Germanwings Flight 9525: The unusual nature of the crash

The Germanwings Flight 9525 that crashed in the French Alps on Tuesday took off from Barcelona en route to Dusseldorf around 10 a.m. local time in good weather.

The Airbus A320, with 144 passengers and six crew members on board, leveled out at about 38,000 feet. Shortly after reaching cruising altitude, it went into a steady descent, which it continued on until it was pulverized on the side of a mountain in a remote part of the Alps, killing all on board.

While officials have so far ruled out terrorism as a likely cause of the crash, no explanation has been eliminated. If it wasn't terrorism, the circumstances of the crash are still unusual, according to numerous experts.

Below is a breakdown of the factors that have made the crash stand out so far.

Crashing After Cruising

CBS News national transportation safety expert Mark Rosenker, a former NTSB chairman, reports that passengers have the most to fear when it comes to plane crashes right after takeoff and right before landing.

"It's extraordinarily rare for an airplane to get into trouble and end up in a crash after reaching cruising altitude," Rosenker reports.

An estimated 80 percent of plane accidents happen within three minutes after takeoff and eight minutes just before landing. The Associated Press reports that just 10 percent of all plane accidents happen at cruising altitude.

The French Civil Aviation Authority said Flight 9525 lost radio contact at around 10:30 a.m. local time, or about 30 minutes after takeoff, and well before it was supposed to begin its descent into its destination.

At that point in a flight, most planes are in cruise control along a predetermined route.

The Slow Descent

So far, officials have not indicated Flight 9525 went into a wild tailspin or dive before crashing.

CBS News aviation and safety expert Capt. Chesley "Sully" Sullenberger, who famously landed an Airbus A320 safely in the Hudson River in New York City in 2009, said there are many reasons the plane could have descended -- from instrument malfunctions to smoke in the cockpit.

While it will be impossible to know what happened until the black boxes are analyzed, Sullenberger said the long descent time "suggests that at least to some extent the pilots were able to control the airplane."

David Learmount, writing for flightglobal.com, reports that within a few minutes of leveling off at cruising altitude, the plane "entered a steady descent profile without altering its ground speed to any significant extent from the 420-450kt (780-830km/h) adopted in the cruise. It was not a dramatic descent, but was very steady all the way to impact."

Additionally, Learmount points out the crash site was not far off from the planned flight route, meaning the plane didn't veer significantly to the left or right during it's descent from 38,000 feet to 6,550 feet, a distance of more than six miles.

Airbus A320 History

CBS News transportation correspondent Jeff Pegues reports the Airbus A320 is considered the "workhorse" of the aviation industry.

The single-aisle, twin-engine plane is used typically for flights no more than five hours, and is certified to fly up to 39,000 feet. Airbus says there are 3,606 A320s in operation, including many in the U.S.

The A320 family also has a sterling record of safety, according to a Boeing analysis, with just 0.14 fatal accidents per million takeoffs.

The aircraft that crashed was delivered to Lufthansa in 1991, and had more than 58,000 flight hours in some 46,700 flights, Airbus said. The plane last underwent a routine check in Duesseldorf on Monday, and its last regular full check took place in the summer of 2013.

Pilot Experience

Germanwings CEO Thomas Winkelmann said the pilot, who hasn't been identified yet, had more than 10 years' experience working for Germanwings and its parent airline Lufthansa.

"We cannot say at the moment why our colleague went into the descent, and so quickly and without previously consulting air traffic control," said Germanwings' director of flight operations, Stefan-Kenan Scheib.

No Distress Call

It was a control center and not the plane itself that sent out the alert about trouble on board Flight 9525.

Eric Heraud of the French Civil Aviation Authority said the plane lost radio contact at 10:30 a.m. Tuesday, but "never declared a distress alert itself." He said the combination of loss of radio contract with a control center and the plane's quick descent prompted the control service to declare a distress situation.