Heading into the north Atlantic to sink Allied shipping was not an easy prospect for German submarine crews. Before the invasion of France the U-boats had to sail the northern route or straight down the channel to access the ocean shipping lanes. After the invasion the Germans had ports on the Bay of Biscay which in theory made access to the Atlantic far easier. This was true for a while, but innovations in anti-submarine tactics, intelligence and airborne radar detection made the passage through the Biscay just as dangerous as the other two routes had always been. It finally was so hazardous for U-boats in the Biscay that Dönitz ordered all submarines to cross the bay submerged.

It was not always the U-boat that became the victim however, sometimes it was the attacker. Such was the case on May 9, 1943 when a Handley-Page Halifax "N" (HR743) of 58 Squadron R.A.F. Coastal Command attacked U-666. One might think the number of the boat was unlucky, but this time they came out on top. After locating the boat, the aircraft made one strafing run, came around and made a second run, by this time the submarine crew were manning the machine guns quite effectively and managed to damage one of the engines and perhaps wounded the pilot. After the second run (according to the U-boat crew) the aircraft turned and crashed into the sea. None of the flight crew survived.

U-666 was undamaged and continued on the patrol, but the Halifax was the only success they had. No shipping was sunk or damaged in the two months at sea during this war patrol. Perhaps it was an unlucky number to carry, in over 200 days at sea they had only one combat victory (other than the Halifax) and in Feb. of 1944 the boat went missing, one of the few submarines lost with no known cause.