On January 10, 1954, British Overseas Airways Corporation (BOAC) Flight 781, a de Havilland Comet, crashed into the Mediterranean Sea following an inflight breakup. It was the second of three fatal Comet accidents in less than a year; the third accident, Flight 201, could not be recovered, yet displayed similar characteristics to Flight 781. Both accidents were investigated by Great Britain’s Ministry of Transport and Civil Aviation (MTCA), who authored the report.
Even though Flight 781 had come apart over the sea, a large portion of the aircraft was salvageable; the investigatory board re-assembled most of the pieces. Early on, the investigation focused on evidence of a mid-air fire. Modifications were made to the aircraft and, within a month, the Comet was back in commercial service. Three months later, Flight 201 crashed, ending the Comet’s career.
The first and second accidents were unalike; the countries where they occurred were far apart. In 1954, with the Jet Age in its infancy, accident investigation techniques were non-existent; inspection techniques were being developed. Pre-WWII aircraft that survived the war were not capable of the speeds, altitudes, ranges, or pressurization capabilities of the new jet-age aircraft. There were no baselines for the MTCA to compare Flight 781’s circumstances to.
The MTCA report claimed in its Conclusions that the aircraft crashed due to “failure of the cabin structure, owing to metal fatigue.” Stress cracks emanating from an Automatic Direction Finder Window provided the ‘weakest link’ point for the original break-up, the window corners particularly.
With modern aircraft windows, stress flows freely around curved edges; it isn’t concentrated. However, with the Comets’ squarish windows, stress cannot smoothly flow around the abrupt corners; the corners develop stress concentrations, which occur in a restricted area, because the stresses are higher than the surrounding areas. The window frames’ squarish shape provided a point for cracks to start.
Between internal air pressures and temperature differentials, fatigue easily weakened the metal. What does that mean? How can metal fail so catastrophically? To put in perspective: a total of 10.9 pounds per square inch of pressure (PSI) is placed on the hull at 27,000 feet of altitude; this is the altitude where Flight 781 came apart, climbing into extreme temperature variations. That’s 10.9 pounds … per … square … inch of pressure forcing out, with next to no pressure pushing in. Consider that pressurizing an aircraft is like inflating a metal balloon. Every time the aircraft climbs results in pressure pushing out on the skin and joints; each time it descends, the skin and joints flex inward. The metal skin attaching hardware, e.g. rivets, screws, attempts to hold the balloon together while 10.9 psi is acting to force it apart. The balloon eventually fails at the weakest point. If this point is at a pre-established crack, the pressure rips the fuselage open with explosive force, tearing the metal like tissue paper, continuing unimpeded over the length and width of the fuselage; to quote the MTCA report, “like a 500-pound bomb.”
Investigators were slowly coming to understand that flight cycles and flight hours were two separate factors, especially when considering pressurization. Manufacturers were concerned with the lifetime flight hours, i.e. the number of hours an airliner is flying, NOT flight cycles.
A flight cycle is vastly different from a flight hour; each time an aircraft takes off and lands is one cycle. During this time, the aircraft is pressurized and depressurized one time. By comparison, an A320 flying directly Fort Lauderdale to Boston may use three flight hours, but log only one flight cycle. An identical A320 may fly Fort Lauderdale to Dallas, stopping off in Tampa and New Orleans; this aircraft logs three flight hours, but it also logs three flight cycles during that same three-hour period; three pressurizations/depressurizations.
What is interesting about the MTCA report is the statement, “During the period 1949 to 1951, there had been growing among all aircraft designers and users, a realization that the life of the essential structure is not unlimited.” This statement is incredible: MTCA’s report suggests engineers in 1951 were aware of structural limitations. Ignoring these concerns is indicative of an industry today that did not learn from the past.
Move ahead 34 years; in 1988, Aloha 243, a B737 flying between the Hawaiian Islands, experienced a catastrophic structural failure when the fuselage’s crown peeled off in mid-flight. Boeing had overestimated the B737 airframe’s lifetime, forecasting a longer life without realistically studying the effects of flight cycles on the structure. Following this accident, all manufacturers were forced to re-evaluate the life limits of all their products.
In many ways, Aloha 243 was unique; it stood separate from other B737 models flown by many US operators. Aloha employed its B737s in short-range, ‘up-and-down’ island hops in a very hostile environment: humid air over a salty ocean; a great combination to aid corrosion. Did Aloha and Boeing ignore the lessons of BOAC 781? The consequences should have been obvious: inflating an overstressed and abused fuselage should have either dictated a more robust inspection program or limited the number of flight cycles in the corrosive environment.
But what changed in that 34-year gap? For one, the engineers who designed the Comet, the B707 and the DC-8 were gone, replaced with engineers ignorant of the past. Second, the same arrogance that doomed the Titanic and the Challenger was present, a culture that felt they could not fail. Third, the industry was captivated by new technologies, fooled into complacency by trusting too much to new sciences.
This type of failure played out again fourteen years later when a rapid decompression destroyed China Air 611 on May 25, 2002. A repair that was not tracked for reinspection turned into a deep crack; the end of the new crack in the B747 passed the repair’s perimeter; the fuselage tore open killing all 225 aboard. China Air 611 came at a time when lessons from other events, added to breakthroughs in non-destructive inspection technologies were commonplace. The fact that it was a different country is irrelevant; China Air 611 should never have happened, while Aloha 243 could have been prevented.
The takeaway of Lessons Unlearned from these accidents’ findings is that we are never ‘too advanced’ to learn from history. Thirty years of technological advances and improved inspection techniques between the Comet accidents and Aloha 243 did not prevent the outcome: the structure, aided by enormous internal air pressures, failed in flight. As we continue moving quickly into the Composite Age, we should review what we learned – and failed to learn – the last times; search for anomalies that could replay disasters of the past. Armed with this knowledge, we can prevent the preventable next time.