The case of Malaysia Airlines Flight 370 fixated people all over the world in early 2014. The plane mysteriously veered off course on a flight between Kuala Lumpur and Beijing and vanished over the Indian Ocean, along with its 239 passengers and crew [source: Associated Press].
As this article was being completed, searchers were faced with scouring an area of more than 22,000 square miles (57,000 square kilometers) in the Indian Ocean. Finding the plane seems like a long shot, let alone solving the mystery of why it disappeared in the first place [source: Associated Press].
It's hard to grasp that a plane could go missing nowadays with all the technology we have at our disposal. Over land, for example, air traffic controllers can use two different types of radar to track planes.
When an aircraft is over the ocean, out of the reach of ground radar, it uses yet another system, Automatic Dependent Surveillance, which transmits a signal to satellites to indicate position. Aircraft are also designed to notify the Aircraft Communications Addressing and Reporting System, if they develop mechanical failures. Investigators believe that in the case of MH 370, those systems were turned off, perhaps deliberately by saboteurs [source: Topham].
But while the Malaysia Airlines case is deeply puzzling, it is far from unprecedented. Since 1948, more than 100 other aircraft have gone missing while aloft and have never been found, according to records compiled by the Aviation Safety Network, an international organization that tracks airliner accidents, hijackings and safety issues [sources: Topham, ASN]. Still other planes have crashed under circumstances that have not been fully explained. Here's a look at 10 of the most puzzling aviation mysteries ever.
In June 1937, celebrated aviator Amelia Earhart set out to become the first woman to fly around the world, a trip of 29,000 miles (46,671 kilometers). She'd completed all but the final 7,000 miles (11,265 kilometers) by July 2, when she and her navigator Fred Noonan took off from Lae in Papua, New Guinea, for a 2,556-mile (4,114-kilometer) flight to Howland Island, a small island in the Pacific.
A U.S. Coast Guard cutter, the Itasca, was stationed just off shore, and two other ships were positioned along the flight route as markers for Earhart's plane. Nevertheless, the plane ran into overcast skies and intermittent rain showers that made celestial navigation, Noonan's favorite method, difficult. The next morning at 7:42, the Itasca picked up this radio transmission from Earhart: "We must be on you, but we cannot see you. Fuel is running low." About an hour later, Earhart radioed, "We are running north and south" [source: AmeliaEarhart.com].
That was her last transmission, and her plane never arrived. Rescuers mounted, what was at the time, the most massive search in aviation history, searching 250,000 square miles (647,497 square kilometers) of ocean in a fruitless attempt to find her. Some believe the plane crashed in the ocean and the passengers were killed; others, that they survived the crash but died of thirst on a remote island, or else that they were taken alive as hostages by the Japanese who though they were spies [source: Roach and Than]. To this day, Earhart and Noonan's fate remains a mystery.
In August 1947, a British airliner containing 11 people took off from Buenos Aires on a flight to Santiago, Chile, and vanished, apparently just a few minutes before landing. The only clue it left was a puzzling Morse code message, "STENDEC," which was the final transmission from the plane. The word was transmitted three times.
Fifty-three years later in 2000, an expedition of searchers finally found the missing plane, which had crashed into a mountain about 31 miles (50 kilometers) from its destination. A glacier had entombed it in ice. Examination of the engines showed no mechanical failure, but accident investigators hit upon another possible explanation. They decided that the plane probably had flown high to avoid bad weather and run into a jet stream, a high-speed wind whose existence had not yet been discovered. That wind would have slowed down the aircraft, so that when it started descending it was not as close to the airport as the pilot thought and so headed toward the mountain. But even so, nobody has ever figured out the meaning of the last message sent by the plane, which for years has defied myriad efforts to decipher it [sources: BBC News, BBC News].
In January 1948, Capt. Thomas Mantell, a Kentucky Air National Guard pilot, and some other pilots took off in P-51 Mustang fighters from the airfield at Fort Knox, pursuing a rapidly moving, westbound, circular object. The other pilots called off the chase when the large, metallic object elevated to 22,500 feet (6,858 meters) in altitude, and they returned safely. (The danger zone for flying without oxygen begins at 14,000 feet, or 4,267 meters.) But Mantell kept following this strange aircraft. His plane tumbled from the sky and crashed in Kentucky.
The official explanation was that he lost consciousness from a lack of oxygen, but questions remain about why he kept going and what he was following so intently. Air Force officials initially said Mantell had mistakenly followed planet Venus, but some witnesses thought he had seen a spaceship or some other UFO. The object is now believed to be a Skyhook weather balloon used to measure radiation levels that was part of a secret project [sources: National Guard History eMuseum, Randle].
During a routine training mission off the coast of Georgia in February 1958, a B-47 bomber accidentally collided with an F-86 fighter jet whose pilot didn't see the bomber on his radar. The crash tore the left wing off the fighter and severely damaged the bomber's fuel tanks. The bomber pilot, Air Force Col. Howard Richardson, was confronted with an agonizing dilemma. His aircraft happened to be carrying a 7,000-pound (3,175-kilogram) H-bomb, and Richardson was worried that the bomb would break loose from his damaged plane when he tried to land.
To avoid that situation, he went with his only other option, and dumped the bomb into the waters off Tybee Island, Ga., before landing at Hunter Air Force Base outside Savannah. The Navy searched for the lost bomb for more than two months, without success, and for decades, its location has remained a mystery. The Air Force says the bomb presents little hazard if left undisturbed [source: NPR].
