An air intake guide for a jet propulsion power plant for a supersonic aircraft comprises an intake aperture, an intake center body and an intake adjustment device. The intake aperture has an intake lip, an intake center body is positioned within the aperture, and an intake adjustment device is positioned on a radially inwardly facing surface of the air intake guide downstream of the intake lip. The intake adjustment device comprises a flexible panel and an actuator with the actuator being adapted to deflect the flexible panel in a radially inwardly direction so as to reduce a cross-sectional area of the intake aperture and thereby to position a shock wave at the intake lip.