Aircraft Accidents and Lessons Unlearned X: BOAC Flight 781

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 … persquareinch 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.

2 thoughts on “Aircraft Accidents and Lessons Unlearned X: BOAC Flight 781”

  1. Stephen, another great article. I see you have incorporated the Titanic and Challenger in this article too. It made me smile, and rightly so, because it does have a bearing in our culture that advocates that we do the right thing. Seriously? Well, let’s look at the FAA. It’s an agency that insists on collecting data, but some how fail to review this data in preventing catastrophic failures or other incidents from happening. With the new personnel structure and data systems in place, today’s inspector is overwhelmed in trying to figure out what is the best course of action to take based on his “risk-analysis” information only to be over-ruled by someone higher in authority. So, is it in our culture to do the right thing? This new data system is called Safety Assurance System (SAS) that is supposed to capture all aspects of a surveillance or investigation. Unfortunately, there are operators who don’t adhere to the regulations and act ignorant to them so they can get a counseling or an on-the-spot-correction that goes hand in hand with the FAA’s new Compliance Philosophy. With this new FAA Compliance Philosophy, it has become a game that is well played by some operators in avoiding any enforcement cases. I know as a previous inspector the best way to find problems is at the facilities during these inspections. But with SAS tying up some of the inspectors trying to to the database right, what is this data going to provide? Since the last 10 years or so there was another collection database called Air Transportation Oversight System (ATOS) that was supposed to minimize risks, but it cannot be joined with SAS, so what good is that data?
    As pointed out in Stephen’s article about new technologies and advances in them, how is that collected data or knowledge from ATOS going to prevent future issues if inspectors are finding similar findings as documented in SAS? To me, the trends are there, and changing infrastructure or data collection systems are meaningless unless data is reviewed as history and operators are held accountable.
    Since I’m on a roll, I will point this out. The FAA preaches about safety and wants operators to ensure safety is always considered in their systems and processes. Well, here’s a recent issue that makes no sense to me and other FAA Inspectors that I’ve spoken to about this as well in various departments (Academy, Designee’s, AEG, and FSDOs). Restricted Category aircraft are aircraft that basically fly modified aircraft in order to contain wild fires. They work for the forestry by contract and they are properly certified in meeting all FAA requirements.
    These pilots who fly these various aircraft cannot get a pilot rating for these aircraft after a stringent written and verbal test as well as flying proficiency in that aircraft by a Designated Pilot Examiner certified in that aircraft? But, pilots can qualify for their annual training requirements in these aircraft.
    So, operators who employ pilots to do this type of job, they must file for an Exemption to conduct this type of training. However, these Exemptions expires every year causing a burden on the operator when this type of training was always accomplished this way for the last 50 years. How does that help safety?
    So as you can see there are examples of previous history that shows there have been no problems whatsoever, and yet, someone has created this burden on the operators who employ pilots who do the impossible in supporting the general public.
    This issue needs to be revisited with common sense and not burden the operators. As long as there are processes in place with qualified persons overseeing the operations I don’t see a problem.

    1. Wow, I don’t think I could have said it better myself. This is some deep insight into the aviation industry; it also shows that to trust just to a bloated government bureaucracy guarantees nothing as far as safety. Thanks Jose.

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