A plane without a functioning pilot doesn’t always fall out of the sky right away.

With enough fuel, altitude, running engines, and an autopilot that works correctly, the plane can keep flying for a surprisingly long time as if nothing has gone wrong. That appears to be what happened on June 4, 2023, when a Cessna Citation 560 with one pilot and three passengers aboard flew over Washington, D.C., after more than an hour of radio silence.

The Citation had taken off from Elizabethton, Tennessee, bound for Long Island’s MacArthur Airport. But about 15 minutes into the flight, as the jet climbed toward 34,000 feet, controllers tried to stop it at 33,000 feet for crossing traffic. The pilot didn’t respond, and no one heard from the cockpit again.

About 80 minutes after takeoff, the jet reached Long Island, turned around, and began flying southwest at 34,000 feet. By the time Air National Guard F-16s from Joint Base Andrews were sent to intercept it just before 3 p.m., the problem had turned into something much more serious than just a missed radio call. A private jet had crossed into the nation’s capital region, and nobody inside was answering.

From the outside, the plane still looked like it was flying high, fast, and in a steady line through the sky. As Joe Pappalardo explains in the Pop Mech feature, “The Hunt for a Deadly ‘Ghost Plane’ Over the Nation’s Capital,” it turns out the plane was likely still obeying old instructions:

Whether the pilot is alive or dead, the business jet is not under human control and may have been flying that way for more than an hour. Its arrow-straight flight path supports the idea that the jet is on autopilot; the sharp turn for home over Long Island likely was instigated by the flight control computer. In the absence of any other prompts, the autopilot has apparently taken control and is steering the plane back to Tennessee as the pilot had previously programmed.

Autopilot isn’t as smart as you may think. It doesn’t judge the flight like a copilot, but rather, follows instructions like “hold this altitude,” “maintain this course,” and “track this route”. In normal flying, this lets the pilot focus on everything else happening in the cockpit. But if the cabin loses pressure and the people onboard are running out of oxygen, autopilot can keep the plane on course even after no one is able to take over.

That’s what investigators saw in the flight data. According to the NTSB’s final report, the Citation’s automatic location data showed that it followed flight-plan waypoints at the last altitude assigned by air traffic control. Investigators determined that autopilot was likely determining the flightpath until the plane could no longer stay under control.

While autopilot can keep a plane pointed along the route, it can’t fix what investigators believe happened inside the Citation: the cabin lost pressure while the jet was cruising at 34,000 feet. At that altitude, the air is too thin to breathe safely without help. If the cabin loses pressure, the pilot has to quickly notice, get oxygen, and bring the plane down. But maintenance records showed that the pilot-side oxygen mask wasn’t installed before the flight.

That likely meant the Citation’s pilot, Jeff Hefner, didn’t have much time to understand what was happening before hypoxia—a lack of usable oxygen in the body—started to affect his brain. Hypoxia can make you feel slow, calm, and confused before you realize you’re in danger. So Hefner may have been sitting at the controls and still breathing, not aware that his judgment was slipping away. Investigators later found he was incapacitated by the loss of cabin pressure, unable to tell the plane what to do, even as the autopilot kept it flying.

The F-16 pilots try to reach the pilot until the business jet’s engines begin to fail. The autopilot tries to hold altitude and keep the plane on course. When this becomes impossible, the Citation begins a sharp, spiraling descent.

Minutes later, the jet crashed into a forest outside Montebello, Virginia, at about 3:23 p.m. The first report to Virginia State Police came in around 3:50 p.m. Search teams took hours to reach the wreckage, and there were no survivors.

This wasn’t the first case of a plane continuing to fly without a responsive crew. In 1999, a Learjet carrying golfer Payne Stewart and five others stopped responding to controllers, was intercepted by military aircraft, and kept flying for hours before it crashed in South Dakota. Investigators found that the crew had been incapacitated after the cabin lost pressure—another tragic instance where the systems kept working after the pilots stopped.

For the haunting complete story of how a ghost plane caused havoc in American airspace, what the F-16 pilots saw when they caught it, and why hypoxia remains such a dangerous aviation threat, read the full Pop Mech feature now.