Federal authorities are investigating the cause of Sunday’s private plane crash in rural Virginia, which prompted a response by military jets that caused a sonic boom heard across the Washington region.

The aircraft took off from Elizabethton, Tenn., bound for Long Island, but turned back and eventually flew over Washington before crashing in Virginia. The crash killed the pilot and three passengers, according to the Federal Aviation Administration.

The National Transportation Safety Board said contact with the plane was lost about 15 minutes after departure, as it was passing over Virginia. Air traffic controllers also could not establish contact with the Cessna Citation as it crossed Washington’s restricted airspace, but one of the pilots of the military F-16 flying at supersonic speeds to intercept the jet saw its pilot slumped over, according to two people familiar with the investigation.

The FAA has said the crash occurred “under unknown circumstances” and has not identified a cause. But experts have said that the pilot could have fallen unconscious because of decompression of the cabin — possibly caused by hypoxia, a lack of oxygen that can lead to unconsciousness — and that the plane was likely running on autopilot until it ran out of fuel.

F-16 flier sent to intercept plane saw pilot slumped over before crash

Here is what you need to know about hypoxia on planes.

Hypoxia occurs when low levels of oxygen in body tissues prevent organs from working properly. It can result from inadequate oxygen delivery to the tissues caused by low blood supply or low oxygen in the blood.

It is often caused by underlying diseases that affect blood flow or breathing, including chronic heart and lung conditions like bronchitis, congenital heart defects, emphysema, asthma and pneumonia.

Body cells require oxygen to produce energy and help organs and tissues function properly. While some tissues can adjust to temporary dips in oxygen levels, prolonged hypoxia — oxygen starvation — can cause organ damage. Brain and heart damage are the most dangerous and can lead to death. A lack of oxygen to the brain, known as cerebral hypoxia, can cause unconsciousness and the death of brain cells within minutes.

Among some of the most common symptoms of hypoxia are confusion, restlessness, difficulty breathing, headache, rapid heart rate and bluish skin. Although symptoms can vary among people, other common ones include lightheadedness, dizziness, tingling, sweating, impaired judgment, tunnel vision and euphoria.

Why does hypoxia happen on airplanes?

In aeronautics, hypoxia typically occurs from decompression or lack of pressurization in the aircraft cabin. It can happen when flying a non-pressurized aircraft above 10,000 feet without supplemental oxygen; during a rapid decompression in flight; or when the pressurization and oxygen systems fail, according to the Federal Aviation Administration.

Hypoxia occurs within a few minutes if the cabin pressure altitude rises to between 16,000 to 20,000 feet, according to the Flight Safety Foundation. Oxygen pressure decreases as altitude increases and is the reason planes are pressurized.

In the case of an extremely rapid depressurization in the aircraft — if there is a cabin rupture, for instance — hypoxia can occur within seconds.

The FAA warns that the brain is the first part of the body to reflect a diminished oxygen supply, usually causing a loss of judgment, making it particularly dangerous for pilots and flight crews.

Unless it is detected and additional oxygen is quickly available, hypoxia “can be a real killer,” according to an official FAA brochure.

He kissed his family goodbye. Their private plane never made it home.

What safety procedures exist to prevent oxygen loss?

When flying at high altitudes, supplemental oxygen is the only solution to provide enough oxygen to meet the demands of the body while allowing a breathing rate that expels the right amount of carbon dioxide, according to the FAA.

For safety purposes, the FAA requires all general aviation operations’ flight crews to use supplemental oxygen for any portion of the flight that exceeds 30 minutes above 12,500 feet, up to and including 14,000 feet. The flight crew must use supplemental oxygen for the entire flight in operations above a cabin pressure altitude of 14,000 feet.

For night flights, because vision is particularly sensitive to reduced oxygen, FAA guidelines require supplemental oxygen when flying above 6,000 feet.

Hypoxia symptoms can greatly vary from person to person, and it can be hard for someone to recognize they are experiencing it, according to the FAA, making it particularly dangerous for pilots traveling alone. Some people, for instance, facing an oxygen deficiency in high altitudes have experienced a sense of euphoria.

“Such is the insidious nature of oxygen deprivation. It sneaks up on the unwary and steals the first line of sensory protection — the sense that something is wrong — dreadfully wrong,” the agency said in the brochure.

Mapping the Cessna Citation’s flight path over D.C. and sonic boom from F-16s

The key to flying safely at high altitude is to know the flight conditions in which aviators may become hypoxic, be able to recognize personal hypoxia symptoms and get additional oxygen before losing one’s ability to help themselves, experts and aviation authorities say.

Aviators can learn their personal hypoxia symptoms by taking formal physiology courses and altitude chamber tests in which high-altitude flight conditions are duplicated.

The FAA’s Civil Aerospace Medical Institute offers one-day aviation training courses with altitude chambers to civilian aviators who have a medical certificate to experience the effects and learn their symptoms of hypoxia in high altitudes.

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