Narrative:

While practicing aerobatics, I thought I smelled gas. I landed the aircraft at ZZZ for inspection an found nothing wrong. I added fuel and continued to practice aerobatics. After about 10 mins I again thought I had the faint odor of AVGAS so I discontinued the flight and returned to ZZZ1 (my home base). I then opened all access doors and panels looking for signs of leaking fuel and found no evidence of same. I did a test flight and everything appeared to be fine. I then flew back to ZZZ took on more fuel and went to my practice area over the water in the area to continue aerobatic flight. This time everything went normally until a snaproll at the top of a loop was performed at approximately 2300 ft. At this time fuel fumes filled the cockpit and raw fuel was seen in the cockpit. As the aircraft accelerated down to the backside of the loop, my vision was blurry and the fumes were so bad I was holding my breath. I tried to open the canopy but could not get it to move, and then secured all electrical power, pulled the emergency fuel shutoff valve and also secured the engine with the magnetic switch. My speed at this point passed 250 mph and a low G pull-up was initiated. Recovery altitude was extremely low so as to avoid high G loading at extremely high speed simultaneously. Airspeed was then traded for altitude in a zoom climb. I determined that I could not glide to the nearest airport, so the engine was restarted with all electrical power off. Landing safely at ZZZ, I discovered a (now) quite significant crack in the aerobatic fuel tank. A temporary repair was performed on this crack and the aircraft refueled and flown back to ZZZ1 without further incident. The fuel tank was then fully repaired by an a & P mechanic. Post incident inspection determined that a static grounding tab welded to the tank had more than likely had excessive stress applied to it from a bonding strap that did not have enough slack. During repeated high G aerobatic flight, the lack of slack in the bonding strap caused it to pull on the welded on tab attached to the fuel tank which then caused the beginning of the crack (and resultant fuel leak) to form. The faint initial fuel fume smells that I experienced earlier were when the crack was just beginning to form and under heavy G loading a very small amount of fuel leaked out. The design of the tank and the location of the crack caused it to finally leak badly only when it was nearly full and negative G was applied. There was nothing wrong with the aircraft canopy, it just was never intended to be opened at extremely high airspeed and there is in fact a warning about attempting to do so, however my fear of an in-flight fire, coupled with partial loss of vision, with the aircraft on a vertical down line and accelerating quickly dictated that an attempt be made to open it. Callback conversation with reporter revealed the following information: the reporter stated that while practicing aerobatics the aircraft was landed and inspected twice for a fuel smell with no leaks detected. The reporter said in a snap roll at 2300 ft the cockpit filled with gas fumes and raw fuel was running under his feet. The reporter stated on the back side of the loop vision became blurry and breathing became impossible. The reporter said the airspeed was 250 knots and recovery altitude was between 50 to 75 ft above the water. The reporter stated after landing maintenance found the aerobatic fuel tank grounding tab weld cracked causing the fuel leak. The reporter said the tank was found shifted from its mountings straining the tank ground strap attached to the tank grounding tab and every heavy 'G' loading applied excessive stress to the tank tab. The reporter stated there are estimated to be 35 to 50 of these yak 50 airplanes operating. The reporter said the tank material is very thin aluminum and should be made of heavier gauge material.

Related reports:

Google
 

Original NASA ASRS Text

Title: A YAK 50 PLT LOST VISION AND BREATHING WAS IMPAIRED BY AVGAS LEAKING INTO THE COCKPIT. CAUSED BY A LEAKING AEROBATIC FUEL TANK LOCATED FORWARD OF THE COCKPIT.

