Friday, November 06, 2015

Falling Out of the Sky - The Mystery of the de Havilland Comet

de Havilland Comet
Square windows were the problem

This week's inflight breakup and crash of a Metrojet Airbus A-321 reminded me of the story of the first jet powered airliner of the postwar era, the de Havilland Comet. The Comet, first flown in 1949, was to be the first airliner to offer a pressurized cabin to passengers. Remember that without pressurization or supplemental oxygen, most humans will suffer hypoxia symptoms above 10,000 ft. Pressurized and heated cabins are the one thing that makes flying long distances at altitude acceptable to the general public.

The mysterious inflight breakup of three Comets shortly after their introduction into commercial service served to highlight the importance of metal fatigue in aircraft design. While consideration was given to the issue in the design stage, it was the design of the windows, which were square, which proved to be the problem. It was at the corners of the windows where stress was concentrated and where metal fatigue caused structural failure which brought the airplanes down.

The full story, which is quite interesting, can be found here. It's a neat engineering whodunit.

Investigators have not as yet officially released the cause of the Metrojet crash, but it is believed that the aircraft suffered a catastrophic structural failure. An onboard bomb is now being suggested as the cause of the crash though consideration is also being given to structural failure due to an old repair. No matter the reason for the failure, any significant structural failure of an aircraft at altitude can sometimes but not always result in the loss of the aircraft.

Why So Much Damage?

So you may ask why does a hole in the fuselage whether caused by metal fatigue or a bomb cause so much damage?

Pressurization of an aircraft is achieved by pumping air under pressure into the fuselage while restricting the outflow. Think of the airplane as one of those large inflatable jump houses at a carnival. A fan blows air in while vents let only some of the air out. Airplanes work in the same way with compressed air coming from the engines acting to inflate the fuselage or "balloon" and an outflow valve to control the amount of pressurization.

Now think about what happens when you drop a shaken can of soda on the ground. Sometimes a small hole in the pop top just squirts out a bunch of soda, but at other times the whole can might rupture and spray everywhere. I've seen flight attendants drop a can of soda in the galley that explodes where soda covers just about everything instantly. You haven't seen mad until you see that.

The same principle applies to a pipe bomb. Burn a small pile of gunpowder in the open and it makes a brief poof. Put that same powder in a pipe and it expands explosively when the metal in the pipe bursts.

The point is, expanding gas when trapped in a pressure vessel can result in tremendous damage when the pressure vessel fails. This is why bombs aboard aircraft are so dangerous. Regardless of where the bomb is placed, it is the pressure shock wave which will find the weakest point in the structure. When the structure fails, the release can do tremendous damage. In the case of the Metrojet crash, the tail section was found separate from the rest of the wreckage. It may have separated due to the failure of the pressure bulkhead at the aft part of the cabin.

Barring 100% prevention of bombing attempts, which seems unlikely, future aircraft design may have to incorporate some sort of predesignated pressure relief panels. They might be designed to fail at a lower pressure than the rest of the fuselage pressure vessel. It's the world we now live in.

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Capt Rob