When do sinus blocks typically occur during flight?

Sinus blocks typically occur during descent due to the rapid change in atmospheric pressure. As an aircraft descends, the external air pressure increases, and this can create a pressure differential between the external environment and the air trapped in the sinuses. If the Eustachian tubes, which help equalize pressure in the ears and sinuses, are not functioning properly or are blocked, it can result in discomfort, pain, or a sensation of fullness, known as a sinus block. Understanding this mechanism is crucial for crew members, as it emphasizes the importance of pressure equalization and awareness of passengers' potential discomfort during descents.