Refer to figures.
Note: this question has had to be slightly modified from the original feedback for clarity and correctness. Please send any further exam feedback to us in the comments or by email to [email protected], thank you.
Learning Objective 081.01.10.01.02: Describe the effect of spoilers on the C_L-alpha graph and stall speed.

Spoilers are aerodynamic surfaces that exist on the top surface of some wings, and are used for many reasons. They are primarily used to increase drag and "dump" lift. (If they are just for drag then they are called speedbrakes, but aircraft manufacturers do blur these distinctions).

They are called spoilers because they "spoil" the airflow, sticking up into the airflow, causing a lift reduction similar to an up-going aileron and a similar drag increase to that of a split flap.

Of course, the spoiled airflow is not as effective at producing lift, and the C_LMAX of wings with spoilers extended is much lower, so the stall speed therefore increases.

The effect on the critical angle of attack is much more complicated, however, and is not always easy to predict. As the spoiler pushes up into the airflow, the adverse pressure gradient over it (which usually causes the stall) is reduced and the airflow is re-energised, but the area behind the spoiler (most do not go all the way to the trailing edge of the wing) is automatically stalled after a small spoiler deflection, due to the gap created. So there are two opposing forces at play:

• Spoilers act a little bit like an up-going aileron, re-energising the airflow and increasing the critical AoA,
• The large stalled area behind the spoiler decreases the critical AoA.

Therefore, it is difficult to say what effect spoiler deflection will have on the critical AoA. Some sources mention a slight decrease in critical AoA overall, but it appears that EASA have published other questions where the overall effect is an increase in the critical AoA due to spoiler extension, so we have stuck with this across all questions, as it seems likely that the examiner wants this answer (and feedback has agreed with this in the past). In this case though, the graph given in the question removes all uncertainty, and an increase in critical AoA is the only possible correct answer.

Of course, the real answer is certain to depend on the proportion of the wing, the size and location of the spoiler, and most importantly, the amount of spoiler deflection. As this is so complex, the examiner’s choice of answer is all that truly matters here.