Which statement describes how Vmo changes with altitude?

• A. It remains constant at 263 KIAS
• B. It decreases from 263 KIAS to 194 KIAS between 21,000 and 35,000 ft
• C. It increases with altitude
• D. It becomes undefined at altitude

Answer: (B)

Vmo in knots indicated is influenced by the Mach limit as you climb. The MMO (maximum operating Mach number) stays roughly constant, so the IAS corresponding to that Mach decreases with altitude. To stay under the Mach limit, the indicated speed limit must drop as altitude increases. That’s why Vmo is seen as decreasing with altitude. The statement reflects this by showing Vmo falling from about 263 KIAS at lower levels to about 194 KIAS between 21,000 and 35,000 ft.