In typical horror trope fashion, Bob is the only person on the plane who notices these shenanigans. His fevered warnings are repeatedly dismissed by uniformed crew and fellow passengers alike. Alas, I forget how the story ends (what can I say? The Twilight Zone is on late and it was a school night). What's important is that the episode's immortal catch phrase There's a man on the wing! has since become enshrined in pop culture, as surely as Don't have a cow, man! or Stuff for smart cats!
Well, there's several things wrong with this.
Drink and Derive has a proud tradition of applying a keen technical acumen to the big screen, but small-screen fare is also fair game. So let's have a look at what science has to say about Bob Wilson's wing gremlin. At first glance, it seems plausible enough -- wing walking has been a thing since the earliest days of aviation. Daredevils like Charles Lindberg climbed up on their Curtiss Jennys for a bit of wing ballroom dancing or wing juggling, their exploits captured by a film crew in a chase plane. So who's to say there aren't intrepid adventurers crisscrossing the skies this very day looking to entertain the folks back in coach with an impromptu game of wing tennis, or at least until the TSA locks them up in Gitmo for the rest of their lives.
As I see it, the three biggest challenges facing a wing gremlin are hypothermia, hypoxia, and wind loading. We need to quantify these challenges using numbers. Get the data, Robert McNamara admonishes us in The Fog of War (an aphorism that apparently slipped his mind whilst encouraging Kennedy to become embroiled in Vietnam. But I digress). So it is with data we begin our analysis.
From the engine nacelle shape and cabin interior details depicted in the episode, the aircraft in question appears to be a Lockheed Constellation, a popular commercial model in 1963. My copy of Jane's All the World's Aircraft (yes, I have one of those) lists a cruising speed and altitude of 340 mph at 22,600 feet for the Constellation (I assume narrarator Rod Serling simply rounded down in the title). These numbers contain a wealth of information, if just a little imagination and thinking are applied.
Chill Bill
Nominal air temperature at 20,000 feet is about -12 degrees Fahrenheit (source: engineeringtoolbox.com) or -25 Celsius. While harp seals and Canadians may view this as balmy, there are nonetheless limits on cold survivability. A wealth of literature exists on the subject, in no small part because the world's militaries have a vested interest in what to expect when sending troops into extreme climes (one only has to think of the brutal Russian weather that halted the Wehrmacht offensive of 1941. General Winter, the Red Army called it).
Here, extrapolation will be necessary. For obvious ethical reasons you can't freeze a volunteer to death, at least not since informed consent became a thing. Furthermore, survivability is critically dependent on clothing and wind speed (the "wind chill" familiar to impatient public transportation patrons everywhere). I couldn't locate any studies that examined hypothermia at 340 mph -- presumably, study designers figured you had larger problems then -- let alone while prancing about on the wing of an airplane in a carpet suit. Still, I did manage to find a helpful study by Peter Tikuisis presented at the Sixth International Conference on Environmental Ergonomics, and by "helpful" I mean "germane to our topic and not protected by some kind of DOD firewall that will require a Wikileaks to penetrate." In Figure 1 of the article we find this handy summary:
The abscissa is air temperature and the ordinate is predicted survival time in hours. The six curves represent various states of undress, with the right-most being a nude subject. Mercifully, the gremlin was not nude, or maybe it was, but in any event we do not know what degree of protection its exocarpeting provided. However, note there are some hours of survivability exhibited in all conditions at -25 Celsius. We are forced to concede that, unless Bob was on a transatlantic flight (unlikely, given that by 1963 the Constellation had been largely supplanted by jet engine aircraft such as the Boeing 707 for international service), cold exposure would not necessarily have been a limiting factor for the gremlin. As such, we may cross hypothermia off the list.
Gremlin Blues
A threat more pressing than hypothermia is hypoxia. O2 partial pressure drops with altitude -- it's one reason normal people find it hard to breathe in Denver (the other reason is their marijuana legalization. But I kid the hippies). You can breathe inside a commercial airliner because the cabin is pressurized. Outside, not so much. (Interestingly, the cabin is not fully compensated, which is why you may have experienced ear popping and headaches. The pressure differential is also designed to make infants shriek, apparently.)
We need to compute how long the average gremlin could survive the oxygen deficit present at 20,000 feet. I suppose I could look up Boyle's Law or whatever and do the necessary calculations. Or, I could Google. Choosing the latter path, we find a helpful app from the nice folks at altitude.org. Plugging in 20,000 feet, it tells us:
At 20000 ft, the standard barometric pressure is 49 kPa (365 mmHg). This means that there is 48% of the oxygen available at sea level.Cool beans, but that doesn't say much about physiological effects. For those, we turn to their physiological effects page:
If you were transported suddenly from sea level to [20,000 ft], you would immediately feel profoundly short of breath and disorientated. Within 10 minutes, you would lose consciousness, and within 30 minutes you would probably die...It would appear our gremlin won't be doing much after the first commercial break. But wait -- the calculator attaches a mysterious caveat (indicated by my scare ellipsis):
Amazingly, with proper acclimatisation [sic], it is possible to walk to this altitude perfectly safely.Clearly, the gremlin did not walk to this altitude, but it seems reasonable to assume there must be some sort of gremlin school which, in addition to teaching one how to evade the TSA, includes "proper acclimatisation" as part of the curriculum. As such, we conclude that neither hypoxia nor hypothermia present insurmountable obstacles to a trained and motivated gremlin.
