Sociolinguistics has never been a closed or autonomous discipline, but rather has considered relations between linguistic variables and so-called real-world variables, such as age, sex, and social class. Developments in discourse analysis now permit the analysis of the effectiveness of utterances in their real-world context. This study of communicative success uses as its data transcripts of eight aviation accidents, as well as transcripts of fourteen flight simulator sessions. The linguistic variable considered is mitigation; the real-world variables are success or failure of the individual communication, and peer judgments of the overall effectiveness of the simulator crews. To quantify the use of mitigation, a four-degree scale was established by using the judgments of several linguistic analysts and was validated against the judgments of members of the aviation community. Using this scale, a number of hypotheses were confirmed: (1) Utterances going up the chain of command are more mitigated than those going down, showing that mitigation is sensitive to social rank. (2) Utterances introducing a new topic are more likely to fail if they are mitigated than if they are direct. (3) Suggestions by a crew member to the captain are more likely to fail if they are mitigated than if they are direct. These results show a strong effect of mitigation on several measures of communicative success. This study shows that a quantitative study of communicative success is possible and suggests the necessity for further studies of this type as a necessary direction for discourse analysis. (Pragmatics, discourse analysis, communicative effectiveness, politeness, mitigation, perlocutionary force, man-machine interface)