These lost men have never had their names added to the Vietnam Veterans Memorial in Washington. Even more oddly, no government agency — not the Army, Air Force, Pentagon, State Department, National Archives or CIA — admits to possessing any records about the mission. Yet in March 1962, Flying Tiger Line Flight 739, containing 93 U.S. military advisers, three South Vietnamese officers and 11 crew members, vanished between Guam and the Philippines on the way to Vietnam. No wreckage was ever found, though some witnesses reported seeing a vapor trail and explosion in the sky on the night it disappeared [source: Burke].
Some speculate that the aircraft was downed by Viet Cong sabotage, which would make the passengers among the first casualties of the Vietnam War. Others believe it was brought down by friendly fire. Eerily, at least three soldiers told their family members they had a premonition that they would not return from this mission alive [source: Burke].
A Boeing 707 cargo plane containing 153 paintings by famed Brazilian-Japanese painter Manabu Mabe worth about $1.2 million, took off from Narita International Airport in Tokyo in January 1979. The plane, operated by Varig Airlines, was scheduled to stop in Los Angeles and continue on to Rio de Janeiro.
But it never got there. Instead, when the plane was north of Tokyo, about 30 minutes after takeoff, air traffic controllers lost contact with the pilot. No trace of the plane, the cargo or its six-person crew has ever been found [sources: ASN, Hastings].
This one sparked an international controversy. On Oct. 30, 1999, EgyptAir Flight 990 took off from Kennedy Airport in New York bound for Cairo with 217 people aboard. A senior pilot named Ahmad al-Habashi was in command of the Boeing 767 jetliner, and a veteran co-pilot, Gameel al-Batouti, was assisting him.
The plane climbed to 33,000 feet (10 kilometers) and flew normally for 30 minutes, before plummeting into the Atlantic Ocean 60 miles (97 kilometers) south of Nantucket, Mass., killing everyone aboard [source: Langewiesche].
So what caused the accident? Egyptian officials believed that mechanical failure was to blame, stating that suicide is against Islam. But the U.S. National Transportation Safety Board (NTSB) concluded that the co-pilot had deliberately caused the crash, citing that the autopilot was off, the plane was in a wings-level dive and the co-pilot muttered calmly to himself, "I rely on God" many times. The final report said that the crash was due to the co-pilot's "manipulation of the airplane controls," a claim Egypt still disputes [source: Wald].
However, one EgyptAir pilot told the FBI that Batouti had just been demoted because of accusations of sexual misconduct, and might have crashed the plane to commit suicide or to take revenge against his supervisor, who was a passenger on the flight [source: Wald].
In May 2003, a Boeing 727 owned by a Florida-based leasing company, was undergoing repairs at an airport in Luanda, Angola. Suddenly, it rolled onto a runway without getting permission from the tower and took off to the southwest in an erratic fashion, with lights off and no transponder signaling.
On board were Ben Charles Padilla, an experienced mechanic and flight engineer who was supervising the repairs, and his recently hired Congolese assistant, John Mikel Mutantu. Neither was trained to fly a 727 – Padilla had only a private pilot's license, and Mutantu had nothing. But both men and the plane disappeared and were never seen again.
This was one of the most horrifying incidents in aviation history. In August 2005, Helio Airways Flight 522 was on a short trip from Cyprus to Greece, when it veered slightly off course. Nineteen radio requests were sent for an explanation but there was no response.
Finally, two F-16s caught up with the plane to see what was going on, and their pilots saw that the captain's chair was empty and the co-pilot appeared to be unconscious, with his oxygen mask dangling from the ceiling. All the passengers were frozen to their seats, at temperatures of minus 58 F (minus 50 C).
The fighters followed the "ghost plane" as it flew on autopilot until it ran out of fuel and crashed into a hillside. The official report by Greek authorities concluded that the pilots had somehow incompetently left the plane's pressurization system in manual mode after takeoff, so the plane didn't automatically repressurize when cabin pressure was lost. Further, the pilots failed to recognize the warning signs that the cabin was losing pressure -- and oxygen -- until it was too late. But that explanation fails to satisfy those who suspect that the aircraft was haunted [sources: Daily Mail, Krisch].
In an accident reminiscent of Malaysia Airlines Flight 370, Air France Flight 447 disappeared over a stretch of the ocean where there was no radar coverage. In June 2009, this Airbus A330-200 airliner carrying 216 passengers and 12 crew members disappeared over the Atlantic en route from Rio de Janeiro to Paris.
For several days, no one knew what had happened. But even after wreckage was spotted and recovered, the circumstances remained murky. The state-of-the-art jet was equipped with the latest tracking and communication equipment, yet it hadn't emitted a distress signal at any point. Two years later, the plane's black boxes were finally recovered from the ocean [source: Smith].
French air safety investigators concluded in a 2012 report that the tragedy likely had been caused by an odd cascade of errors. Ice crystals accumulated on a probe, causing it to give incorrect speed readings and the autopilot system to disengage. The aircraft's two co-pilots, who were in charge at the time because the captain was taking a break, apparently became confused by the malfunction. By the time the captain rejoined them 90 seconds later, the airplane already was in a stall that he could not avert. The plane crashed two minutes and thirty seconds later [source: BEA].
Why it took the captain so long to respond to the co-pilots' frantic calls for help has never been explained. News media reported that the captain was accompanied on the flight by a female companion, and that he'd only slept one hour the night before [sources: Battiste, Smith].
From mechanical failure to pilot error, large passenger jets crash for many reasons. HowStuffWorks looks at five.
Author's Note: 10 Unsolved Airplane Mysteries
I've long been fascinated with the Amelia Earhart case, ever since I read a book as a boy that argued that she had been captured by the Japanese and possibly executed as a spy. I was surprised to discover, however, that so many aircraft had disappeared over the decades.
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