There is limited information available in the literature about flow co
nditions in axial compressors during surge. This article presents deta
iled measurements from a low-speed test rig instrumented to pick up de
tails Of axial and circumferential flow disturbances. The results show
that surge is initiated by rotating stall, and that the ensuing surge
cycle is a sequence of well ordered cause-and-effect events. The diff
erences in cycle behavior between ''classic surge'' and ''deep surge''
are investigated, and it is shown that the shape of the compressor ch
aracteristic determines which of these will occur. From the results it
is also concluded that some important factors, such as overall pressu
re rise and size of hysteresis loop, have not received sufficient atte
ntion in existing techniques for predicting the rotating stall/surge b
oundary. In line with these findings an Appendix by E. M. Greitzer pre
sents a more general version of the ''B Parameter,'' which takes into
account the influence of compressor design variables on the stalling b
ehavior of the compressor.