United Airlines Flight 175

United Airlines Flight 175 was an American domestic passenger flight from Logan International Airport in Boston to Los Angeles International Airport in California that was hijacked by five al-Qaeda terrorists on the morning of September 11, 2001, as part of the September 11 attacks. The hijacked airliner was deliberately crashed into the South Tower of the World Trade Center complex in New York City, killing all 65 people aboard the flight, and resulting in the deaths of more than 600 people in the South Tower's upper levels, in addition to an unknown number of civilians and emergency personnel on floors beneath the impact zone.

The hijacking and crash of Flight 175 is the second-deadliest plane crash in aviation history, surpassed only by American Airlines Flight 11, which struck the North Tower 17 minutes prior to Flight 175's crash. The aircraft involved was a Boeing 767 operating United Airlines' daily scheduled morning transcontinental service from Boston Logan International Airport to Los Angeles International Airport.

Flight 175 departed from Logan Airport at 08:14. Twenty-eight minutes into the flight, the hijackers injured several crew members, forced their way into the cockpit, and killed both pilots while moving anyone who remained to the rear of the aircraft. Lead hijacker Marwan al-Shehhi, who had trained as a pilot for the purposes of the attacks, took over the flight controls once the hijackers killed the captain and first officer. Unlike the team on American Airlines Flight 11, the terrorists aboard Flight 175 did not switch off the plane's transponder when they took over the cockpit. Thus, the aircraft was visible on New York Center's radar, which depicted the deviation from its assigned flight path before controllers took notice four minutes later at 08:51 EDT. Upon realizing, the ATC workers immediately made several unsuccessful attempts to contact the cockpit of the hijacked airliner, which twice nearly collided with other planes as it flew toward New York City. In the interim, three people were able to get through to their family members and colleagues on the ground, passing on information to do with the hijackers as well as casualties suffered by the flight crew.

No more than 21 minutes after the hijacking began, al-Shehhi crashed the airplane into the South Tower's south face from floors 77 through 85 as part of an attack coordinated with the takeover of Flight 11, which had struck the upper floors of the World Trade Center's North Tower at 08:46. Media coverage of the disaster that began in the North Tower 17 minutes earlier meant Flight 175's impact at 09:03 was the only one of the four attacks to be televised live around the world. The damage done to the South Tower by the crash and subsequent fire caused its collapse 56 minutes later at 09:59, killing everyone who was still inside. During the recovery effort at the World Trade Center site, workers uncovered and identified remains from some Flight 175 victims, but many victims have not been identified.

Hijacking

The hijacking occurred sometime between 08:42 and 08:46. The 9/11 Commission concluded that the al-Ghamdi brothers forced the passengers and crew to the rear of the aircraft using knives and bomb threats, while also spraying mace into the confines of the cabin. A flight attendant was stabbed, and both pilots were killed by Fayez Banihammad and Mohand al-Shehri when they breached the cockpit, allowing Marwan al-Shehhi to commandeer the flight controls. The first operational evidence that something was abnormal on Flight 175 came seconds after the North Tower was struck, when the plane's assigned transponder signal changed from "1470" to "3020" and then one minute later to "3321", and the aircraft began deviating from its assigned course. However, Bottiglia did not notice until minutes later at 08:51. Unlike Flight 11, which had turned its transponder off, Flight 175's flight data could still be properly monitored. Also, at 08:51, Flight 175 changed altitude. Over the next three minutes, the controller made five unsuccessful attempts to contact Flight 175 and worked to move other aircraft in the vicinity away from it. At 08:55, a supervisor at the New York Air Traffic Control Center notified the center's operations manager of the Flight 175 hijacking. Now tasked with handling Flight 175, Bottiglia remarked, "We might have a hijack over here, two of them."

Crash

At 08:58, Flight 175 was over New Jersey at 28,500 ft, by which point al-Shehhi would have seen the smoke pouring from the North Tower in the distance. The airplane was in a sustained power dive of more than 24,000 ft in the five minutes and four seconds between approximately 08:58 and the moment of impact, at an average rate of over 5,000 ft per minute. Bottiglia later said that he and his colleagues "were counting down the altitudes, and they were descending, right at the end, at 10,000 feet per minute. That is absolutely unheard of for a commercial jet."

Two minutes before impact, New York Center alerted another nearby Air Traffic Facility responsible for low-flying aircraft, which was able to monitor Flight 175's path as it flew over New Jersey, followed by Staten Island and Upper New York Bay as al-Shehhi lined the plane up to strike the South Tower. The aircraft was in a banking left turn in its final moments, as it appeared the plane might have otherwise missed the building or merely scraped it with a wing. Therefore, those on the left side would have had a clear view of the towers approaching, with one burning. The airplane crashed nose-first into the South Tower's southern facade at a speed of around 587 mph, striking floors 77 through 85 with approximately 9100 U.S.gal of jet fuel on board. All passengers and hijackers on board were killed instantly.

By the time Flight 175 struck the South Tower at 09:03, multiple media organizations were already covering the first plane crash in the North Tower 17 minutes earlier, with millions watching all around the globe. The image of Flight 175's crash was thus caught on video from multiple vantage points on live television and amateur video, while close to a hundred cameras captured Flight 175 in photographs before it crashed. Video footage of the crash was replayed numerous times in news broadcasts on the day of the attacks and in the days that followed, before major news networks put restrictions on use of the footage. The initial assumption was that the crash of Flight 11 had been an accident, a mistaken belief that also hindered the process of immediately evacuating the South Tower after the first plane struck the North. This was dispelled when Flight 175 crashed into the South Tower.

The Port Authority initiated an immediate full-scale evacuation of the North Tower within moments of Flight 11's impact. However, the same was not done for the South Tower during the 17-minute window between the two impacts; instead, the Port Authority instructed workers to remain in place. An estimated 2,900 people descended from floors that were later cut off, before the tower was struck.

Nonetheless, more than 600 people were still present on floors 77–110 when the plane struck. The impact killed hundreds, including everyone on the plane and many more inside the South Tower. Unlike at the North Tower, however, a single stairway was mostly intact from top to bottom after Flight 175 crashed into the South Tower. This was because Flight 11 crashed almost directly midway into the North Tower's central core and severed all escape routes from the 92nd floor up, but al-Shehhi flew the plane into the eastern half of the South Tower's southern facade near the southeast corner while also banking at a severe angle, narrowly missing Stairwell A in the northwest corner. Only 18 people passed the impact zone through the available stairway and left the South Tower safely before it collapsed. One of these survivors, Stanley Praimnath, saw the plane coming toward him. Smoke, isolated fires and hot gases ventilating through the stairs compelled those who were trapped to either avoid using them entirely, or to head upward in the hopes of a rooftop rescue, while it has been suggested that others may have been in the process of descending from the impact zone when the tower collapsed at 09:59. In any case, those who did not make it out succumbed to the fire and smoke or the tower's eventual collapse. Three people were spotted falling to their deaths from the upper floors of the South Tower, two of whom had jumped to escape conditions inside; firefighter Daniel Suhr reporting to the South Tower was killed when one of these two landed on him.

The impact of Flight 175 also did some minor damage to the already-burning North Tower, as some windows on the east face nearest to the South Tower blew out the moment the pressure wave generated by the fireball hit them, worsening the fires in the North Tower. After the plane passed through the tower, part of the plane's landing gear and fuselage came out the north side of the skyscraper and crashed through the roof and two of the floors of 45–47 Park Place, between West Broadway and Church Street, 600 ft north of the former World Trade Center. Three floor beams of the top floor of the building were destroyed, causing major structural damage.