Flight experiments with a microprocessor control system have been conducted. The purposes of these tests were to provide information which will assist the development of flying qualities criteria for digital control systems and to investigate engineering characteristics of the research system. Four test pilots evaluated the effects of digital control system parameter variations on a research aircraft's longitudinal flying qualities during tracking and landing tasks. Critical ranges of sampling rate, quantization level, and time delay were identified as functions of the piloting tasks. In the subject aircraft, the limits for satisfactory control appear to be sampling rates of 4-10/s, control word lengths of 4-8 bits, and equivalent time delays of 50-250 ms, depending upon the task. Satisfactory flying qualities at lower sampling rates and resolution can be achieved with command and stability augmentation.