From Airbus Safety First:
- How is VMO/MMO determined?
VMO/MMO is established with regards to the aircraft’s structural limits and it provides a margin to the design limit speed/Mach number VD/MD. VD/MD must be sufficiently above VMO/MMO to make it highly improbable that VD/MD will be inadvertently exceeded in commercial operations. Several certification criteria exist. As a result, on Airbus aircraft, MD is usually equal to MMO+ 0.07 and VD approximately equal to VMO + 35 kt.
The applicable VMO/MMO are indicated in each Aircraft Flight Manual. For example, VMO/MMO and VD/MD are given in the following table.[img]images/tab-1-Controlyourspeedincruise.jpg?r=0.6150412419197809[/img]