angles blades building city dark decorating decorative functional gray grey lines metal metallic modern ornamental ornamentation rooftop round surface ventilators wind wind operated yellow aircraft anchored balloon cable checks circular colors colours control controlled duty-pilot engineers fabric filled flight-operation gondola height helium helium-balloon high inspected inspections large limited mesh metal-cable metal-line monitored monitoring motion movement net passengers pilot piloted registered ride safe safety secure secured sights sightseeing Singapore tethered tourists views weather-limits wind-limits