As the aircraft accelerates, the shock waves are moving rearwards of the aircraft and generate what is called Mach tuck. In fact, with the aircraft’s increasing speed the shockwave over the wing will strengthen and travel backward, which increases the lift component further back along the wing. This rearward movement of lift implies that the aerodynamic center of pressure moves backward which will the aircraft’s nose down. The Mach trim system helps the aircraft to counteract the pitch down tendency caused by Mach tuck at a high Mach number. The system increases the airplane’s static longitudinal stability by adjusting the horizontal stabilizer as a function of the Mach number. The Mach trim will only operate above a pre-determined Mach number (typically above M 0.615 for the 737 classic).