Mach Tuck and Mach Trim

As speed increases beyond the Critical Mach number (M_CRIT), shock wave formation at the root of a swept-back wing will:

• reduce lift forward of the CG (CP moves rearward), and
• reduce downwash at the tail plane.

Together, these factors will generate a nose-down pitching moment.

At high Mach numbers, an aircraft will become unstable with respect to speed; instead of an increasing push force being required as speed increases, a pull force becomes necessary to prevent the aircraft accelerating further.

A small increase in Mach number will give a nose-down pitch which will further increase the Mach number. This in turn leads to a further increase in the nose-down pitching moment. This unfavourable high-speed characteristic, known as “Mach Tuck”, “High Speed Tuck” or “Tuck Under” would restrict the maximum operating speed of a modern high-speed jet transport aircraft.

To maintain the required stick force gradient at high Mach numbers, a Mach trim system must be fitted. It is operational at high Mach numbers in the transonic speed range.

In the event of this equipment failing, the speed is limited to specified Mach number, so as not to encounter the region of instability without Mach Trim assistance.

Now, if this failure were to occur in flight, the response is simple, slow down below the required Mach No., descending if necessary. However, before flight, whether the aircraft can dispatch or not will depend on the aircraft's MEL (Minimum Equipment List), which is set out by the manufacturer and sometimes modified by the operator (to make it more stringent for their own policies or flight regions, etc.).

Taking a look at some MELs, it appears that many aircraft are approved to dispatch with passengers onboard with inoperative Mach Trim systems, as long as they apply certain speed limits.

This will likely depend on the aircraft type though, as the Boeing 737 we have the best reference for, has quite manageable Mach Tuck tendencies, and with the system inoperative is limited to 280 kt/M0.82, but these tendencies can vary quite a bit depending on the aircraft design. We believe this is the most likely answer to this question though, particularly as the other usual "reference aircraft" for EASA is the A320, which has Fly By Wire flight controls, so handles Mach Tuck as part of it's normal flight controls.