What is the difference between Speed Mode in CLM or DES versus IAS?

What is the difference between Speed Mode in CLM or DES versus IAS?

• A. CLM/DES provides a 50 ft buffer
• B. IAS provides a 50 ft buffer
• C. CLM/DES provides a 100 ft buffer
• D. IAS provides a 100 ft buffer

Answer: (A)

Speed control modes that guide the aircraft through a descent use a small altitude tolerance around the target altitude to decide when to command an altitude change. When you’re in CLM or DES speed modes, the system uses a tighter envelope—about 50 feet—so the airplane stays very close to the assigned altitude as you descend. This tighter buffer helps you hit altitude constraints with greater precision and gives a steeper, more exact descent profile. Using an IAS-based speed reference, the autopilot allows a larger altitude deviation before initiating corrections—about 100 feet. That looser buffer can make the descent feel smoother but trades some precision in hitting exact altitude constraints. So, the key difference is the vertical tolerance the automation uses: 50 feet in CLM/DES versus 100 feet with IAS.

Speed control modes that guide the aircraft through a descent use a small altitude tolerance around the target altitude to decide when to command an altitude change. When you’re in CLM or DES speed modes, the system uses a tighter envelope—about 50 feet—so the airplane stays very close to the assigned altitude as you descend. This tighter buffer helps you hit altitude constraints with greater precision and gives a steeper, more exact descent profile.

Using an IAS-based speed reference, the autopilot allows a larger altitude deviation before initiating corrections—about 100 feet. That looser buffer can make the descent feel smoother but trades some precision in hitting exact altitude constraints.

So, the key difference is the vertical tolerance the automation uses: 50 feet in CLM/DES versus 100 feet with IAS.