LNDG WITHOUT CLRNC AT BWI; MD.

Narrative

HANDED OFF TO BALTIMORE APCH ON FREQ 119.7. BALTIMORE APCH CLRED US FOR VISUAL APCH TO RWY 15L. ADVISED US WE WERE #2 FOLLOWING SMALL SINGLE ENG CESSNA TO ARPT WHO WAS ON FINAL APCH. APCH CTL FREQ 119.7 WAS VERY BUSY. AS WE SEARCHED FOR SMALL ACFT ON FINAL AMONG CITY LIGHTS; SAW ACFT LAND. WE CONTINUED OUR APCH TO LNDG WITHOUT GETTING PERMISSION FROM TWR TO LAND. I DO NOT BELIEVE APCH HANDED US OFF TO TWR FREQ 119.4. I KNEW BALTIMORE TWR FREQ WAS 119.4; AND AT A QUICK GLANCE MAY HAVE MISTAKEN 119.7 FOR 119.4; AND THOUGHT TWR HAD CLRED US TO LAND ON RWY 15L; REMEMBERING THE VISUAL APCH CLRNC FOR RWY 15L. BOTH THE PF AND MYSELF NEVER REALIZED WE HAD MADE THE LNDG MISTAKE UNTIL GND CTL TOLD US THE TWR NEVER CLRED US TO LAND ON RWY 15L. IT SEEMS WE WERE STILL ON APCH CTL FREQ 119.7. I FEEL THE TWO CLOSELY NUMBERED FREQS CAUSED A CONFUSION ALONG WITH THE BUSY APCH CTL FREQ (119.7). APCH FORGOT ABOUT US ALSO; BY NOT HANDING US OFF TO BALTIMORE TWR; 119.4. WE DID PHONE THE TWR AFTER PARKING THE ACFT TO DISCUSS THE SIT AND MADE OUR APOLOGIES. SUPPLEMENTAL INFO FROM ACN 466593: WE WERE HAVING TROUBLE MAINTAINING VISUAL CONTACT WITH TFC DUE TO BACKGROUND LIGHTING (GND LIGHTS). HAVING HAD A COUPLE OF TA'S ON THE TCASII; WE WERE CONCENTRATING ON SPACING WITH THAT TFC. WE DO NOT USE THIS ARPT FREQUENTLY SO UNFAMILIAR SURROUNDINGS; NIGHT; AND SPACING OF ACFT AHEAD WERE CONTRIBUTING FACTORS.