Which definition correctly describes the angle of incidence?

• A. The angle between the wing's chord line and the relative airflow
• B. The angle between the wing's chord line and the thrust vector
• C. The angle between the wing's chord line and the lateral axis
• D. The angle between the chord line of the wing and the longitudinal axis

Answer: (D)

The question tests the distinction between angle of incidence and angle of attack. Angle of incidence is the fixed geometric angle between the wing’s chord line and the aircraft’s longitudinal axis (the nose-to-tail line). It’s a mounting angle set on the airplane and does not change with flight conditions. This is why the correct description is the angle between the chord line and the longitudinal axis.

In contrast, angle of attack is the angle between the wing’s chord line and the direction of the oncoming air (the relative wind). That would be describing angle of attack, not incidence, which is why the other options don’t fit. The thrust vector and the lateral axis are not the standard reference for the wing’s incidence, so they aren’t correct either.

So, the angle of incidence is defined by the angle between the wing’s chord line and the aircraft’s longitudinal axis.