When did Max kill Ari in the book Maximum Ride The Angel Experiment?

She killed him in the sewer tunnel after they found "The Institute" in New York City, also know as "The big Apple". She did not mean to kill him. Ari and Max were fighting - as usual - and Ari slipped in the sewage system when he went after Max. She ended up going after him and knocking him over and that's when he hit his head at an awkward angle, broke his neck and killed him. In other books he returns and Max thinks she's hallucinating, but she isn't. It turns out that Jeb had the power and technology to bring Ari back to life.