This article explores the issues of pronunciation and comprehension in the English as a lingua franca (ELF) context of pilot--air traffic controller radiotelephony communication, and how these are handled in the proficiency rating scale globally used to assess pilots and air traffic controllers engaging in international flight and air traffic control. Based on an analysis of live radiotelephony discourse, feedback from aviation experts on sources of miscommunication, and an investigation of the phonetic features of key turns in the discourse, we present results indicating that L1-influenced pronunciation is one of the major factors causing miscommunication on the part of the speaker, together with unfamiliarity with the pronunciation on the part of the listener. Findings also suggest the descriptors for the "Pronunciation" and "Comprehension" criteria in the rating scale do not reflect the ways language users deal with these matters in their work. This study therefore argues that a new conceptualization of proficiency is needed for taking up the real matters in the assessment of ELF in the aviation context and other ELF contexts at large.