If the plane takes off from airstrip B then it is southbound. The plane did not take off from airstrip B, therefore it is not southbound. Identify the formal fallacy in the above argument.

The argument denies the antecedent and wrongly concludes the consequent is false.

The conditional makes taking off from airstrip B sufficient for being southbound.

A sufficient condition can be met by other routes, so the plane could be southbound without using airstrip B.

From the plane not taking off at B nothing follows about its direction.

Denying the if-part to reject the then-part is the fallacy of denying the antecedent.