How can I figure out critical altitude?

In Vmc situation (minimum speed where pilot can maintain altitude), How can pilot figure out the Vmc? And also, why critical altitude is important to know?

• That should all be jn the operations manual. Feb 24 '19 at 8:43
• If you have 2 questions (the Vmc and the critical altitude), you ask them separately Feb 28 '19 at 9:36