AbstractA requirement for a nonflammable hydraulic fluid was defined in the 1970s to eliminate losses attributable to hydraulic system fires. In order to increase the possibility of success of the nonflammable hydraulic fluid development programme, requirements for compatibility with current hydraulic fluids, seals, and system designs were not established. All previous attempts to develop a compatible nonflammable hydraulic fluid had been unsuccessful. Extensive screening of many chemical classes of candidate base fluids led to the selection of a Chlorotrifluoroethylene (CTFE) oligomer. Considerable hardwarel system development was performed for hydraulic system applications, including development of high‐pressure actuators, pumps, and motors. The major thrust of the programme, a totally new system design, was demonstrated in the Lockheed High Technology Test Bed air craft and in the McDonnell Douglas Aircraft 55.2 MPa (8000 psi) technology ‘Iron Bird’ demonstrator. Possible retrofit applications for current systems include a C135E fireproof brake system and the M1/M1A1 Abrams tank gun mount. This paper also discusses material compatibility requirements, design considerations, and the results of pump testing of the nonflammable hydraulic