The Square Window Corner

De Havilland Comet Fatigue Cracks and Stress Concentration in Pressurized Cabins

Twenty minutes after departing Rome's Ciampino Airport for London on January 10, 1954, British Overseas Airways Corporation (BOAC) Flight 781 climbed through 26,000 feet above the Tyrrhenian Sea. Without a radio distress call, the de Havilland DH.106 Comet 1 jetliner—registered G-ALYP—disintegrated in mid-air. Wreckage and thirty-five victims fell into the Mediterranean waters off the island of Elba. Grounding the fleet briefly to armor the turbine rings and modify control linkages, British aviation authorities found no structural defect and resumed passenger service on March 23. Sixteen days later, on April 8, 1954, a second Comet (G-ALYY, operating as South African Airways Flight 201) climbed out of the exact same Rome airport at night. It broke apart at 35,000 feet near Naples, killing all twenty-one people aboard.

Powered by four Ghost turbojet engines buried inside its wing roots, the Comet was the world's first commercial jet airliner. It cruised at 36,000 to 40,000 feet—twice the altitude of piston-engine transports—to fly above weather and reduce aerodynamic drag in thin air. At 40,000 feet, outside atmospheric pressure drops to 2.7 pounds per square inch. To keep passengers breathing comfortably without oxygen masks, the Comet's cabin was pressurized to an internal differential of $\Delta P = 8.25\text{ psi}$, roughly double the pressure differential of earlier propeller aircraft. To save weight, de Havilland skinned the fuselage in thin 22-gauge D.T.D. 546 aluminum-alloy sheet measuring just $0.028\text{ inches}$ ($0.71\text{ millimeters}$) thick. Large square passenger windows and square overhead navigation hatches were cut directly through that thin metal skin.

Prime Minister Winston Churchill ordered Arnold Hall, Director of the Royal Aircraft Establishment (RAE) at Farnborough, to determine the cause at any cost. Royal Navy salvage vessels used underwater television cameras to recover 70 percent of G-ALYP's airframe from the seabed off Elba. Simultaneously, Hall constructed a 112-foot outdoor steel water tank around an intact Comet fuselage, G-ALYU (Yoke Uncle). Engineers filled both the fuselage cabin and the surrounding tank with water. Every few minutes, hydraulic pumps forced water into the cabin to simulate the $8.25\text{ psi}$ flight pressure differential while outdoor jacks flexed the protruding wings. Using incompressible water prevented a catastrophic pneumatic explosion when the metal skin finally fractured.

In a thin-walled cylindrical pressure vessel of radius $R$ and wall thickness $t$, internal pressure $\Delta P$ generates a nominal circumferential hoop stress $\sigma_0 = \Delta P \cdot R / t$. De Havilland engineers had calculated that baseline stress at roughly $14,000\text{ psi}$—well below the $65,000\text{ psi}$ ultimate tensile strength of the aluminum alloy. However, as British engineer Charles Inglis proved in 1913, any cutout of half-width $a$ with a corner radius of curvature $\rho$ multiplies local stress at the corner tip:

As the corner radius $\rho$ becomes sharp and small relative to the cutout width $a$, the square-root factor drives local peak stress steeply upward. Worse, instead of drilling and reaming the bolt holes around the window frames, factory workers had punch-riveted the fasteners. Punch riveting left microscopic pre-cracks around the hole rims right where the square corner concentrated the cabin load. Strain gauges mounted by Hall's team inside the water tank measured peak stresses of $45,700\text{ psi}$ at the corner of the forward square Automatic Direction Finder (ADF) window cutout. That load exceeded 70 percent of the alloy's static fracture limit on every single climb.

After 1,830 simulated flights in the Farnborough tank (3,057 total pressurization cycles), Yoke Uncle's cabin skin ripped open for eight feet starting from a punch-rivet hole at the corner of the forward escape hatch. That fracture matched the Elba wreckage tear originating at the corner of the roof ADF window after 1,290 flights. Thereafter, every commercial jetliner was engineered with rounded oval windows and crack-stopping circumferential tear straps.