The last unperformed Stockhausen opera lifts off

It's a Thursday afternoon and, outside the Argyle Works in Digbeth, it is raining very, very hard. Inside, the rain hammers against the former chemical plant's corrugated roof, the sound as inescapable as the strange, slightly acrid smell that still lingers, years after its former occupiers packed up and left.

But there are more pressing matters than the weather, not least getting a variety of musicians harnessed to chairs and hoisted up into the air using a system of pulleys, the better for them to rehearse Orchester-Finalisten, the second scene in Stockhausen's mid-90s opera Mittwoch aus Licht (Wednesday from Light). In just under a couple of weeks' time, Birmingham Opera Company will stage a world premiere of this six-hour opera; it is the only Stockhausen work never to have been performed in its entirety.

"It's a kind of dream, a rather charming one," explains Graham Vick, artistic director of the BOC. "They are all ego-driven, ambitious orchestral musicians, who want to join an orchestra in the sky. Each has a virtuoso solo, based on the characteristics of the instrument and the physical characteristics of the players. So the cellist has it written into the score that they have to toss their head in this real, intense sort of Jacqueline du Pré way, and the bass player has to have a fit and lie on his back with his double bass on top of him. They look down over various bits of the landscape and the events that happen. They look down on a cathedral, on a kindergarten, on some little goats playing, on Marrakech, on the plains of Africa – and the things they look down on get bigger and bigger in scale and space, so you're kind of getting higher and wider."

Those unfamiliar with the latterday operatic work of Karlheinz Stockhausen should note that this is one of the more easily explained scenes in Mittwoch aus Licht. Nevertheless, it is still a deeply odd experience watching a cellist being dangled from the ceiling of a vast, crumbling concrete hall, instrument and all. For me, as a rock critic, this is lent a faint tang of familiarity by the fact that getting him airborne requires a couple of roadies. Opera roadies, it turns out, look exactly the same as their rock counterparts the world over: burly men in big shorts with tattoos and heavy-metal T-shirts. And when the music gets underway, the soloists augmented by an octophonic recording of electronics and sound effects controlled by the late composer's muse and collaborator Kathinka Pasveer, these roadies wear the same inscrutable expressions as the roadies I once saw working with the Backstreet Boys: never letting it show they'd rather be listening to Opeth or Impaled Nazarene.

The musicians, too, seem to be bearing it all with good grace. I say as much to Vick, who is watching proceedings, as he expects at least some of the audience to, recumbent on the floor. He nods, then reconsiders. "I don't think Bruce on the trumpet's very happy about it," he says. He has a point: swaying gently a few feet from the ground, Bruce on the trumpet is wearing the kind of rictus grin that conveys abject terror.

But if he is terrified, he seems to be alone. Everybody else behaves as if flying musicians around a former chemical works is the acme of normality, which I suppose for them it now is. The rehearsal period – 12 hours a day, seven days a week – seems to have immunised them to the sheer strangeness of the work. Walking through another part of the Argyle Works, I spot a man making repairs to a giant camel costume. I mention this to Jean Nicholson, the company's general manager. "Ah yes," she nods, "Lucicamel. It's elected President of the Universe in scene four, and it shits planets and does a dance."

Vick and the Birmingham Opera Company have form when it comes to experimentation, staging Beethoven's Fidelio in a rave tent pitched next to Aston Villa's ground in 2002 (the audience were required to wear black bags over their heads during part of the second act); Monteverdi in the Bull Ring shopping centre in 2005; and La Traviata at the 12,000-capacity National Indoor Arena in 2007, with a cast featuring 300 local volunteers.

But this is an experiment on a different scale altogether. Stockhausen's score is both incredibly detailed and contradictory, Vick says. "There are lots of physical instructions, but you can't do half the stuff that's asked for when the musicians are flying. I think he wanted it to be staged, but I don't think he ever bothered rethinking all the practicalities." Arguably, scene three will be more difficult than scene two: the Helikopter-Streich-quartet, which requires a string quartet to perform mid-air in four helicopters, playing a synchronised, polyphonic composition in which the sounds of the instruments meld with the sounds of the rotor blades, while the audience inside the Argyle Works watch live on video screens.