Narrative: WHILE PRACTICING AEROBATICS, I THOUGHT I SMELLED GAS. I LANDED THE ACFT AT ZZZ FOR INSPECTION AN FOUND NOTHING WRONG. I ADDED FUEL AND CONTINUED TO PRACTICE AEROBATICS. AFTER ABOUT 10 MINS I AGAIN THOUGHT I HAD THE FAINT ODOR OF AVGAS SO I DISCONTINUED THE FLT AND RETURNED TO ZZZ1 (MY HOME BASE). I THEN OPENED ALL ACCESS DOORS AND PANELS LOOKING FOR SIGNS OF LEAKING FUEL AND FOUND NO EVIDENCE OF SAME. I DID A TEST FLT AND EVERYTHING APPEARED TO BE FINE. I THEN FLEW BACK TO ZZZ TOOK ON MORE FUEL AND WENT TO MY PRACTICE AREA OVER THE WATER IN THE AREA TO CONTINUE AEROBATIC FLT. THIS TIME EVERYTHING WENT NORMALLY UNTIL A SNAPROLL AT THE TOP OF A LOOP WAS PERFORMED AT APPROX 2300 FT. AT THIS TIME FUEL FUMES FILLED THE COCKPIT AND RAW FUEL WAS SEEN IN THE COCKPIT. AS THE ACFT ACCELERATED DOWN TO THE BACKSIDE OF THE LOOP, MY VISION WAS BLURRY AND THE FUMES WERE SO BAD I WAS HOLDING MY BREATH. I TRIED TO OPEN THE CANOPY BUT COULD NOT GET IT TO MOVE, AND THEN SECURED ALL ELECTRICAL POWER, PULLED THE EMER FUEL SHUTOFF VALVE AND ALSO SECURED THE ENG WITH THE MAG SWITCH. MY SPEED AT THIS POINT PASSED 250 MPH AND A LOW G PULL-UP WAS INITIATED. RECOVERY ALT WAS EXTREMELY LOW SO AS TO AVOID HIGH G LOADING AT EXTREMELY HIGH SPEED SIMULTANEOUSLY. AIRSPEED WAS THEN TRADED FOR ALT IN A ZOOM CLB. I DETERMINED THAT I COULD NOT GLIDE TO THE NEAREST ARPT, SO THE ENG WAS RESTARTED WITH ALL ELECTRICAL POWER OFF. LNDG SAFELY AT ZZZ, I DISCOVERED A (NOW) QUITE SIGNIFICANT CRACK IN THE AEROBATIC FUEL TANK. A TEMPORARY REPAIR WAS PERFORMED ON THIS CRACK AND THE ACFT REFUELED AND FLOWN BACK TO ZZZ1 WITHOUT FURTHER INCIDENT. THE FUEL TANK WAS THEN FULLY REPAIRED BY AN A & P MECH. POST INCIDENT INSPECTION DETERMINED THAT A STATIC GROUNDING TAB WELDED TO THE TANK HAD MORE THAN LIKELY HAD EXCESSIVE STRESS APPLIED TO IT FROM A BONDING STRAP THAT DID NOT HAVE ENOUGH SLACK. DURING REPEATED HIGH G AEROBATIC FLT, THE LACK OF SLACK IN THE BONDING STRAP CAUSED IT TO PULL ON THE WELDED ON TAB ATTACHED TO THE FUEL TANK WHICH THEN CAUSED THE BEGINNING OF THE CRACK (AND RESULTANT FUEL LEAK) TO FORM. THE FAINT INITIAL FUEL FUME SMELLS THAT I EXPERIENCED EARLIER WERE WHEN THE CRACK WAS JUST BEGINNING TO FORM AND UNDER HEAVY G LOADING A VERY SMALL AMOUNT OF FUEL LEAKED OUT. THE DESIGN OF THE TANK AND THE LOCATION OF THE CRACK CAUSED IT TO FINALLY LEAK BADLY ONLY WHEN IT WAS NEARLY FULL AND NEGATIVE G WAS APPLIED. THERE WAS NOTHING WRONG WITH THE ACFT CANOPY, IT JUST WAS NEVER INTENDED TO BE OPENED AT EXTREMELY HIGH AIRSPEED AND THERE IS IN FACT A WARNING ABOUT ATTEMPTING TO DO SO, HOWEVER MY FEAR OF AN INFLT FIRE, COUPLED WITH PARTIAL LOSS OF VISION, WITH THE ACFT ON A VERTICAL DOWN LINE AND ACCELERATING QUICKLY DICTATED THAT AN ATTEMPT BE MADE TO OPEN IT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT WHILE PRACTICING AEROBATICS THE ACFT WAS LANDED AND INSPECTED TWICE FOR A FUEL SMELL WITH NO LEAKS DETECTED. THE RPTR SAID IN A SNAP ROLL AT 2300 FT THE COCKPIT FILLED WITH GAS FUMES AND RAW FUEL WAS RUNNING UNDER HIS FEET. THE RPTR STATED ON THE BACK SIDE OF THE LOOP VISION BECAME BLURRY AND BREATHING BECAME IMPOSSIBLE. THE RPTR SAID THE AIRSPEED WAS 250 KNOTS AND RECOVERY ALT WAS BTWN 50 TO 75 FT ABOVE THE WATER. THE RPTR STATED AFTER LNDG MAINT FOUND THE AEROBATIC FUEL TANK GROUNDING TAB WELD CRACKED CAUSING THE FUEL LEAK. THE RPTR SAID THE TANK WAS FOUND SHIFTED FROM ITS MOUNTINGS STRAINING THE TANK GND STRAP ATTACHED TO THE TANK GROUNDING TAB AND EVERY HEAVY 'G' LOADING APPLIED EXCESSIVE STRESS TO THE TANK TAB. THE RPTR STATED THERE ARE ESTIMATED TO BE 35 TO 50 OF THESE YAK 50 AIRPLANES OPERATING. THE RPTR SAID THE TANK MATERIAL IS VERY THIN ALUMINUM AND SHOULD BE MADE OF HEAVIER GAUGE MATERIAL.

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.