Narrative:

I was on an instrument flight plan in good VMC conditions from hagerstown, md to reading, PA. The air was smooth. About 40 mins into the flight at about 20 mi south of reading, there was a single event of severe turbulence. The aircraft abruptly skewed sideways and fell about 200 ft. This was accompanied by a thumping sound which I attributed to 'oil-canning' of the aluminum airframe. In my opinion, the event was not so rough as to induce structural damage. All engine gauges performed normally. However, within a few mins, I notice both fuel gauges were reading empty. I had just topped both tanks at hagerstown prior to this flight. The aircraft carries 36 gals of fuel yielding 4 hours at 75 percent power at typical cruising altitudes (4000-8000 ft). Since I was already in contact with reading approach, I informed them of my condition without declaring an emergency. Reading asked me if I wished to continue inbound or divert to the nearest airport. I requested the nearest airport which was morgantown, PA, a grass strip primarily used for glider operations. Reading gave me a vector of about 7 mi east which was confirmed by my GPS 'nearest airport function.' as I descended to morgantown for landing, an observer on the ground with a hand-held transceiver advised me I was trailing 'smoke.' the landing at morgantown was without incident and an overhead aircraft asked my status to relay to reading approach control. After shutting down and inspecting the aircraft, I found the r-hand fuel tank cap was off the filler neck and captive only by its safety chain. This cap is an approved vented fuel cap for the aircraft. The reported 'smoke' was atomized gasoline trailing from the open tank. Apparently, the sound I had heard during the turbulence was the fuel cap coming off and hitting the wing. I would have expected the cap to continue to make banging sounds but it had not. Just prior to takeoff at hagerstown, I had personally secured both fuel caps. However, at morgantown, I did notice the r-hand cap seemed a bit looser fit when replaced on the filler neck. There are 2 metal retaining ears on the cap which may be slightly bent to tighten the friction locking action of the cap. After landing at morgantown, there was only a total of 12 gals of fuel remaining. The left tank had fed through to the siphoning right tank. Since I had used about 9 gals of fuel in flight, some 15 gals had siphoned away in only a few mins. Accordingly, I believe total fuel exhaustion would have occurred before reaching reading. Landing at morgantown was definitely the right choice. The lesson learned here is to be more vigilant of observing fuel quantity even during a very short flight with ostensibly full tanks. When I heard the 'thump' sound, I checked all engine gauges but did not immediately check the fuel gauges. It was several mins later when I thought about the fuel gauges and by then, almost all the fuel was gone. My mechanic has examined the fuel cap and has determined it to be airworthy. Callback conversation with reporter revealed the following: reporter states the aircraft was a cessna 170B. His best guess regarding the problem is that the horizontal displacement of the aircraft in the turbulence may have caused the raised portion of the cap, which is somewhat like a small sail, to be turned by the wind and pop off. The reason he heard only 1 bang was probably due to the fact they were at cruise and forward motion kept the cap directly behind the fuel tank entrance. He fueled the aircraft himself and knows that he placed both caps on properly. The only other explanation he can come up with is the 'ears' mentioned in the narrative. All in all a very strange experience.

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Original NASA ASRS Text

Title: C170B HAS GAS CAP COME LOOSE IN FLT RESULTING IN GAS SIPHONING. EMER LNDG.

