How does Ari die in Maximum Ride?

Ari actually dies twice in the series

1. The first time that he dies is due to breaking his neck. Max and Ari are in the subway tunnels when they slip and fall on the slippery ground. Ari then lands on his neck. Before he dies, he tells Max that he would have never hurt her the way she hurt him.
2. The second time that Ari dies is in Germany. After the scientists bring him back to life by repairing his spinal cord, Ari saves the flock from being killed by Ter Borcht. Max then invites him to stay with them. Fang does not trust Ari and the flock splits up. Max, Nudge, Angel, Total, and Ari go to Germany. In the middle of the "epic fight", Ari collapses. He had an expiration date on the back of his neck, which is the day in which he expires. Ari dies for a second time in Max's arms.