The International Civil Aviation Organization (ICAO) is currently implementing a program to improve the language proficiency of pilots and air traffic controllers worldwide. In justifying the program, ICAO has cited a number of airline accidents that were at least partly caused by language factors. Two accidents cited by ICAO are analysed in this paper: the mid-air collision above Zagreb in 1976, and the runway collision at Tenerife in 1977. The paper examines the linguistic factors involved in each accident, such as code switching and L1 interference, and uses the "Swiss cheese" model of accident causation developed by Reason (1990) and adapted by Wiegmann and Shappell (2003) to put these factors into a broader aviation context. It is shown that, while linguistic factors were in each case significant, both accidents occurred as the result of multiple causal factors, many of which were non-linguistic. Furthermore, stress and fatigue played a decisive role in exacerbating the linguistic factors in each accident. Finally, the paper suggests lessons that may be drawn from the analysis for the training of pilots and air traffic controllers, both native-speaker and non native-speaker. (Contains 4 endnotes, 1 table and 3 figures.)