Narrative: I WAS ON AN INST FLT PLAN IN GOOD VMC CONDITIONS FROM HAGERSTOWN, MD TO READING, PA. THE AIR WAS SMOOTH. ABOUT 40 MINS INTO THE FLT AT ABOUT 20 MI S OF READING, THERE WAS A SINGLE EVENT OF SEVERE TURB. THE ACFT ABRUPTLY SKEWED SIDEWAYS AND FELL ABOUT 200 FT. THIS WAS ACCOMPANIED BY A THUMPING SOUND WHICH I ATTRIBUTED TO 'OIL-CANNING' OF THE ALUMINUM AIRFRAME. IN MY OPINION, THE EVENT WAS NOT SO ROUGH AS TO INDUCE STRUCTURAL DAMAGE. ALL ENG GAUGES PERFORMED NORMALLY. HOWEVER, WITHIN A FEW MINS, I NOTICE BOTH FUEL GAUGES WERE READING EMPTY. I HAD JUST TOPPED BOTH TANKS AT HAGERSTOWN PRIOR TO THIS FLT. THE ACFT CARRIES 36 GALS OF FUEL YIELDING 4 HRS AT 75 PERCENT PWR AT TYPICAL CRUISING ALTS (4000-8000 FT). SINCE I WAS ALREADY IN CONTACT WITH READING APCH, I INFORMED THEM OF MY CONDITION WITHOUT DECLARING AN EMER. READING ASKED ME IF I WISHED TO CONTINUE INBOUND OR DIVERT TO THE NEAREST ARPT. I REQUESTED THE NEAREST ARPT WHICH WAS MORGANTOWN, PA, A GRASS STRIP PRIMARILY USED FOR GLIDER OPS. READING GAVE ME A VECTOR OF ABOUT 7 MI E WHICH WAS CONFIRMED BY MY GPS 'NEAREST ARPT FUNCTION.' AS I DSNDED TO MORGANTOWN FOR LNDG, AN OBSERVER ON THE GND WITH A HAND-HELD TRANSCEIVER ADVISED ME I WAS TRAILING 'SMOKE.' THE LNDG AT MORGANTOWN WAS WITHOUT INCIDENT AND AN OVERHEAD ACFT ASKED MY STATUS TO RELAY TO READING APCH CTL. AFTER SHUTTING DOWN AND INSPECTING THE ACFT, I FOUND THE R-HAND FUEL TANK CAP WAS OFF THE FILLER NECK AND CAPTIVE ONLY BY ITS SAFETY CHAIN. THIS CAP IS AN APPROVED VENTED FUEL CAP FOR THE ACFT. THE RPTED 'SMOKE' WAS ATOMIZED GASOLINE TRAILING FROM THE OPEN TANK. APPARENTLY, THE SOUND I HAD HEARD DURING THE TURB WAS THE FUEL CAP COMING OFF AND HITTING THE WING. I WOULD HAVE EXPECTED THE CAP TO CONTINUE TO MAKE BANGING SOUNDS BUT IT HAD NOT. JUST PRIOR TO TKOF AT HAGERSTOWN, I HAD PERSONALLY SECURED BOTH FUEL CAPS. HOWEVER, AT MORGANTOWN, I DID NOTICE THE R-HAND CAP SEEMED A BIT LOOSER FIT WHEN REPLACED ON THE FILLER NECK. THERE ARE 2 METAL RETAINING EARS ON THE CAP WHICH MAY BE SLIGHTLY BENT TO TIGHTEN THE FRICTION LOCKING ACTION OF THE CAP. AFTER LNDG AT MORGANTOWN, THERE WAS ONLY A TOTAL OF 12 GALS OF FUEL REMAINING. THE L TANK HAD FED THROUGH TO THE SIPHONING R TANK. SINCE I HAD USED ABOUT 9 GALS OF FUEL IN FLT, SOME 15 GALS HAD SIPHONED AWAY IN ONLY A FEW MINS. ACCORDINGLY, I BELIEVE TOTAL FUEL EXHAUSTION WOULD HAVE OCCURRED BEFORE REACHING READING. LNDG AT MORGANTOWN WAS DEFINITELY THE RIGHT CHOICE. THE LESSON LEARNED HERE IS TO BE MORE VIGILANT OF OBSERVING FUEL QUANTITY EVEN DURING A VERY SHORT FLT WITH OSTENSIBLY FULL TANKS. WHEN I HEARD THE 'THUMP' SOUND, I CHKED ALL ENG GAUGES BUT DID NOT IMMEDIATELY CHK THE FUEL GAUGES. IT WAS SEVERAL MINS LATER WHEN I THOUGHT ABOUT THE FUEL GAUGES AND BY THEN, ALMOST ALL THE FUEL WAS GONE. MY MECH HAS EXAMINED THE FUEL CAP AND HAS DETERMINED IT TO BE AIRWORTHY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATES THE ACFT WAS A CESSNA 170B. HIS BEST GUESS REGARDING THE PROB IS THAT THE HORIZ DISPLACEMENT OF THE ACFT IN THE TURB MAY HAVE CAUSED THE RAISED PORTION OF THE CAP, WHICH IS SOMEWHAT LIKE A SMALL SAIL, TO BE TURNED BY THE WIND AND POP OFF. THE REASON HE HEARD ONLY 1 BANG WAS PROBABLY DUE TO THE FACT THEY WERE AT CRUISE AND FORWARD MOTION KEPT THE CAP DIRECTLY BEHIND THE FUEL TANK ENTRANCE. HE FUELED THE ACFT HIMSELF AND KNOWS THAT HE PLACED BOTH CAPS ON PROPERLY. THE ONLY OTHER EXPLANATION HE CAN COME UP WITH IS THE 'EARS' MENTIONED IN THE NARRATIVE. ALL IN ALL A VERY STRANGE EXPERIENCE.

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.