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The investigation into the disappearance of AirAsia Flight 8501 has centered on one moment: a request by the pilot to climb 6,000 feet because of weather, which was denied by air traffic control.
But what the crew was experiencing before that — which led them to make the request — is unknown. And what happened to the plane in the following minutes also remains a mystery, two days after the aircraft vanished from radar.
Indonesian officials have said they believe the Airbus jet went straight to the bottom of the sea, presumably killing all 162 on board. But Greg Feith, a former investigator for the National Transportation Safety Board and NBC News' aviation expert, says there are many questions about the incident that still need to be answered.
Did weather really take down the jet?
It's possible, of course, but a half-dozen other planes passed through the storm-struck area where the AirAsia jet vanished. "What did those pilots know that the accident crew didn't know?" Feith wonders. "What were they using for decision-making that allowed them to get through that line?"
The weather system was big, so climbing from 32,000 to 38,000 feet would not have allowed the QZ8501 pilots to completely avoid it, he said. But the request for a 6,000-foot change — as opposed to, say, 2,000 feet — had to be rooted in a concrete concern.
He noted that it's customary for pilots to report unusual conditions to air traffic control and that any other cockpit on that frequency would hear them. Feith wants to know what warnings the AirAsia crew heard about turbulence or heavy rain and whether they prompted the altitude change request.
Or did they — along with the air traffic controllers — think they could safely get through the storm, just like the other planes did, but conditions deteriorated too quickly? "Thunderstorms like this are very dynamic," Feith said. "It's a high-energy situation."
Who was in control of the plane?
Typically, the crew of a commercial airliner will fly it on autopilot as long as possible, even when making an altitude change. But the jet could have hit turbulence from the storm that became too much for autopilot to handle — forcing the pilot to take over, Feith said. Or the pilot could have decided he could do a better job of responding to the changing weather conditions.
Why haven't they been able to find the jet?
While controllers know where the plane was last seen on radar, that doesn't mean that's where it went down. A very dense thunderstorm could have prevented the signal from bouncing back, Feith said. A change in the plane's attitude — whether it is pitching up or down or banking to the left or right — could also interfere with radar. As a result, the plane could have flown on for some minutes off radar before it crashed.
After the crew lost control, the aircraft would not necessarily have plunged straight down into the water. It could have gone into a spiral, reversed course or shot off in any direction before hitting the water, making for a large search area. If the plane was intact when it hit the water, there will be less debris than if it broke up in midair.
Did the plane have ACARS?
The Aircraft Communications Addressing and Reporting System sends flight data from sensors to ground stations in short bursts, providing vital clues in the case of an accident. When Air France 447 crashed into the Atlantic in 2009, putting its black boxes out of reach until 2011, it was ACARS messages that revealed the pilots were getting incorrect speed data.
"I would love to know if AirAsia had an ACARS subscription and if not, why not?" Feith said.
But in the case of another flight, Air France Flight 447, which disappeared off the coast of Brazil in 2009, investigators had access to a cascade of error messages that were sent over the ACARS system that indicated, among other issues, a problem with the plane's airspeed sensors that ultimately disabled the plane's autopilot system.
AirAsia and local officials have so far not divulged much technical information about the flight, but Indonesia on Monday asked U.S. investigators, via the State Department, to assist in the search — so perhaps more knowledge will be forthcoming.