Here’s what you’ll learn when you read this story:

• On August 14, 2005, Helios Airways Flight 522 crashed into the hills of the Greek town Grammatiko, killing all 121 passengers onboard.
• A subsequent investigation revealed that a series of errors led to depressurization of the airplane, causing passengers and the pilots to pass out from critical hypoxia.
• While the autopilot kept the plane aloft over Athens, Greece, for more than an hour, the engines eventually flamed out, causing the crash.

Aviation is one of the most remarkable achievements in human history, but sadly, flying eventually became the safest way to travel only because of piloting and engineering principles learned through disasters, mishaps, and tragedies. In 1956, for example, the collision of two passenger aircraft led to improvements in air traffic control systems in the United States. In 1983, a fire aboard a DC-9 caused the FAA to mandate smoke detectors and automatic fire extinguishers in all aircraft lavatories. The deadly crash of TWA Flight 800 in 1996 created changes that reduced sparks from faulty wiring. And on the morning of August 14, 2005, Flight 522 of Helios Airways, a low-cost airline with regular service between Cyprus and many European and African destinations, would join this hallowed list of aviation disasters that eventually helped make aviation safer for all of us.

At 11:05 a.m. on August 14, 2005, two Greek Air Force F-16 fighter jets were met with a bone-chilling sight. As the fighter jets approached Helios Airways Flight 522, which had been circling in a continuous loop over Athens in what some thought might be an impending terrorist attack, the pilots peered through the passenger aircraft’s windows into its main cabin and cockpit. Everyone inside the passenger jet appeared to be slumped in their chairs while wearing oxygen masks—including the pilot and the co-pilot. For more than an hour, Nicosia air traffic control had tried to hail the pilots to no avail, and now they knew why.

The Boeing 737—and the 121 people on board—only had one hope, a flight attendant named Andreas Prodromou, who had remained conscious thanks to a portable oxygen supply. While Prodromou held a U.K. Commercial Pilot License, he had no training in landing an aircraft of this size. Reports from the F-16 crew state that Prodromou briefly waved to the F-16 pilots from the cockpit window, tried to wake the Boeing’s pilots, and even sent a few mayday messages, but he was too late. The plane engine’s soon flamed out and the aircraft began its deadly descent, crashing into hills near the Greek village of Grammatiko just after noon local time. No one survived, leaving bewildered officials and grieving families to pick through the pieces.

A subsequent investigation uncovered the crucial mistake—Helios Airways Flight 522 left Larnaca, Cyprus, with its cabin pressurization system set to “manual” instead of “auto.” When the aircraft had arrived from London earlier in the morning, the flight crew reported a problem with a door seal. To investigate the cause, the pressurization system was set to “manual” so that ground crew didn’t need to restart the engines, but, unfortunately, they forgot to switch the system back to “auto,” and the flight crew overlooked this issue during preflight checks.

The flight departed from Larnaca shortly after 9 a.m. local time, and only five minutes later the cabin altitude warning sounded as the plane reached 12,040 feet. But the pilots dismissed the alarm because they mistook it for an identical-sounding “takeoff configuration warning,” which usually only goes off while on the ground. These compounding errors eventually caused critical hypoxia throughout the cabin, leading to what’s been dubbed a “ghost plane.” With passengers and pilots quickly falling unconscious, the plane entered an autopilot holding pattern above Athens, the aircraft’s scheduled layover, prompting the Greek military to jump into action.

The crash resulted in lawsuits against both Helios Airways and Boeing, and in 2006, the airline, already known for its bad aviation reputation before the crash, ceased operations. Eventually, the FAA in the U.S. required all Boeing 737 aircraft to include additional cockpit warning lights to differentiate between takeoff configuration and pressurization warnings, as well as increased hypoxia and pressurization awareness among flight crews. While the lives lost on Helios Flight 522 now protect everyone who flies around the world, for the families of the 121 onboard, the loss is felt every day.

“His dream was to become a professional pilot,” Prodromou’s father, Kostantinos Prodromou, said in a 2007 documentary about the crash. “He left a very big gap. We will never get over it.”