Footnote: I once had opportunity to observe an hypoxia experiment firsthand, albeit in a decidedly non-academic setting. Several years of my misspent youth were spent at a not-to-be-named dojo, where we were sometimes joined by a spectacularly frightening woman who, in addition to taking great pride in bleeding anyone who was bigger than her, took great joy in being on the receiving end of any authentic martial experience. One night this took the form of her demanding one of us "choke her out" (her words) because she wanted to know what it felt like. One of us did so, and it's rather unnerving how quickly it happens. (Granted, the subject was a willing crazy lady and not a thrashing adversary. Still, we spend our every waking moment not too many seconds away from oblivion.) Locker room gossip hinted her brash adventurism translated, how shall we say, to the bedroom, which I never confirmed, which was probably for the best. I called her Bubbles. I like to think she found that charming, although I suspect in her heart-of-hearts she rued the day my shadow ever darkened the stoop of her beloved dojo.
Consider a Cylindrical Gremlin
Finally, there is the issue of wind load. If I recall my physics, force is just pressure times area, a formula I found verified in a structural engineering handbook. However, there is an important twist required when considering dynamic loading:
F = P ⋅ A ⋅ Cd
Here, F is force in pounds, P is the wind pressure in psf, A is surface area in square feet, and Cd is an empirical drag coefficient dictated by the geometry of the object in question. Alas, we don't have wind pressure, we have wind speed. I tried to derive a conversion from first principles, but gave up once Avagadro's number and beer appeared. Fortunately, the conversion was given in my book on the next page:
P = 0.0026 x V^2
where V is wind speed in mph.
The gremlin was played by Nick Cravat, a Hollywood personality of some renown, or at least renown enough to merit an IMDB page, which lists his height as 5' 4''. Performing a cursory metromorphic analysis of the gremlin using a still frame from the episode (see attached), I obtained an aspect ratio of 1.8. As such, let us model the gremlin as a 5' 4'' tall cylinder with radius of 18 inches. We place the cylinder upright in a slipstream of 340 mph and compute how much force it experiences. Looking up the drag coefficient for a cylinder (1.2) and entering all this into the equations gives:
F = 0.0026 ⋅ 340^2 ⋅ 64/12 ⋅ (2⋅18)/12 ⋅ 1.2
for a wind force of 5770 pounds.
This is bad news for suspension of disbelief. The moment the gremlin appears on the wing, it's going to be swept away like it was common sense.
However, let us suppose the gremlin somehow manages to affix itself to the wing, say, using the power footies in its carpet suit. We now have another problem. Specifically, we have an unbalanced yaw force of 5770 ⋅ y foot-pounds acting on the airframe, where y is the distance of the gremlin to the center of mass. Although unbalanced yaw can be corrected by rudder application, the gremlin is not particularly aerodynamic and its presence would likely create some controllability issues. (For your consideration, the unbalanced forces created when one of the thrust reversers was accidentally deployed mid-flight brought down Laudia Air Flight 004 in 1991, a (much larger) 767 with much more advanced avionics.)
In short, a gremlin attached to the wing need not bother pulling out wires and whatnot to crash a Constellation; all it would probably have to do is stand up, if it could stand at all.
Epilogue
So it would seem it was all in Bob's head. If you had any lingering hopes for what could have been a truly epic installment of Punk'd, I'm afraid they have been undone by that cruel mistress physics. With the benefit of hindsight, we understand now my earlier wing-walking observation was a red herring, the cruising speed of a biplane being 80 mph, not 340. A large enough quantitative change brings a qualitative change, as they say. Don't expect to be seeing Steve-O or Johnny Knoxville recreating N@2K in Jackass IV.
For all of the fun we've poked, Nightmare at 20,000 Feet remains a genuinely creepy half-hour of television. The story plays on the very real sense of helplessness and dread air travel evokes. It should be noted early commercial flights were a fright compared to today, with slower prop planes, lower cruising altitudes, unreliable weather forecasting, and relatively primitive technology all around making for frequent white-knuckle rides. Madison Avenue may have painted the friendly skies as carefree and glamorous back then, but the reality was more Dramamine and sawdust. There's a reason people used to wear their Sunday best when flying; it wasn't hard to imagine it's what you were going to be buried in.
Still, there's no denying the drama loses some of its punch because of an unfortunate monster costume. This is a recurring theme in the franchise, with the top-notch creative talent The Twilight Zone employed struggling against crippling budget and time constraints imposed by network television (host and head writer Rod Serling admitted the show eventually exhausted him). While many classic episodes like "To Serve Man" and "It's a Good Life" have aged surprisingly well, others would be a hard sell for modern audiences. In some sense it's an inversion of the current state of affairs, the science fiction and fantasy genres increasingly dominated by CGI and special effects at the expense of good writing. It is a rare show or film indeed that hits on all cylinders, either then or now.
I'm told a remake appeared in the 1983 Twilight Zone movie featuring a redesigned gremlin. I have not sought it out.
These days, the people inside the plane are scary enough.
Image Credits: Still frames from Nightmare at 20,000 Feet copyright CBS Productions and claimed here as fair use as they illustrate a critical or scholarly article describing the work in question.
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