A study of the problems posed by the use of natural language in various aspects of aviation is presented. The study, part of a larger investigation of the feasibility of voice input/output interfaces for communication in aviation, looks at representative real examples of accidents and near misses resulting from language confusions and omissions. An intelligent voice interface is proposed for both real-time safety in the air traffic control/aircraft communications circuit and in the training of pilots and controllers. Such a device would monitor communications and filter out potential linguistic confusions, requiring clarification before conveying messages. The intelligent voice input component would consist of a voice word recognizer, to divide raw speech into meaningful elements, and a language filter to make interpretive decisions such as disambiguation and reference determination. The filter would use three components of natural language understanding: the lexicon, grammar, and knowledge base of air traffic control. The output component would be designed to check back with the speaker and provide required callouts and readbacks. (MSE)