In view of the overall losses occurring in the bypass ducts of bypass engines a study has been made of the losses taking place at sudden changes of section which occur as the flow passes over compressor and turbine casings not properly faired. It has been shown that the normally estimated losses occurring at sudden contractions and expansions are greatly increased by the low discharge coefficient. When allowance is made for this, the losses at obstructions can be estimated reasonably accurately. It is shown that a fairing ahead of the blockages has to be quite extensive to produce the normally calculated loss and that all these results are reconcilable with the known losses of obstructions in semiinfinite flow. It is shown, however, that where obstructions are placed in series so that the flow areas are roughly in line, losses are much less than the total of the losses caused by the obstructions separately.