Emergency Descent Name-of-the-game
The objective during an emergency descent is to discipate energy as fast as you can. This means drag - lots of it. Drag is increased by increasing speed and by increasing load factor. The steeper the bank, when maintaining a relatively constant vertical flight path angle, the higher the g and thus the greater the drag.
The FARs include a requirement for maximum cabin altitude:
- never above 40K feet
- above 25K feet for no more than 120 seconds
The crux is to make sure that you can get down to 40K feet before the cabin altitude increases to that level and to get down to 25K feet in less than two minutes.
While a turning descent would improve performance (i.e., reduce time to descend) commercial transports are certified using a straight ahead method. The first part is throttles to idle and a moderately agressive pitch down (0.5g) to quickly accelerate to Mmo/Vmo. You have to be careful approaching the altitude of the Vm0/Mmo corner lest you overshoot Vmo. Descent gradient along Mmo is much steeper than along Vmo.