Freighter conversions: Boeing or the other guy? Bulgarian cargo carrier switched 737-800 order from original equipment manufacturer for reasons beyond quality 0 32 7 minutes read Aeronautical Engineers Inc. cut a big hole in the side of this Boeing 737-800 jet and installed a wider door so containers can be loaded in the main cabin. (Photo: Cargo Air) Some car owners will only go to the dealer for maintenance, while others are perfectly happy with an independent mechanic. The same thinking holds true in aviation when used airplanes are turned into freighters. A Bulgarian air cargo company will soon start transporting packages for an express carrier with a 737-800 newly converted to carry containers instead of passengers after ditching Boeing (NYSE: BA) for a smaller engineering outfit to do the job.