A compressor or fan is controlled to avoid stall, surge or flutter. The control system includes a look-up table from which is retrieved a margin-adjusted limiting value of a control parameter of the compressor, using inputs representing inlet guide vane angle (IGV), non-dimensional speed of the compressor (N/R√T), vehicle Mach number (Mn) and inlet geometry (IG). A variable stall, surge or flutter safety margin about a nominal limiting value of a control parameter is pre-established to reflect threats to the working, stall, flutter and surge lines, for example as a result of measurement inaccuracies, and from this the margin-adjusted limiting value is derived. An actual value of the control parameter is determined in real time, and the actual value is compared with the margin-adjusted limiting value. An output signal indicating stall, surge or flutter risk is generated if the actual value falls within the stall, surge or flutter safety margin.