What is true about equilibrium flight?

Equilibrium flight is characterized by a condition where the forces and moments acting on an aircraft are balanced, resulting in stable and unchanging flight conditions. When an aircraft is in equilibrium, its speed, altitude, and flight path are constant.

Trimmed flight is a specific state where the aircraft is balanced in such a way that no control inputs are needed to maintain that state. Thus, when the aircraft is in equilibrium, it is invariably in trimmed flight, meaning the aerodynamic and weight forces are in balance without the need for continuous control surface deflection.

The other options do not accurately describe the characteristics of equilibrium flight. A negative angle of attack is not required; instead, the angle of attack can be positive, zero, or negative depending on the aircraft design and specific flight condition. Similarly, while equilibrium flight can occur in a climb, it does not require a constant climb rate—it can also occur in level flight or even descending flight. Lastly, zero roll rate is not a necessary condition for equilibrium flight; the aircraft could be in equilibrium while still experiencing a roll, as long as the forces are balanced appropriately. Thus, the statement about equilibrium flight being always in trimmed flight is the most accurate depiction of this flight condition.