(Bloomberg) -- Crash investigators face a difficult task dissecting what happened aboard the private jet that cruised up and down the US East Coast on Sunday with an incapacitated pilot, especially if the cause was a lack of oxygen.

The Cessna 560 Citation V carrying a Hamptons real estate agent, her daughter and a nanny flew for about two hours without responding to radio calls before finally slamming into mountainous terrain in Virginia at high speed.

Investigators may only be able to outline the most likely scenarios without being able to pinpoint their origins, according to aviation experts and prior accidents reviewed by Bloomberg.

“It’s very difficult,” said Roger Cox, a former investigator with the National Transportation Safety Board who was an airline pilot. “When you have a high-energy impact, there’s so little evidence remaining that you usually can’t find anything useful in the wreckage to help explain the accident.”

A pilot of a fighter jet that intercepted the Cessna because it traversed some of the most sensitive government sites near Washington without permission saw its pilot slumped over, according to preliminary information from the NTSB.

That suggests either he suffered a medical issue or lost consciousness because there wasn’t enough oxygen in the cabin as the jet flew at 34,000 feet (10,366 meters). A health emergency seems less likely because there is no evidence the passengers tried to intervene, Cox said. 

But there’s no way to test bodies for a lack of oxygen, or hypoxia, after an accident. 

In the 1999 fatal crash that killed golfer Payne Stewart, three other passengers and two pilots, NTSB concluded their Learjet 35 lost pressure as they climbed, a similar scenario to Sunday’s crash. After taking off from Orlando on a flight to Dallas, the pilots lost consciousness and the plane flew more than 1,000 miles to Aberdeen, South Dakota, before running out of fuel.

But investigators couldn’t determine why. They also weren’t able to say why the two pilots weren’t able to use the emergency oxygen system, which should have given them time to address the issue and descend to safer altitudes. 

After a Sept. 5, 2014, crash into the Caribbean killed a New York real estate developer and his wife, NTSB investigators concluded that he suffered hypoxia while flying at 28,000 feet from Rochester, New York, to Naples, Florida. 

In that case, the NTSB recovered aircraft computer components under the sea that indicated problems with the single-engine, turboprop Socata TMB 700’s pressurization system. The pilot also was confused during radio calls, a symptom of lack of oxygen, the NTSB said. But they weren’t able to say why he didn’t don his oxygen mask when the problem began. 

The Cessna jet that crashed Sunday apparently had a crash-proof cockpit sound recorder installed on it, though it hasn’t been found yet, the NTSB said Wednesday. It could provide clues about the state of the pilot and the sound of any alarms. But recorders typically capture two hours of sound, so it might have recorded over the initial failure.

‘Kind of Loopy’

Such accidents are rare. The NTSB reports only a handful of fatal accidents due to pressurization-system problems in the past 15 years. 

When they occur, however, they can quickly turn deadly because the lack of oxygen to the brain can confuse pilots and give them a false sense of security. 

“Hypoxia is insidious,” Cox said. “If you continue, after a while you lose the ability to make judgments.”

Jim Hensley, a charter pilot who flies the same Cessna jet model involved in Sunday’s crash, said as part of his training he went into a special chamber that simulates high altitudes for training. 

Hensley, owner of America Jet Charter Inc. in Bethany, Oklahoma, said he had difficulty doing the most basic math problems, but little sense he was impaired. 

“You’re kind of loopy,” he said. “It’s like you’re intoxicated. You have a really good feeling. You don’t feel there’s a problem, but your brain isn’t getting enough oxygen to compute things.”

Potential Problems

While the NTSB is looking at the potential for hypoxia in Sunday’s crash, spokesman Eric Weiss cautioned that investigators haven’t reached any conclusions and will also examine a range of other potential problems. 

The plane was carrying Adina Azarian, who sold real estate in New York and Long Island, and her two-year-old daughter, Aria, according to a statement from Keller Williams NYC, a franchise of Keller Williams Realty Inc. Also aboard was an unidentified nanny and pilot Jeff Hefner. John Rumpel, whose Florida-based Encore Motors of Melbourne Inc. owns the plane, had adopted Azarian, he told the Washington Post.

The first indication of an emergency occurred about 15 minutes after the jet took off from Elizabethton Municipal Airport in Tennessee on a flight to Long Island, according to NTSB. 

As the plane reached about 31,000 feet, an air-traffic controller attempted to radio the pilot, according to NTSB. There was no response. 

The risks grow dramatically as planes climb. At that altitude, a pilot would have to respond quickly to a lack of air pressure, the NTSB wrote in its report on Stewart’s crash.

“Research has shown that a period of as little as eight seconds without supplemental oxygen following rapid depressurization to about 30,000 feet may cause a drop in oxygen saturation that can significantly impair cognitive functioning and increase the amount of time required to complete complex tasks,” the report said.

Warning System

If it does turn out to be an issue with the cabin pressure, there should have been multiple warnings in the cockpit, Hensley said. 

A red light directly in front of the pilot known as a master caution would have illuminated, he said. That would have directed the pilot to a second, specific warning on the cabin pressure. They are set to go off if the cabin reaches an altitude of 10,000 feet or higher, he said. 

Separately, a cockpit gauge shows the cabin pressure levels and oxygen masks would automatically drop for passengers if the cabin went to the equivalent of 14,000 feet, Hensley said.

Unlike in some recent accidents investigated by NTSB, Hefner, the Cessna pilot, had extensive professional experience. He had flown earlier in his career for Southwest Airlines Co. and would have regularly been drilled on how to respond. 

(Updates with NTSB statement on cockpit voice recorder in 12th paragraph)

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