How is stall margin computed during approach in landing configuration?

• A. Stall margin = stall speed minus approach speed.
• B. Stall margin = approach speed plus stall speed.
• C. Stall margin = approach speed in landing configuration minus stall speed in the same configuration.
• D. Stall margin is unrelated to configuration.

Answer: (C)

Stall margin is the cushion you have above the speed at which the aircraft would stall in the current configuration. During approach with landing configuration, both your stall speed and your approach speed are determined with flaps and gear in the landing setup, so the safe margin is the difference between those two speeds. In other words, stall margin equals your approach speed in landing configuration minus the stall speed in that same configuration. This gives a clear, meaningful number that represents how much faster you are flying than the stall speed, which is crucial for staying safe in gusts and during maneuvering near the stall.

For example, if the stall speed with landing configuration is 100 knots and your approach speed is 110 knots, the margin is 10 knots. This margin should remain positive and comfortable; if it shrinks toward zero, you’re getting dangerously close to the stall.

The other ideas don’t fit because they either combine speeds in an inappropriate way or ignore the fact that stall speed changes with configuration. stall speed and approach speed must be measured in the same configuration to yield a meaningful, safe margin.