Narrative:

I fly an small aircraft. The practice area is 500-800' high terrain so I set my altimeter so that at 1000' MSL the altitude reads zero. This gives me terrain clearance. I was in the process of a 'push out' hammerhead, (approximately 4 1/2 negative G) when the weight of the gas under negative G load pushed the cap out of the tank. My enclosed cockpit was immediately doused with gas rushing out of the tank (directly ahead of the windshield) and blasted on me by the propeller wash. It looked like I was under water and I couldn't see anything. I rolled up right using control inputs from memory and the windshield cleared immediately as the gas quit coming out of the tank. I was shook up, worried about fire as the entire fabric airplane was soaked with gas , and most of all... Worried about running out of gas on the way home as I had lost a lot. The fastest way home was through the arsa so I called approach control. They told me to hold 2500' which I did. Unfortunately I was still using the terrain clearance setting on the altimeter and was actually at 3500'. Between the two of us, we finally figured out what was happening and I got back uneventfully. This is how the problem arose. Contributing factors were diversion of attention to my possible problems and forgetting to reset the altimeter. It was discovered when approach said my blind mode C said 3500' and I showed 2500'. I corrected the altimeter and he cleared me to 3500'. Callback conversation with reporter revealed the following information. Reporter states that rubber seal on gas cap had apparently worn significantly. He further states that the locking mechanism appeared normal but in fact was not capable of holding the cap in place during negative G maneuvers. This cap is adjustable and possibly could have been checked more closely during preflight. However this is a standard category piece of aircraft hardware and reporter knows of no mfr information or warning that would alert user to the possibility of failure of the cap during flight. Given the potential for disaster this is a serious problem.

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Original NASA ASRS Text

Title: ACFT GAS TANK FILLER CAP FAILURE DURING AEROBATIC FLT. ALT DEVIATION. EXCURSION FROM ASSIGNED.

Narrative: I FLY AN SMA. THE PRACTICE AREA IS 500-800' HIGH TERRAIN SO I SET MY ALTIMETER SO THAT AT 1000' MSL THE ALT READS ZERO. THIS GIVES ME TERRAIN CLRNC. I WAS IN THE PROCESS OF A 'PUSH OUT' HAMMERHEAD, (APPROX 4 1/2 NEGATIVE G) WHEN THE WT OF THE GAS UNDER NEGATIVE G LOAD PUSHED THE CAP OUT OF THE TANK. MY ENCLOSED COCKPIT WAS IMMEDIATELY DOUSED WITH GAS RUSHING OUT OF THE TANK (DIRECTLY AHEAD OF THE WINDSHIELD) AND BLASTED ON ME BY THE PROP WASH. IT LOOKED LIKE I WAS UNDER WATER AND I COULDN'T SEE ANYTHING. I ROLLED UP RIGHT USING CONTROL INPUTS FROM MEMORY AND THE WINDSHIELD CLRED IMMEDIATELY AS THE GAS QUIT COMING OUT OF THE TANK. I WAS SHOOK UP, WORRIED ABOUT FIRE AS THE ENTIRE FABRIC AIRPLANE WAS SOAKED WITH GAS , AND MOST OF ALL... WORRIED ABOUT RUNNING OUT OF GAS ON THE WAY HOME AS I HAD LOST A LOT. THE FASTEST WAY HOME WAS THROUGH THE ARSA SO I CALLED APCH CTL. THEY TOLD ME TO HOLD 2500' WHICH I DID. UNFORTUNATELY I WAS STILL USING THE TERRAIN CLRNC SETTING ON THE ALTIMETER AND WAS ACTUALLY AT 3500'. BTWN THE TWO OF US, WE FINALLY FIGURED OUT WHAT WAS HAPPENING AND I GOT BACK UNEVENTFULLY. THIS IS HOW THE PROB AROSE. CONTRIBUTING FACTORS WERE DIVERSION OF ATTN TO MY POSSIBLE PROBS AND FORGETTING TO RESET THE ALTIMETER. IT WAS DISCOVERED WHEN APCH SAID MY BLIND MODE C SAID 3500' AND I SHOWED 2500'. I CORRECTED THE ALTIMETER AND HE CLRED ME TO 3500'. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. RPTR STATES THAT RUBBER SEAL ON GAS CAP HAD APPARENTLY WORN SIGNIFICANTLY. HE FURTHER STATES THAT THE LOCKING MECHANISM APPEARED NORMAL BUT IN FACT WAS NOT CAPABLE OF HOLDING THE CAP IN PLACE DURING NEGATIVE G MANEUVERS. THIS CAP IS ADJUSTABLE AND POSSIBLY COULD HAVE BEEN CHKED MORE CLOSELY DURING PREFLT. HOWEVER THIS IS A STANDARD CATEGORY PIECE OF ACFT HARDWARE AND RPTR KNOWS OF NO MFR INFO OR WARNING THAT WOULD ALERT USER TO THE POSSIBILITY OF FAILURE OF THE CAP DURING FLT. GIVEN THE POTENTIAL FOR DISASTER THIS IS A SERIOUS PROB.

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.