Mach Tuck

What Causes Mach Tuck?

Mach tuck arises as an aircraft approaches and exceeds its critical Mach number (MCRIT). At this point, shock waves develop on the wing, altering the pressure distribution and causing the center of pressure (CP) to shift rearward. This rearward CP movement produces a nose-down pitching moment, known as 'tuck under' or Mach tuck.

Mach Tuck Explained

The onset of Mach tuck typically begins just above MCRIT. As speed increases, the shock-induced separation of airflow over the wing intensifies, further moving the CP aft. This shift can become so pronounced that the aircraft's nose pitches down uncontrollably if not properly managed. The phenomenon is more severe in aircraft not specifically designed for high-speed flight, and it can limit the maximum safe Mach number.

Mach Tuck Symptoms and Handling

Pilots may notice a tendency for the nose to drop unexpectedly during high-speed flight, requiring increasing nose-up control inputs. This reversal of normal stick force stability can be disorienting and dangerous. Modern jet transports use a Mach trim system, which automatically adjusts the horizontal stabilizer incidence to counteract the nose-down moment, maintaining normal control feel and safe handling.

Mach Tuck Prevention and Mach Trim Systems

To prevent Mach tuck, aircraft are equipped with Mach trim systems that sense increasing Mach number and automatically trim the tailplane nose-up. This compensates for the rearward CP movement. If the Mach trim system fails, operational Mach limits are reduced to avoid entering the Mach tuck regime. Some specialized aircraft, like Concorde, used fuel transfer to adjust the center of gravity in step with the CP movement.

Mach Tuck vs. Other Pitching Moments

Mach tuck is distinct from low-speed nose-down tendencies, as it is directly linked to transonic aerodynamics and shock wave formation. It is not caused by high angles of attack or low-speed stalls, but by the aerodynamic changes unique to high Mach numbers.