The project POSCOS within the Swedish National Aeronautical Research program (NFFP) aims at method development regarding strength prediction and sizing-methods for buckling-critical composite structures. The test results are reported for a panel where aluminium-stringers are bolted to the flat composite skin (POSCOS-1) loaded in compression. The panel was produced by SAAB AB, and the testing was performed at FOI. The test-results contain load-, strain and full-field measurements using the optical system Aramis. The measurement shows that the collapse load is approximately three times higher than the buckling load for the skin. It is also found that a limited number of load-cycles well above the buckling load do not cause any observable damage or measurable residual strain. These results can be compared with those for a similar test performed on an integrally stiffened slightly curved composite panel (POSCOS-2).