What is "helikopter-streichquartett"

The Helikopter-Streichquartett is one of Karlheinz Stockhausen's best-known pieces, and one of the most complex to perform. It involves a string quartet, four helicopters with pilots, as well as audio and video equipment and technicians. It was first performed and recorded in 1995. Although performable as a self-sufficient piece, it also forms the third scene of the opera Mittwoch aus Licht ("Wednesday from Light").