How can one tell if a heliport is closed?

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A heliport that is closed can typically be indicated by a yellow "X" marked through the "H" symbol on the surface. This visual cue serves to clearly communicate to pilots that the heliport is not available for landing or takeoff. The yellow "X" is a universal signal in aviation indicating that an area should be avoided, which is critical for maintaining safety and situational awareness.

The other options do not provide standard indicators of a closed heliport. A red flashing light above the "H" may signify other conditions but does not specifically denote closure. Black curtains are not a standard practice for marking a closed heliport and would not be a recognized signal by aviators. Similarly, a white light being turned off does not directly indicate closure of the heliport and can be influenced by various other factors. Thus, the yellow "X" through the "H" is the definitive marker for a closed heliport, making it the correct answer.