This was an Allied 'civil' air operation linking the UK and neutral Sweden (1943/May 1945).
The eastern terminus for this undertaking was Bromma airport outside the Swedish capital, Stockholm, and this airport saw very considerable incoming and outgoing traffic associated with both the Germans and the Allies. German Lufthansa transports found it straightforward to undertake their services as they operated over German-controlled and neutral airspace. It was more problematical for the Allies, however, for their aircraft had to cross German-controlled airspace across the eastern part of the North Sea and occupied Norway to reach Swedish airspace, and then return through this same area in which there was the chance of German fighter interception.
Allied military transports, marked as civilian aircraft, nonetheless arrived in a steady stream from Leuchars in Scotland on moonless nights. They brought with them VIP and diplomatic personnel, vital machine parts, film and photo-chemicals, books and even the recent newspapers, and on departure from Bromma carried departing VIP and diplomatic personnel, Allied military aircrews who had come down in Sweden and been interned before being repatriated, roller bearings, special steel products, and other compact or light products.
The standard Allied transport used in 'Sonnie' was the Consolidated C-87 Liberator Express, the transport version of the B-24 bomber. It is reported that on landing, one such C-87 was found to have a cracked a cylinder head. A spare could have been despatched on the next flight from Leuchars. The celebrated aviator Bernt Balchen, who was one of the major figures in the 'Sonnie' undertaking, knew that the Douglas DC-3 aircraft which Lufthansa was operating between Berlin and Stockholm used the same type of engine, and asked his friend Carl Florman, of the Swedish airline ABA, to borrow a spare cylinder from the Lufthansa representative at Bromma. The German did not have the item available in Stockholm, but agreed to arrange for one to be sent up from Berlin on the next flight, which arrived on the following day with a cylinder salvaged from a B-24 which had crashed in Germany. Balchen installed this in his C-87’s damaged engine and flew back to Leuchars. There he obtained a spare cylinder and flew it to Stockholm day to replace the item borrowed from the Germans.