When walking aboard an airliner, the cockpit door is usually open, and passengers get a glimpse of the seemingly rational and error-proof array of technology.
Would that it were so. In truth, the cockpit of a modern airplane is a minefield of what might be called “pilot traps” that can cause confusion or worse at precisely the wrong moment.
For example, consider the throttles on the Eclipse EA-500, a new Very Light Jet that carries four passengers. Last June, in a landing at Chicago’s Midway Airport, the pilots found that the engines would not respond to throttle commands, had no emergency procedures, and were only able to successfully land the airplane by shutting down one of the jet’s two engines. It turns out that when the engines are firewalled, the computer logic assumes the last command dominates, and so left the engines at full power while the pilots pulled the throttles to idle. The case is under investigation by the National Transportation Safety Board (NTSB) and Eclipse has already rushed a fix to the field.
Another, more subtle pilot trap pertains to the attitude directional indicator (ADI), which basically tells the pilot which way the airplane is headed with respect to the horizon. The instrument is critical when the view out the windscreen is nil, as is often the situation at night, in fog, snow, over terrain with few ground lights, over water, and so forth. In such conditions, the pilot is at real risk of becoming disoriented and must rely on the ADI.
There is a long list of fatal accidents and near-catastrophes in which misinterpretation of the ADI played a role. One would think that the science of ADI design would be resolved. This is not the case.
In Western-built jets, a little aircraft symbol is superimposed over a horizon line that separates the sky from the ground. The airplane symbol is fixed, with its wings level with the cockpit, and the horizon line moves in concert with aircraft pitch and roll. When the pilot begins a climb, the horizon line behind the airplane symbol moves down the ADI. If the pilot rolls his aircraft, he will see the horizon rotate around the miniature aircraft.
In Soviet or Eastern-designed jets, the opposite convention applies: the horizon line remains fixed and the little airplane symbol moves. In other words, the horizon line, not the airplane, stays level with the cockpit. Thus, when a pilot trained on the Russian ADI makes a control input, he will see the miniature aircraft respond by moving on the ADI. If he initiates a climb, the little aircraft in the foreground will rise above the fixed horizon line.
One would think that the proper display, the one with minimal chance of confusion, would be a long-settled issue. Not so. Back in the late 1920s and 1930s, the famed Jimmy Doolittle, and other pilots of his day, reportedly experienced difficulty in remembering that the moving bar represented the horizon and not the airplane. It was realized there was a problem. A few radical thinkers advanced the notion that the horizon should be fixed and the airplane symbol should move, just as the pilot perceives his own airplane moving against the horizon. The Russian design followed that philosophy.
Problems result from having the two displays in use throughout the world. In the Western-style display, there is an initial tendency – particularly among inexperienced pilots – to roll the horizon bar back to wings level, which puts the airplane into a steeper bank. In a steep bank, the nose drops. To maintain altitude, the pilot pulls back on the stick, which tightens the turn and steepens the spiral dive.
In the Russian-type display, rolling left aligns the wings in the ADI with the horizon, helping to avoid control reversal and spiral dives.
Pilots initially trained on Russian equipment, who find themselves later flying Western-built jets, have occasionally become confused, particularly when faced with the need to instantaneously interpret a dynamic unusual attitude in a loss of control upset. When this happens, there is a 50% chance that a pilot will react instinctively and wrongly, a development that usually equates to a 100% chance of a subsequent accident.
There is a third display, propounded by some, in which both the airplane symbol and the horizon line move. (See box below)
Attitude Indicators: What Moves?
The three display modes all indicate a bank to the right. On the left above, the widely-used Western convention, in which a bank to the right is indicated by the horizon line moving counterclockwise and the aircraft symbol remains fixed. In the middle, the Eastern convention, the horizon remains fixed but the airplane symbol rotates clockwise. On the right side, both the horizon line and the airplane symbol move. With this display, pilots maintain wings-level flight merely by aligning the airplane symbol with the horizon line, a natural response.
If the airplane rolls to the right, the horizon bar rolls left. The pilot notices this, and applies left aileron to align the airplane symbol with the horizon bar, causing the plane to start rolling back to wings level. As this is going on, the pilot gradually reduces the left aileron input until wings are level. The display, if you will, is a more natural and realistic depiction of what is going on.
This improved display cannot easily be incorporated into the old mechanical cockpit instruments, but certainly can be fitted to the new “glass cockpit” displays, where the imagery is generated entirely electronically.
Some industry trainers claim that the display in which both airplane and horizon move would be a simple, affordable way to save hundreds of lives worldwide, every year. This idea has been around since about 1975, which makes the deaths since all the more inexcusable.