Tom Székely, P.E., LEED AP



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July 5, 2007

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The Wackiest Ship in the Army, or, Of Airplanes and Buildings


The first clause of the above headline is the title of a 1965-66 TV series loosely based upon actual events, revolving around a two-masted schooner presented to the U.S. by New Zealand early in World War Two.


One episode I saw at my buddy Doug’s house featured the shoot-down of a Japanese Zero by one of the protagonists (Jack Warden)  using a Thompson submachine gun.  Since a Thompson uses a .45 caliber ACP   pistol round, and we were all very familiar with firearms (a necessary skill for growing up in the Bushwick section of Brooklyn), we began thinking that Hollywood had done it again, putting something on the screen which had no basis in reality.


I mean, shoot down an airplane with a pistol? 




Actually yeah, depending upon how close you get.  I did the ballistics calculations  on a 230 grain (about half an ounce) bullet exiting the muzzle at 850 feet per second (about 580 mph).


Re-doing the calculations now, I find it has a muzzle energy of about 370 foot-pounds, and about 183 foot pounds by the time the bullet has slowed down to about 600 feet per second (maybe one or two hundred yards downrange).  Try dropping a 180 pound weight, which has a contact area about the size of the tip of your thumb, on to a thin aluminum sheet from a foot away and you’ll discover it will punch right through it. 


Since a Thompson’s rate of fire is between 600 and 800 rounds per minute, it’s punching through the aluminum (positing the shooter’s ability to hold on target) at least 10 times in a one second burst, tearing up all kinds of systems under the skin as it does so.


The math behind all this is the Physics definition of kinetic energy, or how hard something hits, which is half of an object’s mass times the square of its velocity, and that leads me to the second clause of the headline to this piece.


A couple of weeks back, the New York Times (I just gotta do that with the typeface) published an article explaining that the TSA was going to look at anti-terror rules for small jets and boats, but in the body of the story, they referred to a Cessna, saying it weighed about as much as a small SUV.


The implication of the story was that a single engine lightplane could be a terror threat.  That is, today’s general reader knows very little about business jets, and next to nothing about Very Light Jets, which are currently only in development, and would naturally think of a single engine lightplane with regard to the Cessna reference, particularly since it didn’t specify a business jet.


Since I fly single engine lightplanes I looked at this as another story by the unknowing about the (to them) unwanted. 


I wrote a letter to the editor doing the math to point out that the reason the Cory Lidle crash had the same effect as the unfortunate who intentionally flew a Cessna into a hi-rise in Florida shortly after 9/11, was that such aircraft simply cannot hit hard enough to do much to a building.


That is, taking two objects of the same mass  if one travels twice as fast as the other, it will hit four times as hard; three times as fast, nine times as hard; four times as fast; sixteen times as hard, etc.


The math works out like this:


A business jet weighs 5 times as much as a Cessna 172, and travels about 5 times as fast, so it’d hit something  5x25 or 125 times as hard as the 172.


A commercial airliner weighs between 100 and 150 times as much as a 172, and can travel up to 6 times as fast, so it can hit as hard as 150x36 or 5,400 times as hard as a 172.  This means it also can hit about 43 times as hard as a business jet.


Let’s look at this in another way.


43 business jets or 5,400 single engine lightplanes would have to fly into a building at exactly the same time to have the effect of a single commercial airliner impact, and even at that, the 9/11 impacts didn’t bring down the twin towers.


The fires, however, did.


Medieval blacksmiths softened iron over coal or charcoal fires with bellows, and wood (8,000-10,000 BTU per pound) is a lousy source of concentrated heat energy when compared to petroleum products (18,000-24,000 BTU per pound or about 124,000 BTU per gallon for gasoline).


Your car’s gas tank might hold between 12 and 25 gallons of gasoline.  A 172 with long-range tanks could bring as much as 50 gallons of avgas to a building impact.


A business jet could bring between 500 and 5000 gallons of kerosene to such a scene, while in the case of a commercial airliner we’re talking between 12,000 and 24,000 gallons.  To put these numbers in some sort of perspective, the fuel tanker trucks you see delivering gasoline to your gas station typically carry about 3000 gallons of fuel, maybe 150 cars’ or an eighth of an airliner’s worth.


Rust or iron oxide, by the way, is the slow burn of the iron in steel by oxygen in the air.  Get a hot enough fire going, and steel will not simply soften or melt, but some of it will, (contrary to Rosie O’Donnell’s monumentally ignorant proclamation as to the impossibility of such) burn.


I guess the point of this piece has been that while having opinions is necessary, one needs to have facts upon which to base those opinions. 


That is, there are medical opinions, legal opinions, engineering opinions, and just plain ignorant opinions.


We live in such an information-rich age that it’s easy to mistake consensus for fact, and easier still to form opinion based upon the common perceptions or beliefs of one’s peer groups.  What should be so obvious as to not need stating is that the danger here is that such beliefs or perceptions may have little or nothing to do with fact.


This gets me back to one of my recurring themes; what defines a professional.  Actually, what I’m aiming towards here is that you keep in mind the fact that neither  legislators nor journalists have any special qualifications which should have you take their words as gospel.


Quite the contrary, in fact.


The dirty word “lobbyist” can be nothing more than someone who can be on call to educate the ignorant that they don’t do  or say something really stupid.





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