One plane was slammed into a mountain by an employee with a history of depression, another was found to have been brought down by pilots who didn't perform as they were trained.
While 2015 is on course to be one of the safest on record in the skies, the year has offered grim reminders that technology is only as safe as the humans who use it.
The Germanwings crash on March 24, in which the co-pilot of an Airbus A320 intentionally triggered a deadly descent into the French Alps, and the AirAsia crash report which revealed confusion and miscommunication in the cockpit, have sent aviation-safety experts back to the drawing board.
They face questions over how an industry requiring regular medical checks missed signs that a pilot was mentally ill, and how a team with years of training managed to lose control of a modern jet.
In doing so, the experts are revisiting an age-old problem of man vs. machine.
"Humans still cause the majority of accidents," said Guy Hirst, a former Boeing 747 pilot who organizes conferences on safety in high-performance workplace environments such as cockpits and operating rooms.
"It's not the machines that are getting it wrong. Since the jet age, planes are mechanically much safer than they ever were," he said. "Something approaching 70 percent of accidents in aviation are down to some form of human error."
That does little to reassure air travelers who traditionally have viewed pilots as the calm and competent voice of authority.
"When an employee of an airline apparently brings an aircraft down, it is time to think carefully about how and why they were allowed anywhere near controls of a plane," said Chris Yates, consultant and former aviation security editor at the specialist publisher, IHS Jane's. "This year has reminded us that there so many things we can't control."
Earlier this month, officials announced plans for tighter rules in the wake of the Germanwings crash — including enhanced alcohol, drug and psychological testing for pilots, improved support and a new international database for pilots' aero-medical data.
The proposals, drawn up by the European Aviation Safety Agency (EASA), are already being considered by airlines and national regulators.
"We need to act quickly if we want to minimize the risk of a catastrophe such as the Germanwings accident to happen again," EASA executive director Patrick Ky said.
It all comes too late for the 144 passengers on Germanwings Flight 9525 who went through stringent security checks at Barcelona's airport yet whose safety was ultimately undone by collective trust in the cockpit crew.
A preliminary report revealed that co-pilot Andreas Lubitz, left alone on the flight deck while the captain used the restroom, adjusted the plane's selected altitude and sent it into a French mountainside, killing all on board.
Lubitz had seen seven doctors within the month before the March 24 crash, prosecutors said, including three appointments with a psychiatrist. None told his employer, because of German patient secrecy laws, but the FAA had previously questioned his fitness to fly after he admitted being treated for episodes of severe depression.
It was the most shocking tragedy in what is otherwise on course to be a record-setting year for aviation safety.
Seventeen fatal crashes — including Germanwings, AirAsia and the downing of a Russian passenger jet over Egypt's Sinai Peninsula — have caused 564 deaths up until Dec. 18. That is significantly below last year's 990 fatalities, and well under the 10-year average of 708, according to website Aviation Safety.
Terrorism threats and fears over the health of pilots might top the list of passenger concerns, but industry alarm bells were sounded in November when the final report into the 2014 crash of an AirAsia Airbus A320 blamed the response of pilots to a computer failure.
As the captain and his first officer struggled to handle an influx of electronic-warning messages, the plane rolled sharply several times, climbed too high and ultimately stalled before crashing into the Java Sea.
Despite the unfolding crisis, the captain did not retake control as required by the airline's procedures. While he pushed forward on his controls to bring the nose down, the co-pilot was still pulling back, causing the plane's automated fly-by-wire system to cancel out the opposing instructions.
The report described a scenario chillingly similar to the one that caused the crash of Air France Flight 447 in the Atlantic Ocean in 2009. In that crash, the crew mistakenly responded to a speed-sensor failure by pulling the nose up, causing a dangerously high pitch and ultimately a deadly stall over the southern Atlantic Ocean.
"The Airbus is a very clever aircraft but it seems clear that pilots are still being trained in a conventional way," Hirst said. "You've only got to read the AirAsia report to see they didn't understand what was happening."
In both cases, he said, pilots seemingly became so fixated on tackling automated messages that they missed or lost sight of how to keep the plane in the air, despite years of experience and intense training.
"Eight-eight percent of crews involved in accidents lose their situational awareness, their mental picture of what's happening. They were taking decision based on the wrong picture," Hirst said.
He said industry training must address the issue of how the human brain responds to ever-increasing automation — a problem summarized in "The Multitasking Myth," a 2009 collection of NASA studies into human performance.
"We were never designed to do the jobs we now do, we're always trying to adapt," he said. "As humans we make mistakes when we're distracted, when we're interrupted, when things happen out of their normal sequence.
"Unlike our computers, we are never going to get a firmware update, so we need to strive to understand our vulnerabilities and develop strategies to compensate for cognitive blind spots," he added.
Part of the problem lies in aircraft design, Hirst believes.
"Aircraft systems are typically designed and built by engineers, often with not enough input from the people actually fly it or use it," he said. "A lot of assumptions are made about how pilots will use the systems, particularly given the differences between fly-by-wire controls on the Airbus, and Boeing."
Others believe the expansion of the industry may have come at the expense of effective training.
"I guess that the problem might be that the growth of low-cost carriers has conspired to almost remove any concern about the adequacy of training," said Yates. "This year has been a bit of a wake-up call. We'd like to believe people are sufficiently trained to operate those aircraft but in the end there are always going to be corners that are cut. The question is whether the corners are being cut far too finely."
How can the industry overcome these problems? It already has pioneered the use of checklists to reduce simple errors and oversights — a practice now widely adopted in healthcare, particularly for surgeons.
"They are useful because they give you a chance to step back, take a deep breath and assess," said Hirst, but they should not be used as a safety system in their own right. "They are an aid to make you safer, that is all."
One key lesson from 2015 is that changes designed to make passengers safer might put them at risk of other danger. In the Germanwings crash, the locked cockpit door could not be overridden from outside by the helpless captain — a protocol introduced in response to the 9/11 hijackings.
"It is often said that the knee-jerk response to yesterday's incident can lead to tomorrow's accident," Hirst said.
Medical rules and training must also address the nature and behavior of those who typically become pilots, he added.
Rates of mental illness and suicide were not significantly different from other professions, he said, and pilots were often "mission-oriented" and less inclined to call in sick over stress or psychological problems.
"Pilots on the whole are completers, finishers," Hirst said. "Sometimes you need somebody to pull you back, perhaps when trying to land in weather outside the limits... You need a second-in-command saying 'hey, this isn't working.'"
Both the AirAsia and Air France crash reports found evidence that pilots had not challenged each other's decisions, even in a worsening emergency. In the latter case, the captain had left the cockpit for a rest despite knowing his Airbus A330 was headed for rough weather. "Good teamwork also involves effective leadership," Hirst noted.
However, Hirst believes that passengers should fly with confidence despite the aviation tragedies of 2015.
"Clearly there needs to be a lot more research in the aviation industry about human factors," he said. "But look at the statistics. You've done the most dangerous bit when you've got to the airport."