the impact zone made their way upward toward the roof in hopes of a helicopter rescue. However, access doors to the roof were locked; in any case, thick smoke and intense heat prevented rescue helicopters from landing. Flight 175's crash into the South Tower was faster and lower down than that of the North Tower, compromising its structural integrity more. With 24 floors above the South Tower's impact zone compared to the North Tower's 10, there was far more structural weight pressing down on a damaged section of the building on fire. The South Tower collapsed at 09:58:59, after burning for 55 minutes, being the first of the two skyscrapers to collapse despite being the second to be hit, and only burning for around half the amount of time as the North Tower did before it too fell. Aftermath The flight recorder for Flight175, as with Flight11's, was never found. Some debris from Flight175 was recovered nearby, including landing gear found on top of a building on the corner of West Broadway and Park Place, an engine found at Church and Murray Street, and a section of the fuselage which landed on top of 5 World Trade Center. In April 2013, a piece of the inboard wing flap mechanism from a Boeing 767 was discovered wedged between two buildings at Park Place. During the recovery process, small fragments were identified from some passengers on Flight175, including a piece of bone belonging to Peter Hanson, and small bone fragments of Lisa Frost. In 2008, the remains of Flight175 passenger Alona Abraham were identified using DNA samples. Remains of many others aboard Flight175 were never recovered. Shortly after September 11, the number for future flights on the same route was changed from 175 to 1525. Since then, United Airlines has renumbered and rescheduled all flights from Boston to Los Angeles. United Airlines Flight311 now operates the Boston to LAX route, leaving at 08:30 and is operated by a Boeing 737 MAX 9. It was reported in May 2011 that United was reactivating flight numbers 175 and 93 as a codeshare operated by Continental, sparking an outcry from some in the media and the labor union representing United pilots. However, United said the reactivation was a mistake and said the numbers were "inadvertently reinstated", and would not be reactivated. The names of the victims of Flight175 are inscribed at the National September 11