Narrative:

Upon rotation, the right fuel cap came loose, retained only by the safety chain. Fearing a loss of the cap that could have fatal consequences to someone on the ground, I kept the airspeed as low as safely possible and told the tower that I needed to land immediately. They put me in a right downwind for runway 20R. With the preoccupation with the fuel cap I misunderstood and landed on runway 20L. There was no traffic conflict as the nearest traffic was a falcon on a 4 mi final for runway 20L. Upon landing, the tower informed me that I had been cleared to land on runway 20R. I taxied clear of runway 20L and contacted ground. The fuel cap came off because the restrictor plate in the fuel tank opening, which the cap attaches to, came loose at its attach point.

Google
 

Original NASA ASRS Text

Title: PLT OF AN SMT TWIN NOTICED A MAIN FUEL CAP COME LOOSE DURING TKOF AND ADVISED TWR THAT A RETURN LAND WAS NECESSARY. HOWEVER, THE RPTR MISTAKENLY LANDED ON THE WRONG PARALLEL RWY. THERE WAS NO CONFLICT SINCE HE TURNED OFF THE RWY PRIOR TO ANOTHER ACFT LNDG.

Narrative: UPON ROTATION, THE R FUEL CAP CAME LOOSE, RETAINED ONLY BY THE SAFETY CHAIN. FEARING A LOSS OF THE CAP THAT COULD HAVE FATAL CONSEQUENCES TO SOMEONE ON THE GND, I KEPT THE AIRSPD AS LOW AS SAFELY POSSIBLE AND TOLD THE TWR THAT I NEEDED TO LAND IMMEDIATELY. THEY PUT ME IN A R DOWNWIND FOR RWY 20R. WITH THE PREOCCUPATION WITH THE FUEL CAP I MISUNDERSTOOD AND LANDED ON RWY 20L. THERE WAS NO TFC CONFLICT AS THE NEAREST TFC WAS A FALCON ON A 4 MI FINAL FOR RWY 20L. UPON LNDG, THE TWR INFORMED ME THAT I HAD BEEN CLRED TO LAND ON RWY 20R. I TAXIED CLR OF RWY 20L AND CONTACTED GND. THE FUEL CAP CAME OFF BECAUSE THE RESTRICTOR PLATE IN THE FUEL TANK OPENING, WHICH THE CAP ATTACHES TO, CAME LOOSE AT ITS ATTACH POINT.

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.