The Airplane - Jay Spenser [36]
On the eve of World War II, the Douglas DC-3 showed what semi-monocoque construction could do. Douglas delivered about 420 DC-3 airliners before the war halted production. It then rolled out 10,000 military variants for use by U.S. and other Allied forces. Yet more were manufactured under license by both the Soviet Union and Japan. All these decades later, hundreds of these rugged transports are still at work hauling cargo in many countries around the world.
World War II provided astonishing endorsements of the strength of stressed-skin construction. No airplane bore more dramatic witness to this than the Boeing B-17 Flying Fortress, which ranks as one of the most rugged airplanes of all time. Wartime photographs depict B-17s returning with the entire nose section blown off, gaping holes in the wings and fuselages, most of the tail gone, and so on. In one famous instance, a Flying Fortress returned sliced nearly in half by the wing of a colliding German fighter.
Of course, success breeds hubris, and with hubris there is often a fall.
On May 2, 1952, the de Havilland Comet I—history’s first jetliner—entered commercial service between London and South Africa. Sleek and futuristic, it looked to be a winner until two fatal crashes showed that people had more to learn about all-metal airplane design.
The de Havilland DH.106 Comet entered ser vice in 1952 as the world’s first jetliner. The prototype, shown here, flew in 1949.
National Air and Space Museum, Smithsonian Institution
On January 10, 1954, a Comet I disintegrated in flight off the coast of Italy. Finding nothing wrong with the design of the airplane, investigators tentatively attributed the tragic event to a bomb. But then on April 8 another Comet mysteriously came apart over the Mediterranean, and a more sinister picture emerged. The Comet fleet was grounded and an investigation of unprecedented scope was launched. In retrospect, people now suspected that a third crash in India, formerly attributed to a violent tropical storm, might also be a case of spontaneous structural failure in flight.
An unseen killer was stalking the Comet, but what was it? Engineers and accident investigators worked around the clock exploring every possibility. Finally, pressure testing of a Comet fuselage in a water tank together with analysis of recovered parts identified the culprit: metal fatigue.
The Comet flew twice as high as most other pressurized airliners of the era, so its pressurized cabin sustained a higher pressure differential between inside and outside. This exposed its metal body to stress every time the cabin was pressurized. The resultant flexing weakened the structure until it failed in a catastrophic rupture at altitude.
Suspicion initially focused on an escape hatch in the roof of the flight deck, which appeared to have failed. Wreckage subsequently brought up by a fishing boat suggested it might instead have been a passenger window. But exactly where the failure occurred was moot; the important thing was that it was occurring at all.
With little prior experience in pressurization, de Havilland’s engineers had not known to avoid designing windows and hatches with square corners. Unlike round metal window frames, or even square ones with rounded corners, square frames concentrate stresses at their corners. This alone had not doomed the jets, however, because these were robust structures. All calculations suggested they were sufficiently strong.
Neither de Havilland nor Great Britain’s certification authority, which had approved the Comet for production, was thus responsible for what had happened. Instead it was a case of simply not enough being known at the time about how metal fatigued under repeated stresses such as pressurization cycles.
With the 707 of 1958, Boeing