The article notes that
Despite several attempts no company has found a way to pull off “Mittwoch” (“Wednesday”), partly because it incorporates Stockhausen’s infamous “Helicopter String Quartet,” in which each member of the ensemble flies in a separate helicopter, but also because the music is so fiendishly difficult.However, while the opera that contains it has not been mounted, the string quartet itself has been. A 1995 article from the Times describes a performance of the work:
As part of this summer's Holland Festival, the composer staged the premiere of the Helicopter Quartet, written for four string instruments and, no less, four helicopters.Here's some video from another performance:
The work took off from a field on the outskirts of Amsterdam, at the Westergasfabriek, a former gasworks that has become a theater. Here, in the gentle evening light last month, Mr. Stockhausen said goodbye to the members of the Arditti String Quartet as each went off to his own helicopter and pilot.
Somewhat bemused, the audience stayed behind in the theater, with Mr Stockhausen in their midst, directing the event. At a large control table, he mixed the video images and the haunting tremolos that were sent down from the heavens and projected into the theater. Other sets of microphones picked up the whir of the motorblades, also delivering them to the composer's mixing desk. The audience could follow the event through banks of loudspeakers and television monitors that carried images from cameras aboard the aircraft.
The concert's four helicopter pilots belong to a stunt team of the Dutch Air Force known as the Grasshoppers, but the composer declared them to be musicians because the sounds of their engines were an intrinsic part of the highly detailed score. The pilots "played," so to speak, as their aircraft changed speed or turned, maneuvers that to the keen ear yielded different timbres from the rotorblades. It had taken several days of testing to know which microphone position on the rotor would yield the most desirable rendering.