Frequency spectrum generated using data from over ten rotor revolutions during stable fan operation (top) and over ten rotor revolutions when stall is incipient (bottom).. The frequency spectrum generated using data when stall is incipient (b) includes features that are associated with the spike-like pressure pulses that occur when stall is incipient. As such, the two frequency spectrums are different enabling one to use them as the basis of a stall detection system.