China Boeing 737's steep (and final) nosedive puzzles experts

Air crash investigators are trying to figure out just why a passenger jet in China nosedived so steeply into the ground, killing all 132 people on board — calling the extreme angle of the crash unusual.

The jet's plunging final trajectory cannot be easily explained, according to air safety experts. "It's an odd profile," former Boeing 737 pilot John Cox told Bloomberg News. "It's hard to get the airplane to do this."

Information about the abrupt final dive of Flight MU5735, a Boeing 737-800 jet operated by China Eastern airlines, might come from its two "black box" flight recorders, one of which was found by investigators on Wednesday (March 23), according to the South China Morning Post.

The "black box" recorder is designed to withstand high impacts, and it contains flight data, records of pilot control inputs and cockpit audio. Investigators found it in the wreckage on a remote and forested mountainside near the city of Wuzhou in the southern Guangxi region. 

Chinese state media reported that the aircraft had been traveling at a cruising altitude of about 29,100 feet (8,870 meters) at about 523 mph (840 km/h) when it started to fall. 

Air traffic controllers reportedly tried to contact the jet's pilots by radio several times before its final impact, but received no response. The jet burst into a fireball upon impact on Monday (March 21) at about 2:.20 p.m. local time. There were no survivors. 

The jet had taken off about an hour earlier from the southern city of Kunming, bound for the city of Guangzhou near Hong Kong. The weather at the time was partly cloudy with good visibility. 

Fatal plunge

Flight MU5735 began plunging toward the ground when it was about 100 miles (160 kilometers) from its destination — about the point where it would normally begin its descent for landing.

But instead of a gradual descent of a few thousand feet per minute, the jet began falling at more than 30,000 feet (9,140 meters) per minute, according to tracking data from Flightradar24. The tracking data reveals the jet seems to have halted its dive after about 10 seconds and climbed briefly, but it soon resumed its fatal plunge.

As well as the flight tracking data, a surveillance video from a mining site in the region seems to show the aircraft in its final moments, in a steep dive directly toward the ground. 

Other videos posted on social media show the aftermath of the crash.

While some other passenger jets have crashed after suddenly dropping from cruise altitude, the MU5735 crash seems to be different.

"It's very odd," Jeff Guzzetti, a former Federal Aviation Administration accident investigator, told Bloomberg.

For instance, Air France 447 — which crashed into the Atlantic Ocean in 2009, killing all 228 people on board — fell much more slowly and more erratically than MU5735; a French inquiry ruled that its speed sensors had iced up and its pilots had become confused.

Final moments

While Guzzetti cautioned that the Flightradar24 data is preliminary, he noted that MU5735's relatively straight final trajectory and the fact that its transponders were still broadcasting suggests it didn't break up in-flight — a consequence of some terrorist bombings.

Another previous air crash that appears to have such a trajectory in its final moments was SilkAir flight 185, which crashed in Indonesia in 1997.

An Indonesian inquiry found that the SilkAir jet fell in a vertical dive from its cruising altitude of more than 38,000 feet (11,580 m) per minute, resulting in a crash that killed all 104 people on board. 

The cause of that crash was ruled to be extreme flight-control inputs "most likely by the captain," although the inquiry found "no concrete evidence" to support allegations that he did it deliberately.

A former U.S. air crash investigator and 737 pilot, Benjamin Berman, told Bloomberg it was too early to draw conclusions about what led to the MU5735 crash.

But he noted the aircraft's final steep trajectory was unusual, because — like other passenger jets — it's designed so that it won't typically dive at steep angles.

That means it could only result from an extreme effort by a pilot or a highly unusual malfunction, he said. 

Accidents like a disabled pilot slumping onto the flight controls, for instance, could be easily counteracted: "You need something to hold the nose down," Berman said.

Originally published on Live Science.