The crash of an American U-2 spy plane over southwest Asia on Tuesday has highlighted issues about the reliability of what is generally regarded as a militarily essential, but extremely uncomfortable and unwieldly aircraft.
The U-2 can fly more than twice as high as airliners
The lack of visibility for the pilot, problems of cruising at high altitude and the considerable assistance required to land the surveillance aircraft mean that, by the US Air Force's own admission, it is the world's most difficult plane to fly.
But recent versions of the U-2 have vastly improved what used to be a dismal safety record.
An astounding 80-90% of the original U-2As crashed or were shot down.
The newer U-2R and U-2S have fared much better. There have only been five crashes and a sixth aircraft badly damaged in the last two decades, according to Jane's All the World's Aircraft.
There are currently thought to be 35 of the aircraft in service.
The U-2, which was developed in 1955 to photograph Soviet military facilities, is still regarded as invaluable to military commanders.
The U-2's significance as a reconnaissance aircraft is that it can cruise at up to 27,430 m (90,000 ft) - more than twice as high as airliners, and in all weather, day or night.
But its flexibility and ingenuity come at a price.
It flies so high that the single pilot has to wear a full-pressure suit similar to those worn in space.
He is required to breathe 100% oxygen for 60 minutes before take-off to avoid getting the bends, and his rest and diet are strictly controlled for days in advance.
When on board, he receives his meals in liquid form squeezed through toothpaste-like tubes, astronaut-style.
Paul Jackson, editor of Jane's All the World's Aircraft, describes flying at this kind of altitude as "balancing on a knife edge".
The speed parameters are so narrow that there is little room for manoeuvre between flying the plane so fast that it disintegrates and flying it so slowly that it falls out of the sky, he says.
To make matters worse, forward visibility is restricted because of the aircraft's extended nose.
Landing is also difficult. As in the space shuttle, the pilot cannot gauge his height above the ground.
A second pilot, himself always a U-2 pilot, follows in a chase car along the runway, providing advice on altitude and alignment.
A number of upgrades have improved the aircraft's performance and safety.
The introduction of the U-2R in 1967 and a $1.7bn modernisation in 1994, leading to re-designation as the U-2S, have served to iron out some of the aicraft's problems.
Though 40% larger, it now has a fuel-efficient F118 engine and no longer needs to refuel in the air on longer missions.
Fibre-optic technology and a state-of-the-art cockpit re-design are other improvements which have help to secure a continuing vital role for the U-2.