At the end of 1942, Dönitz was faced with the appearance of escort carriers, and long-range aircraft working with convoy escorts. To protect his boats against the latter, he ordered his boats to restrict their operations to the Mid-Atlantic Gap, a stretch of ocean out of the range of land‐based aircraft referred to by the Germans as "the black hole." Allied air forces had few aircraft equipped with ASV radar for U-boat detection into April and May 1943, and such units would not exist in Newfoundland until June. Convoys relied on RAF Coastal Command aircraft operating from Northern Ireland and Iceland. The aircraft imposed restraints on U-boat captains, who feared them for their ability to sink a submarine or alert surface warships to their position. In 1942 Coastal Command began forming units combined with ASV and Leigh Light groups to attack U-boats in transit to the Atlantic via the Bay of Biscay, which continued into 1943. The Command was moderately successful after mid-1942.