEL NECK THE OTHER UNDERNEATH. SECURE BUT ASKEW. THE POH STATES THE CAPS ARE NON-VENTED TYPE AND MUST BE PROPERLY SECURED OR FUEL WILL SIPHON OUT AND FEED UNEVENLY DURING FLT. AFTER CLRING IT WITH THE SHERIFFS; FIRE DEPT AND AN FAA/NTSB PERSON ON A CELL I TOOK 10 GALS OF FUEL FROM THE LAND OWNER AND DID A FULL PREFLT. MINUS MY SISTER AND BAGS I RAN UP THE ENG TO TEMP; CHKED MAGNETOS; CARB HEAT AND WENT THROUGH MY SHORT FIELD TKOF CHKLIST. TOOK OFF CLBED AND CIRCLED TO APPROX 5500 FT. THE ENG WAS RUNNING FINE AND FLEW TO ZZZ. I BELIEVE MY FAILURE TO CORRECTLY SECURE THE GAS CAP LED TO IMPROPER GAS GAUGE READING AND POSITIVE AND NEGATIVE PRESSURES LEADING TO FUEL SURGES IN THE WINGS.

Data retrieved from NASA's ASRS site as of January 2009 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.