Two hours into a surveillance mission over Afghanistan, Lt. Col. Kevin Henry felt it begin. A tingling in his joints. A creeping confusion behind his eyes. Then the nausea hit — sudden, violent, disorienting. At 70,000 feet in a Lockheed U-2 Dragon Lady, the nitrogen dissolved in his blood was doing what physics demanded: forming bubbles. Inside his brain.

Within minutes, Henry could not remember how to fly his airplane. He forgot which instruments to read. He forgot which way to turn. He was alone in a single-seat aircraft at the edge of space, flying through the thinnest margin of survivable airspeed in all of aviation, and his brain was being consumed by the same phenomenon that kills deep-sea divers who surface too fast. At 70,000 feet, there is no surfacing. You are already at the top.

The Coffin Corner: Where Physics Turns Against You

To understand why decompression sickness is so dangerous in a U-2, you first have to understand where the U-2 flies. At 70,000 feet, the air is so thin that the difference between the aircraft’s stall speed — the minimum speed at which the wings produce enough lift — and its critical Mach number — the maximum speed before the airframe breaks apart — can be as little as six knots. Pilots call this convergence the “coffin corner.”

Fly six knots too slow, and the U-2 stalls. The nose drops. In the thin air, the aircraft accelerates rapidly. Fly six knots too fast, and you exceed the critical Mach number, triggering Mach tuck — an aerodynamic phenomenon where the nose pitches down uncontrollably, driving the aircraft past its structural limits. Either way, the aircraft comes apart. The U-2 pilot must hold the aircraft in this impossibly narrow band for missions lasting nine hours or more. And he must do it while his brain may be under chemical assault.

Boiling Blood at the Edge of Space

The U-2’s cockpit is only partially pressurized. At typical mission altitudes, the cabin pressure is equivalent to approximately 29,000 feet — the summit of Mount Everest. At that pressure, nitrogen dissolved in the pilot’s blood and tissues can come out of solution and form bubbles, exactly like opening a carbonated drink. This is decompression sickness — “the bends” — and it can strike without warning.

The bubbles can form in joints (causing agonizing pain), in the lungs (causing “the chokes”), or in the brain (causing neurological devastation). When nitrogen bubbles lodge in brain tissue, the results can include hallucinations, severe disorientation, memory loss, vision impairment, and loss of motor control. The pilot may not even realize something is wrong — the first symptom of cognitive impairment is often the inability to recognize that you are cognitively impaired.

Talked Down From the Edge: The Kevin Henry Incident

When Henry’s cognitive functions collapsed over Afghanistan, his squadron commander, Lt. Col. Dave “Super Dave” Russell, was monitoring from Beale Air Force Base in California. Russell recognized the symptoms over the radio and began talking Henry through every action — turn left, turn right, check this gauge, push this throttle. Henry was so disoriented that he initially turned the wrong way, nearly straying into hostile airspace. Russell patiently corrected him, turn by turn, for hours.

Henry landed the aircraft. He survived. But the nitrogen bubbles left permanent marks. Over the following three weeks, he suffered recurrent headaches, short-term memory loss, and what he described as “feeling in the fog.” MRI scans revealed focal lesions on his brain — physical scars from where the nitrogen bubbles had destroyed tissue. The cognitive deficits were permanent.

A Growing Crisis: DCS in the Modern U-2 Fleet

Henry’s case was not isolated. Between 2002 and 2009, the Air Force documented 16 confirmed cases of severe decompression sickness among U-2 pilots, five of which were classified as life-threatening. Nine of those pilots suffered long-lasting or permanent brain damage. Counterintuitively, DCS cases actually increased after the Cold War — the result of longer mission durations and higher operational tempos during the wars in Iraq and Afghanistan.

The U-2 community has responded with enhanced protocols. Pilots now pre-breathe 100% oxygen for at least one hour before flight to purge nitrogen from their bloodstream. They eat a carefully controlled diet to reduce gas in the intestinal tract. Their full-pressure suits — each costing approximately $125,000 — are essentially astronaut equipment, providing a final barrier against decompression. But none of these measures can eliminate the risk entirely. The physics of high altitude are relentless.

Still Flying: The Dragon Lady Endures

The U-2 has been in continuous service since 1956 — nearly seven decades. It has outlived every aircraft that was supposed to replace it, including the SR-71 Blackbird and multiple satellite programs. Today’s U-2S variants fly reconnaissance missions worldwide, carrying sensors that can image vast swaths of territory from altitudes where the sky turns black and the curvature of the Earth becomes visible.

The pilots who fly them are among the most carefully selected and physically monitored aviators in the world. They wear spacesuits. They eat prescribed meals. They breathe pure oxygen for an hour before every flight. And they know that at 70,000 feet, in the coffin corner, with six knots between survival and destruction, the greatest threat may not come from enemy missiles or mechanical failure. It may come from inside their own bloodstream — tiny bubbles of nitrogen, invisible and relentless, hunting for the one organ they cannot afford to lose.