It was a now a very dangerous proposition for any U-boat to be caught on the surface. The ‘mid-Atlantic gap’ was now largely covered by aircraft patrolling from carriers. On the 3rd August U-66 had been caught on the surface and men wounded in an aircraft attack. On 6th August she rendezvoused with the large XB class U-boat U-117, designed for carrying mines and used as a supply vessel. The doctor was able to treat the wounded on U-66 and a new man was transferred onto U-66 to replace one of them.
The following day they were still together in mid Atlantic, as U-66 was re-fuelled. They were spotted from 12 miles away by an Avenger aircraft from USS Card. This was the report of Lt. Asbury H. Sallenger:
While proceeding on a track of 2960 T, time 0946 GCT, altitude 4500 feet, speed 150 knots, true airspeed, I spotted a large white object bearing 150 on my starboard bow, distance about 12 miles. At first I thought it was a merchant ship, but I soon realized it was two submarines close together, fully surfaced, cruising very slowly with neither wake nor bow wave. The bearing from the ship was 2980 T, distance 82 miles.
The weather was slightly hazy, there was no cloud cover, and as the submarines were on a course of 2200 at a speed of about 2 knots, I maneuvered to come out of the sun. They were almost abreast of one another not more than 200 feet apart. There was no indication that fueling operations had been or were going to be conducted. The sides of both submarines were painted white and seemed to be the same size.
I made the first attack out of the sun with two Mk. 47 depth bombs set at 25 feet, selecting the submarine nearest me and slightly astern. I approached at a target angle of 1950, speed 220 knots, releasing at an altitude of 125 feet, plane course 2350 T. The submarines were apparently caught unaware and did not open fire with their AA guns until I was about 400 yards away. This fire was very intense from both U-boats, though the plane was not hit.
The bombs seemed to straddle the U-boat and about 3 seconds later there were two large explosions, one five to ten feet on the starboard quarter half way between the conning tower and the stern and the other just ahead of the conning tower 15-20 feet out. I circled sharply to the left gaining altitude while the turret gunner strafed and the radioman took pictures.
The attacked submarine immediately began to smoke badly throwing off a dark greyish black smoke. It began making erratic turns in a crazy quilt pattern trailing a heavy oil slick. I had made a preliminary contact report to the ship before the attack and now made another giving the bearing and distance. I then cut on my IFF and climbed to 6500 feet.
The undamaged submarine apparently tried to aid the one attacked for about 15 minutes and then started to submerge near the damaged sub. I immediately dove to attack, meeting heavy AA fire from the damaged U-boat. This fire was particularly noticeable because it was necessary to fly directly by the conning tower at 130 knots and 200 feet to drop the Mk. 24 mine about 40 seconds after the undamaged sub had gone down.