Final answer:
A structural icing layer of frost on an aircraft decreases lift because it disrupts the smooth airflow over the wings, leading to less lift and potentially hazardous flight conditions.
Step-by-step explanation:
The occurrence of a structural icing layer of frost on an aircraft's surfaces (like wings and tail) decreases lift. This is because ice disrupts the smooth flow of air over the wing, which is critical for creating lift. As the airflow becomes more turbulent due to the rough ice surface, the wing produces less lift, which can be hazardous during flight operations.