Fifty years after the plane carrying UN secretary-general Dag Hammarskjold crashed in the African bush during a peace mission to Congo, killing all aboard, the accident remains one of the Cold War's greatest unsolved mysteries.
Three investigations, including one by the UN, failed to explain the plane's plunge, feeding conspiracy theories involving superpower rivalries. But aviation experts now say the flight most likely fell victim to a danger foremost in the mind of today's air crash investigators, but unknown 50 years ago: pilot fatigue.
As an obscure Swedish technocrat, Hammarskjold had been an unexpected choice to become the second head of the United Nations in 1953. But he proved surprisingly forceful, personally negotiating the release of American soldiers captured in the Korean War, sending UN peacekeepers to defuse the Suez Canal crisis and trying to resolve the civil war in Congo, which had just won independence from Belgium.
He is still considered the most effective secretary-general the UN has ever had. And he is the only person ever awarded the Nobel Peace Prize posthumously.
In the course of his work, though, he angered many governments. Congo was a land in chaos that 20,000 UN troops were trying to stitch back together. But several countries coveted the mineral wealth of the country's breakaway province of Katanga, and feared the Congolese government would nationalize those resources if the country were reunited.
And so, when the chartered Douglas DC-6B airliner flying Hammarskjold went down on September 18, 1961, a slew of conspiracy theories emerged.
Did the Americans have Hammarskjold killed? Perhaps Belgian agents sabotaged the plane, or maybe British fighters shot it down? Each country had a stake in Congo's mineral resources, and stood to lose if a peace deal was struck. Other theories pointed the finger at the Soviets, who accused Hammarskjold of complicity in the execution of Moscow-backed Congolese independence leader Patrice Lumumba.
Hammarskjold's plane went down just as he was trying to negotiate an agreement between the government of Congo and Katanga. The wreckage wasn't found until 15 hours after the crash, just a few miles (kilometers) short of Ndola Airport in Northern Rhodesia - today's Zambia. Harold Julian, a bodyguard, was the only survivor among the 15 people on board, but he died three days later.
Three inquires, two by Rhodesia and one by the UN, failed to determine the cause of the crash. No evidence of explosives or damage from bullets or shells was found.
But experts now say pilot fatigue almost surely played a significant role. Although not conclusive, their findings have not previously been reported.
"By using modern fatigue models, we can show that the schedule that this crew flew on the day of the accident would have made them tired to a point of impairment when the accident occurred," said William Voss, head of the Flight Safety Foundation.
Over the past 15 years, a dozen fatal crashes have been blamed on pilot fatigue. Safety experts compare its effects to drunk driving. Both the Federal Aviation Administration and the European Union's air safety agency are currently drawing up new, stricter limits on the hours pilots can work daily.
It has never been established precisely how much flying Capt. Per Hallonquist and First Officers Nils-Eric Aahreus and Lars Litton had done in the preceding weeks, but it must have been considerable, said Bengt Rosio, Sweden's representative in Congo at the time. "All UN pilots just flew too much, they flew much more than they wanted to," he said.
Several other UN planes crashed or were damaged on landing during the 18-month long crisis. UN crews routinely flew multi-segment flights averaging well over eight hours a day, Rosio said, in addition to more hours of just waiting at airports or airstrips for their flights.
On that fateful day, the pilots waited 12 hours for Hammarskjold at the airport in Leopoldville - today's Kinshasa - before the seven-hour night flight to Ndola. Today's rules on flight-time limitations pay special attention to such waiting periods, which are added to the actual flight times and other factors as a cause for pilot fatigue.
"Fatigue is a very serious issue today, but in those days there was no understanding of its impact," said Gideon Ewers, spokesman for the international airline pilots association. "There were no duty time limitations. Pilots essentially flew until they fell asleep."
"A 12-hour wait would be considered excessive by today's standards," he said. "You don't get any rest sitting on the plane or an uncomfortable crew room waiting for you're next flight."
Even the official investigation reports acknowledged this saying that, "the co-pilots complained of being tired."
"There was a war, they were flying all the time, and nobody was keeping up-to-date records on how many hours the pilots remained on duty," said Lasse Holst, who has written a history of Transair, the former Swedish charter airline that owned the plane.
"It was like the Wild West; civilian rules were not in force there," he said.
The UN accident report, released in 1962, conjectured that an error by the pilots in keeping track of altitude on their nighttime approach to Ndola airport was the likely cause. But the report failed to explain how or why the pilots had erred so disastrously on the final approach.
The plane, which had its the landing gear down when it struck the trees at a shallow angle, carried no flight data or cockpit voice recorders and the probes found no trace of foul play.
Operating a lumbering, four-engine DC-6B was substantially different from flying a modern computer-controlled jet. It relied more on seat-of-the-pants flying and attention to the plane's myriad mechanical systems - tasks performed automatically in today's airliners.
And it's usually at the end of a long nighttime flight that aviators make mistakes.
Voss said rest-impaired crews often have events of "micro-sleep" when they effectively fall asleep for a few moments.
This is potentially deadly when the pilot is executing a manual approach such as the one in Ndola, where the aircraft descends to the runway in a series of "stair steps," losing a few thousand feet in altitude before leveling off, and then repeating the process several times until making its final approach.
If a pilot zones out and misses one of the level-offs, the plane can hit the ground in seconds.
"Fatigue, high workload, and a challenging approach into an unfamiliar airport ... that's a combination that didn't leave a whole lot of margin for error," said Patrick Smith, an airline pilot and aviation analyst.