Vick is quick to point out that scene four – the Michaelon – is no picnic, either. The only difficulties with the helicopter quartet, he says, are "logistical and financial"; the problems with the Michaelon are all artistic. The music here is so demanding that it has never been recorded. The Birmingham Opera Company has had to fly in "absolute Stockhausen specialists, people who worked with him before he died" to perform it, Vick says; if that seems extravagant, it is cheaper than paying other musicians to learn it, because learning it would take so long. And even with these specialists, he has had a gruelling morning trying to make sense of the music.

"A lot of the work is happening now, because it's only becoming concrete to me now. It was a mess yesterday – I couldn't hear it at all. You try slowing it down and separating it out and then suddenly you hear, and you think: that's what the idea is. I trust my own instinct, is what I do. I know when I recognise that it is what it should be, but I don't always know how to get there." He smiles. "Which is nice. It's very exciting."

It has to be said that Vick doesn't have the air of a man days away from the world premiere of a piece that many people think is unstageable. If he has concerns, these seem to have been smothered by sheer enthusiasm for what he calls "an irresistible opportunity to do something incredibly difficult", which is infectious and inspiring.

Somewhat unexpectedly, Radio 1 DJ Nihal Arthanayake has been cast in the role that Stockhausen intended for himself: interviewing the participants – players and pilots alike – after the Helikopter-Streichquartet, and hosting an audience question-and-answer session. "There's no context as to why they would have approached me," Arthanayake says. "I mean, I've seen an opera before, but I'll tell you how shandy an opera that was: it had Dawn French in it. To be honest, I didn't stop to ask them why, just in case they thought, 'Oh, we've got the wrong fella.' I thought, 'Oh God, it's gonna be one of those things where you're DJing dubstep and a big woman comes out and starts singing over the top of it' – but then Graham started talking about string quartets and helicopters. I don't think I've ever been in a meeting for an hour and half and been so dazzled and confused and inspired. Any new experience for me, I'm in. But it's only just hit me how big a deal it is. It's globally significant." He laughs. "And I'm doing it the same week I'm guest-presenting The Wright Stuff on Channel 5."

He might be even more bemused if he heard what Vick had to say about him. "Nihal is my ideal audience in many ways for this event. Someone who's got nothing to do with this musical culture, but with an open attitude, an open mind, who's going to come and engage non-cynically." There are people flying in for the premiere on Stockhausen's birthday, and 81 people are coming to see all four performances, he says: the Stockhausen equivalent of the kind of fan who takes all their holiday allowance at once in order to follow a band on tour. "They're the hardliners – they'll know far more about it than I will." But he is even more interested in what local people will make of it: "I want a lot of people in Birmingham to find it exciting."

On the face of it, Mittwoch aus Licht seems a big ask of an audience not attuned to Stockhausen. Not really attuned to Stockhausen myself, I watch a rehearsal of Orchester-Finalisten from a suitably prone position, and am surprised by how engaging it is. For all the complexity of the music – and there are moments when, if it were any more challenging, it would be poking you in the chest and offering you outside for a fight – it carries you along with it, even if you have no idea what's going on; this is a rather pleasurable sensation. There is also something utterly charming about the way the musicians perform, something overwhelming about the sheer scale of the spectacle, and something beguiling about the sense of being enveloped by sound in such a vast space.

"There are no rights and wrongs and absolutes," says Vick. "You take away the experience you have and you value it, or not, as much as you do. It's up to you whether it was good or not, it's not up to the person next to you. It's up to you whether you make a phone call or not. We don't make any announcements about switching phones off. We don't forbid the taking of photographs. You know, if our performance isn't good enough to survive a few camera clicks, then it can't be that good a show. And besides, what are they doing with the photographs afterwards? They're putting them on Facebook or Twitter, they're showing them to their friends. And a whole load of people are talking about a Stockhausen opera. Well," he smiles, "that can't be a bad thing." And he heads off, to help push the trombonist across the hall.