What is the Mandatory block altitude?

• A. An altitude depicted on an instrument approach chart with two underscored and overscored altitude values between which aircraft are required to maintain altitude.
• B. An altitude depicted on an instrument approach chart with a single underscored altitude value.
• C. An altitude depicted on an instrument approach chart with an overscored altitude value.
• D. An altitude depicted on an instrument approach chart with two underscored altitude values between which aircraft are required to maintain altitude.

Answer: (D)

Mandatory block altitude is shown on an instrument approach chart as two numbers, both underscored, that define a vertical band you must stay within on that segment. This block ensures you maintain a specific altitude range for obstacle clearance and procedure integrity until you reach the next fix or clearance. So the depiction of two underscored altitudes between which you must hold is the standard symbol for a mandatory block altitude. The other options don’t reflect this two-boundary, underscored format, or describe a single value or a different symbol that isn’t the mandatory block.