The Cessna Skymaster was a clever idea to try and aviod the problems pilots have with twin engined aircraft when one engine fails. With normal twin engined aircraft (ie with an engine on either side of the fuselage) when one engine fails the pilot is left with an unbalanced aircraft. Hence the need for pilots to have a twin-rating on their licence before being allowed to fly one. The Skymasters in-line engines gets around that problem. If one engine fails then the aircraft is still balanced and the pilot continues flying normally (with less power of course). The American FAA recongnised this fact and awarded pilots a special "CLT" rating for Skymaster Pilots (CLT = Center Line Thrust). However the British CAA obviously don't know a good idea if they ever saw one and still instist that British PPL Holders have twin-ratinings on their licenceses.