This study aimed at examining the effect of using an on-job task-based program on developing civil aviation personnel's English language fluency. The study began with a review of pertinent literature and previous studies on task-based language teaching, on-job teaching programs, and language fluency. A suggested list of oral and written fluency components delimited to participants' needs of language use was designed as a guide for the design of the instruments. A pre/post fluency test and a rubric were prepared by the researcher. The proposed program was prepared by the researcher. A random group of participants (N=30) was nominated to participate in this study and get involved in its tasks. Their ages ranged from (28 to 45) years old. The participants were submitted to a pre-posttest. Then, the scores were analyzed both quantitatively and qualitatively. Results showed that the participants' results of post-treatment outperformed their results pretreatment. Thus, the proposed program was highly effective in developing participants' English language fluency.