Essay: Kennedy's Design-induced Sprial
by Stanley N. Roscoe, PhD, published with permission from Aero Innovation Inc., Montreal, Quebec.
Attitude Indicators: What moves?
The three display modes all indicate a bank to the right. The left indicator is rotated counterclockwise. In the centre indicator, the aeroplane symbol is rotated clockwise. In the right indicator, the horizon is the same as on the left, but, in addition, the aeroplane symbol is rotated clockwise by the pilot's aileron input, indicating that the plane will continue to roll to the right. The instrument represented on the left is the standard system in today's aeroplanes, including John Kennedy's Piper Saratoga II. With the display at the right, pilots maintain wings-level flight merely by aligning the aeroplane symbol with the horizon, a natural response.
Flight experiments support the conclusion that John Kennedy Jr. became spatially disoriented in the absence of a visible horizon in conditions of poor visibility. The overwhelming probability is that he wound up in what is known by pilots as a "graveyard spiral." That is exactly what 19 out of 20 similarly trained pilots did with the loss of a visible horizon in an experiment at the University of Illinois 45 years ago (Bryan, Stonecipher and Aron, 1954).
Later experiments at Illinois have shown that a simple addition to the conventional "artificial horizon" indicator on the instrument panel can virtually eliminate this type of accident, one that kills many people every year in general aviation and can even occur in commercial aviation (Roscoe, 1997). For the aviation community to correct this situation, someone in the media has to expose the problem, someone in government has to listen, and someone important has to die.
In conditions of poor visibility, inexperienced pilots get into screaming spiral dives in several ways, but this is the most common: While the pilot is looking for lights on the ground or other horizon reference, the aeroplane slowly rolls into a banked attitude. With no horizon visible, the pilot looks at the "artificial horizon" indicator in the cockpit and notices that the horizon bar is not level. The initial reaction is to roll the horizon bar back to level, which rolls the aeroplane into a steeper bank. This is known as a horizon control reversal.
In a steep bank, the nose of the aeroplane drops, and the aeroplane starts to lose altitude. To hold altitude the pilot pulls back on the wheel, which tightens the turn and steepens the spiral dive. At this point the pilot is confused, totally disoriented and no longer in control of the aeroplane. Such a sequence can and does happen very rapidly, and the resulting crash is invariably attributed to pilot error. No doubt the pilot made the error, but what caused the error is never determined, nor the probable cause reported.
The term "pilot error" is misused when such errors can be prevented by an experimentally proven equipment modification. In the case of flight attitude control, all that is needed is to cause the "little aeroplane" symbol on the artificial horizon indicator to rotate in direct response to aileron control inputs. Thus, to return to a wings-level attitude, the pilot merely has to align the aeroplane symbol with the displaced horizon bar and maintain that alignment as the real aeroplane and the artificial horizon bar, rotating in opposite directions, both return to wings-level.
To illustrate, if the aeroplane rolls to the right, the horizon bar rolls left. The pilot notices this and applies left aileron to align the aeroplane symbol with the horizon bar, causing the plane to start rolling back towards wings-level. As this is going on, the pilot gradually reduces the left aileron input to maintain alignment until the ailerons are neutral when the wings are level. Thus, straight-ahead flight is restored.
Stanley N. Roscoe, PhD, WW II pilot, is emeritus professor of aviation engineering psychology and aeronautical and astronautical engineering, University of Illinois at Urbana-Champaign; emeritus professor of psychology, New Mexico State University; former head of the Display Systems Department of Hughes Aircraft Company; president of ILLIANA Aviation Sciences Limited of McKinleyville, California, and Las Cruces, New Mexico; and senior vice-president of Aero Innovation, Inc., an aviation human factors company of Montreal, Quebec. For more on this topic, visit http://www.aero.ca/e_kennedy%27s_spiral.html.