What if I told you that there is more to the graveyard spiral than most people realize? This isn’t about some esoteric physics factoid, either; this is something that could save your life someday, simply by arming yourself with a little knowledge. Each year, a hundred or so pilots dissipate the last three minutes of their lives engaged in a futile attempt to maintain altitude after they lose outside visual reference and instead tighten their screaming dives to oblivion via that ignominious exit known as the graveyard spiral. The fact is that a significant number are condemned not just by ignorance, but by a misguided precedent in the very design and presentation of the gyroscopic instrument itself. In this case, the culprit is the artificial horizon, otherwise known as the attitude indicator.
A LITTLE KNOWLEDGE CAN BE A DANGEROUS THING What brought Kennedy’s Saratoga down, however, was something other than hubris and having “more money than brains.” In my opinion, we’re dealing with a human factors issue that is not so cleanly explained by the graveyard spiral, but rather more properly explains the graveyard spiral, itself. Perhaps the moving horizon line of the attitude indicator was confused with the airplane’s wings, instead of the gyroscopically stabilized backdrop to their relative motion.
The SETUP: A moment’s inattention, and an airplane slowly rolls at a rate below the vestibular threshold of its human pilot. The next time he sees the attitude indicator, it shows a bank. He rapidly applies ailerons to recover, but it feels as though he is rolling into a bank, instead of out of one. This illusion is actually compelling enough, under circumstances of stress or confusion, to precipitate a reversal of previously ingrained reactions. Because of the inherent conflicts between comparatively unerring gyroscopic rigidity and our own limited vestibular abilities, it is critical that an attitude indicator shows the clearest possible relationship between the airplane and horizon symbols and what happens in the real world. The predominating school of thought at the time of its invention (two thirds of a century ago) was that the artificial horizon be a porthole through which we see a symbolic analog of the horizon (e.g., we roll right, the horizon rolls left).
Research has shown that our ‘instinctive‘ expectation is that a display element should move in the same direction as our control input — and this is not what happens on all the instruments we fly with.
THE TWO SIDES TO THE STORY
In the study of man-machine interactions, the proper modeling of human cognitive functioning is essential for creating displays that are the least likely to be misread and thus contribute to loss of situational awareness … and loss of life. In the case of an attitude indicator, if the earth is chosen as the point of reference, the horizon is stable and it is the aircraft pictogram that moves. In human factors language, this is called an ‘exocentric‘ view. The alternative, which is where the aircraft pictogram stands still and the horizon rotates, is called an ‘egocentric‘ view, and it is this one which actually does provide a more direct ‘cognitive mapping‘ (the ‘porthole theory>‘, previously mentioned). However, as also mentioned, this standard also violates an almost instinctive expectation regarding compatibility of display elements representing what actually moves in the real world.
(Don’t get hung up on the ten-dollar buzzwords, by the way. All exocentric means is that the action of an object is based on what can be seen from another object’s point of view, and egocentric just means the action of an object is based on what can be seen from that object’s point of view.)
Both of these two images represent an aircraft banking to the right. In the first image (on the left) we see what we’re all used to seeing: in a bank to the right, the horizon rotates the other way. The image on the right though, the aircraft pictogram rotates in the same direction that the yoke (or stick) does: to the right, while the horizon stays still. Which frame of reference to choose has remained a subject of controversy. However, experiments conducted by universities, aircraft manufacturers, and the Naval Research Lab favor the “moving airplane” view as the better one.
If it’s the better approach, why isn’t it used? While moving the little airplane instead of the horizon may be easier for most brains to interpret, there are some complications — especially if the little airplane becomes inverted. In this case, if you can’t see outside the cockpit, it’s not as easy to see what the problem is and much more confusing for a pilot to understand what action will correct it. Of course, if he happens to be inverted in the soup that’s when the information needs to be clearest.
SO WHAT COULD THEY DO ABOUT IT?
Ever since Elmer Sperry patented the artificial horizon in Great Britain, back in 1911 (incidentally, the technology was first demonstrated in an aircraft in 1916 — not in that now more famous flight in 1929 by then Lt. Jimmy Doolittle) arguments have gone back and forth as to which was better. Those in favor of the horizon bar remaining faithful with the real world’s horizon won out, that’s the way Sperry built them, and that’s what we fly with today. Doolittle wasn’t too thrilled with their final choice, by the way and the perceptual motor problems associated with it have since been the subject of much experimental attention.
The best solution that has emerged thus far is, in its simplest form, a combination of the two views. If it were incorporated in an electromechanical display, it would look like the image on the right. This represents a compromise between the orientational and dynamic models: for rapid rolls above a certain threshold, where perception of motion dominates, the airplane moves. For gradual turns, the horizon moves. This “frequency-separated” display has proven to be the best means yet for eliminating design-induced bank-reversal errors. There is some promise of innovative improvement, at least in jetliners or experimentals with glass cockpits, by the addition of a “flight path predictor” to the conventional moving horizon. Like the simple composite shown here, such a design would give an immediate and unambiguous indication of aileron and elevator control inputs. You can read about this online in fact, on page 1 of this 1997 issue of GATEWAY (a publication of the Crew System Ergonomics Information Analysis Center).
WHAT YOU NEED TO KNOW… FOR NOW
Be aware of how your mind wants to react to what it sees, and stay aware of it. Consider that, in an emergency, it might help to ignore the little airplane altogether — remember, the attitude indicator is a window. When you’re in the soup, it just happens to be the only window you can see out of. If by now you’ve begun to entertain the notion that this isn’t some Lilliputian pettifoggery over which end of a hard-boiled egg to crack open, well, bravo! This is important material here. They say that whenever there’s a blatant design-induced accident, the media has to first expose it, someone in government has to pay attention, and somebody important has to die. One down… two to go?