Did you know that U-2 decompression sickness cases increased after the end of the Cold War? Here’s why the iconic U-2 is the world’s hardest plane to fly

Did you know that U-2 decompression sickness cases increased after the end of the Cold War? Here’s why the iconic U-2 is the world’s hardest plane to fly

By Dario Leone
May 25 2021
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The U-2 cruises at 70 000 feet or at least twice the height of a typical passenger jet. The sky is dark during the day and the curvature of the earth can just be seen…

Built in complete secrecy by Kelly Johnson and the Lockheed Skunk Works, the original U-2A first flew in August 1955. Early flights over the Soviet Union in the late 1950s provided the president and other US decision makers with key intelligence on Soviet military capability.

The U-2R, first flown in 1967, was 40 percent larger and more capable than the original aircraft. A tactical reconnaissance version, the TR-1A, first flew in August 1981 and was structurally identical to the U-2R. The last U-2 and TR-1 aircraft were delivered in October 1989; in 1992 all TR-1s and U-2s were designated as U-2Rs. Since 1994, $1.7 billion has been invested to modernize the U-2 airframe and sensors. These upgrades also included the transition to the GE F118-101 engine which resulted in the re-designation of all Air Force U-2 aircraft to the U-2S.

The U-2 provides high-altitude, all-weather surveillance and reconnaissance, day or night, in direct support of US and allied forces. It delivers critical imagery and signals intelligence to decision makers throughout all phases of conflict, including peacetime indications and warnings, low-intensity conflict, and large-scale hostilities.

It cruises at 70 000 feet or at least twice the height of a typical passenger jet. The sky is dark during the day and the curvature of the earth can just be seen,’ Shawn Santo, an aviation expert, says on Quora.

Pilots have to wear a space suit because the cockpit is only partially pressurized. The typical pressure in the cockpit is equivalent to 29 000 feet (as high as Mt Everest). Before the flight, pilots have to pre-breath pure oxygen for at least one hour to purge the nitrogen from their systems. This procedure is to prevent decompression sickness (DCS), also known as the “bends”. The nitrogen dissolved in the blood stream will form bubbles at the reduced pressure at cruising altitude.


‘A study in 2011 looked at why the number of DCS cases was increasing after decades of flying the plane. The report found three main causes. Pilots are flying more missions in a given timeframe compared to the Cold War. The missions are longer. Finally, it was found the simple actions (like pressing a pedal, flipping a switch) increase the chance of DCS developing. On missions today, the pilots are “more active” during the mission than they were during the Cold War.

‘While maneuvering on the ground, the main landing gear is not steerable and differential braking is not available. The tail wheels turn about 6 degrees in each direction so a lot of space is needed to maneuver on the ground.’

Santo continues;

‘It climbs like crazy, easily passing 10 000 feet in less than 90 seconds.

‘At altitude, there is a small difference between the stall speed and its max speed [the so called ‘coffin corner’, which is the region of flight where a fast but subsonic fixed-wing aircraft’s stall speed is near the critical Mach number]. The difference can be as small as two and half knots of the U-2. This was increased to almost seven knots in the TR-1. A stall/spin can cause loss of control which may not be recoverable at altitude. Exceeding the maximum speed could overstress the fuselage and cause structural failure. The autopilot is designed to hold the aircraft below the maximum speed.

‘The flight controls are all pushrods, bell cranks and pulleys and cables. No boosted controls. Such a system would have meant more weight. It has a yoke instead of a control stick.

‘For landing, there is an automated gust protection system because the airframe cannot handle heavy gust and maneuvering loads. ‘It has one of the best glide slopes, at 109 knots, it has a glide ratio of 28:1.

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At landing, it comes in nose down at about 75 knots. Any faster and it would cause excessive floating in the ground effect space. Because the wings had fall away wheels on takeoff, during landing the pilot must work at keeping the wings level. There is a roll spoiler on each wing to assist in this. There cannot be more than a 15 knot cross wind for landing.

‘The pilot has to be one foot off the ground before he can stall the plane and touch down. A chase car paces the aircraft to give the pilot a visual reference and call out the final altitude. Also because the center of gravity is behind the main landing gear, it lands tail first.

‘Taxiing is challenging unless you are familiar with tail dragging gliders. It has been likened to either dancing with a lady or wrestling a dragon depending on the wind conditions.’

Santo concludes;

‘Also, each plane is essentially hand made and each of it a little different from the rest. Pilots would talk to each other and pass on warnings of key differences for a particular plane to the next pilot. Efforts have been made to eliminate these but they remain.’

It’s hard to come up with another aircraft that has so many dangers during normal operations.


Photo credit: U.S. Air Force

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Dario Leone

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.

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