Can You Stand the Engineering Heat?

Large scale engineering projects often have to be designed to withstand thunderous events: sometimes in reality and sometimed figuratively. The engineers of such projects need the technical knowledge, the professionalism and, yes, the courage to master the challenges. The Lad is talking here to those who may be thinking about an engineering career. For some of those it is just these challenging aspects that will appeal to them and perhaps give meaning to their careers. Do you look to having the opportunity to address and master major challenges? Do you?

More details are slowly emerging of a tragedy that is an example of a, thankfully rare, concomitant of such challenges: a real thunderous event. It is that of the Airbus A330-203 flight AF 447 which, on 1st June 2009 in the dead of night during storms over the Atlantic Ocean, vanished with every one of the 228 souls aboard. The aircraft was an example of the most modern and enormously powerful technology in flight.

There is still much to discover about, and to learn from, the cause of the accident. here we can only allude to a small aspect. There have been some suspicions voiced that the cause of the destruction was an operating failure of a small device for measuring the aircraft speed. This device is called a pitot tube.

This is a device that could hardly be simpler. The idea that its malfunction could be such a catastrophe requires a particularly wide-ranging imagination. Either that or an innate engineering wariness; a wariness founded on the possibility that ANYTHING could lead to problems. The operating principles of the pitot tube can be shown by the simple diagram below.

Pitot Tube Operating Principle

” Pitot Tube Operating Principle [Apologies for the very bad image quality. I must get a decent CAD application.].

This simple device is vital to the control of the even the modern aircraft. Indeed it has been so since the days of the earliest airliners. It and the details of its detailed design are part of the hundred thousand design decisions that are made as every large scale project, in this case the airliner, takes shape.

The pitot really is so simple. How does it work? There are two tubes that face forward into the air flow and are supported by part of the aircraft structure, probably the fuselage or the wing. One of them, called B in the diagram has an opening facing directly into the airflow. The air in that tube is driven in by the impact of the plane’s forward speed. Thus the pressure in the  tube is raised and measured by the pressure gauge at its inner end. This you might think is a good measure of the total speed but it is not. Part of that pressure is that of the static atmospheric pressure. This is the pressure which at sea level is measured by a barometer. The static atmospheric pressure varies greatly for an aircraft as it climbs and descends in its flight. This is part of that pressure which is measured by tube B but which is nothing to do with the plane’s speed. So Tube A is tasked with measurement of the static air pressure throughout the flight and whatever the air speed. This it does because it has no opening pointing forward but only one at the side of the tube. The side opening does not see the impact pressure due to the speed but only the static air pressure that acts in all directions. This pressure is measured in Tube A by its own separate gauge and is subtracted form the pressure measured by Tube B. So there we have it: the resultant modified pressure is due solely to the speed. Simples.

But watch that neither tube or opening is affected by storms or icing during the flight. Closure or even change in area of any of the openings by ice build up will entirely destroy the accuracy of the airspeed measurement. This, in turn can then baffle the computer controls of the aircraft. The design engineers would not have wanted this to happen but, nonetheless, this is what is suspected to have happened.

The engineers, metallurgists, chemists, physicists and all the other professions do not work as lonely individuals in the bringing of such a project to fruition. They all work together, struggle and debate on solving known and predicted problems as men and women in teams from one company and with others from other companies over periods of months and sometimes years. Despite this each one person is a professional, responsible for the accuracy of her or his own work. It is here that any decision may prove to be pivotal in subsequent events. The decision may be small,: indeed so small as to go unremarked and not reviewed by the others in the team or by its leaders. It can be successful or it can be fatal. Each engineer must consider frequently as he or she works, if he or she can stand up in a court and answer for this decision as being a reasonable and proper thing to do.

This is where the heat is in the kitchen. Can you stand it?

The end of this flight came when all forward speed had drained away and so, inescapably, the sealed, metal vessel weighing around one hundred and fifty tons and full of over two hundred sentient human beings careened vertically downwards at about ninety miles per hour. After three and a half minutes, its whole structure and its entombed passengers were destroyed as it hit the sea, belly first. Poignantly, the stalled plane had slowly rotated at intervals as it fell till, at the end, it was facing almost back the way it had come.

The French Aviation Regulatory Authority is Bureau d’Enquetes et dAnalyses pour la securite de l’aviation civile, known more simply as BEA. Its latest technical report can be found in English at
http://www.bea.aero/en/enquetes/flight.af.447/info27may2011.en.php

The numbing details above of the horrifying plunge from the stratosphere to the chill depths of the Atlantic Ocean come from that report.

See my posting of “The eternal question: can you answer for it?” on December 9 2010.

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