This study investigates the variation in oral proficiency demonstrated by 14 Air Traffic Controllers across two types of testing tasks: work-related radio telephony-based tasks and non-specific English tasks on aviation topics. Their performance was compared statistically in terms of level ratings on the International Civil Aviation Organization (ICAO) scale. The results demonstrate significant differences in the performance of the test-takers across task types, differences that were not fully predictable across subjects. The differences between general English proficiency and specific purpose proficiency were even greater than those we would expect for other LSP situations. We discuss the implications of these findings for fairly and safely assessing Aviation English using ICAO standards in a politicized context. (Contains 3 endnotes and 5 tables.)