Gallery: Forward From 15; Aviation 15 years after 9/11

This gallery begins with a photo of the Tribute in Light between the Twin Towers of the World Trade Center in New York on last year's 9/11 anniversary.

Hijacked United Airlines Flight 175, a Boeing 767-200, flies toward the South Tower of the World Trade Center on Sept. 11, 2001 in New York City. The aircraft was one of four hijacked that morning and the second plane to be flown into the World Trade Center buildings after an American Airlines Boeing 767-200ER hit the North Tower.

The Twin Towers of the World Trade Center billow smoke after hijacked American Airlines Boeing 767-200ER and United Airlines Boeing 767-200 aircraft were deliberately crashed into the North and South towers.

Continental Express aircraft sit at the closed New York Newark Airport the day after the terrorist attacks on the World Trade Center. One of the hijacked planes departed the Newark Airport and later was crashed in a field in Shanksville, Pennsylvania.

The Washington Monument stands in the background as the Pentagon burns after a hijacked American Airlines Boeing 757-200, Flight 77, was crashed into the iconic US military headquarters just south of the US capital.

American flags fly at the memorial site in Shanksville, Pennsylvania, where the United 757 crashed.

A memorial park was opened in 2015 on the Pennsylvania site where United Airlines Flight 93, a Boeing 757, was crashed after passengers attempted to storm the cockpit and take control from that aircraft's hijackers.

Trans World Airlines (TWA) operated from 1925-2001. It was originally formed as Transcontinental & Western Air to operate a transcontinental route from New York City to Los Angeles via St. Louis and Kansas City, as well as other intermediate stops with Ford Trimotor aircraft. The TWA name disappeared in December 2001, after the carrier was acquired by American Airlines.

Fort Worth, Texas-based American Airlines bought TWA out of bankruptcy in 2001 and retired the name.

America West Airlines, 1983-2005, was a US airline headquartered in Tempe, Arizona.

America West’s new CEO came onboard just 10 days before Sept. 11, 2001, and led the small carrier through the crisis. Ultimately, Doug Parker would become the mastermind behind the merger of America West with US Airways, and then of US Airways with American Airlines. He is now CEO of American, the world’s largest airline.

Arizona-based carriers US Airways and America West Airlines completed a merger in September 2005, with the US Airways brand surviving.

March 2007: Florida-based Spirit Airlines adopts the ultra low-cost carrier business model, becoming the first US carrier to commit to a for-fee pricing system, a system now widely adopted by low cost carriers and many legacy carriers worldwide. This Spirit Airbus A319 is painted in the grayscale livery of the mid-2000s.

Not all startups since 2001 have been minimalist. San Francisco-based Virgin America began service Aug. 8, 2007 with a mission statement "to make flying good again, with brand new planes, attractive fares, top-notch service, and a host of fun, innovative amenities that are reinventing domestic air travel." This is a Virgin America A320

Another classic US airline brand that has disappeared since 2001 is Northwest Airlines, which operated from 1926 to 2008.

In April 2008, Atlanta-based Delta Air Lines and Minnesota-based Northwest Airlines announced they would merge to be called Delta Air Lines. FAA approved the merger in September 2008.

US airline consolidation has been a significant feature of the industry in the past 15 years. Chicago-based United Airlines and Houston, Texas-based Continental Airlines announced a merger in May 2010 and closed the transaction Oct. 1, 2010, creating United-Continental Holdings.

Dallas-based Southwest Airlines acquired Orlando-based Air Tran Airways in 2011, in a deal that brought together the world's largest low-cost carrier (LCC) with the third largest LCC in North America.

The US Transportation Security Administration (TSA) launched Pre-Check in October 2011. Since then, More than 1 million people have enrolled in TSA Pre-Check. TSA is currently available at more than 180 airports with 18 participating airlines. In September, Germany's Lufthansa became the first European airline to participate in the expedited screening program.

Fort Worth, Texas-based American Airlines and Tempe, Arizona-based US Airways closed a mega-merger in December 2013. American Airlines had been operating under Chapter 11 bankruptcy protection since November 2011. With the creation of the "New American", the US Airways brand also was retired.

In April 2016, Seattle-based Alaska Airlines and San Francisco-based Virgin America announced plans to merge in a $4 billion deal. In July 2016, Virgin America shareholders approved the proposed merger. The acquisition is expected to be completed by early 2017.

New York-based JetBlue Airways began the first commercial airline passenger service from the US to Cuba in 55 years, with an Aug. 31, 2016 flight from Fort Lauderdale, Miami, to Santa Clara, Cuba.