Police helicopters play a pivotal role in the success of the police force in Los Angeles, and the LAPD helicopter fleet is now the biggest fleet of police helicopters in the United States, with 17 choppers. Over the course of the year, the helicopters do a combined 18,000 hours of flight, observing crime, serving as backup for officers on the ground, and ensuring stability of infrastructure. The Hooper Heliport, west of the Los Angeles River, is where most of the choppers take off and land. LAPD helicopters have helped set up perimeters for ground officers to surround and maintain a crime scene, and they also served as backup for ground officers around 4,000 times each year. While it is hard to measure the exact impact the fleet of choppers has on the city of Los Angeles, they helped recover 6% of recovered vehicles that had been stolen in 2011, and, in the same year, they provided support with one in every seven felony arrests. It costs $20 million to run and maintain the fleet of helicopters each year.