Perils of the Glass Cockpit:

Copyright © 1996-1997 Tim A. Krell. All rights reserved.

The Boeing 737-400 is among a new generation of modern aircraft to utilize a so-called "glass cockpit." Such cockpits replace a myriad of gauges, switches, indicators and the like with several computerized display systems. By using computers to manage the on-board systems, pilots are able to call up what they want to see when they want to see it. This has allowed modern aircraft such as the 747-400 and MD-11 to require only two crewmembers instead of the three needed by these planes' predecessors. But with these great technological advances, a much greater burden is being placed on the designers and users of these glass cockpits.

It was such a glass cockpit which may have confused the crew of a British Midland 737-400. On January 8, 1988, a fan blade in one of the engines failed. The crew, guided by the displays in their glass cockpit, proceeded to shut down the engine. Only too late would they discover, however, that they had powered down the wrong engine. The plane crashed just outside of East Midlands Airport in the UK, killing 47 of the 118 people on board.

Using information obtained from the flight data recorder, scientists at the NASA-Langley Research Center have recreated the cockpit display aboard the ill-fated British Midland. Their purpose in doing so was to contrast the confusing display the crew encountered with a new system, dubbed E-MACS (Engine Monitoring and Control System), which was developed at Langley. This new system offers pilots a new perspective on the same information. Instead of a series of gauges displayed in graphical form, E-MACS utilizes vertical bars that change from green to yellow to red, depending on the severity of the problem. Below that display, the computer's diagnosis of the problem is indicated. Even to an untrained eye, the difference is dramatic. Both the traditional displays and E-MACS utilize and interpret the same data--but the latter may have well prevented a tragedy.

It is in the graphical display of a glass cockpit that crucial information is conveyed to a pilot. If this information is misread, misinterpreted, or misunderstood, the results can be disastrous. Knowing this, the developers of such electronic systems must be sensitive to how human beings interpret--and misinterpret--data displayed on a screen. On January 20, 1992, this lesson would again be learned the hard way.

On that day, an Airbus A320 operated by Air Inter flew into a mountain on a night approach to Strasbourg, France killing 87 of the 93 on board. It was later determined that confusion over the plane's "descent mode" led to this disaster. As the pilots were approaching the airport, air traffic controllers directed them to a secondary runway due to shifting winds. This runway had no electronic landing aids, causing the pilots to perform a standard "non-precision approach" procedure. In this procedure, a pilot proceeds at a level altitude until reaching a given distance, at which time the plane descends through a series of "steps" leading up to the airport. The pilots determined that a descent angle of 3.2 degrees would align them with the steps while keeping them clear of the mountains below. To program this into the flight control unit, the pilots had to turn and pull a particular knob once "3.2" appeared on the display.

But this same knob actually serves two functions. Depending on whether a nearby button had been pushed or not, this knob would command the plane to descend either in degrees or a vertical speed mode. Where the Air Inter pilots thought they had commanded the plane to descend at an angle of 3.2 degrees, they had actually instructed the plane to begin a 32 hundred feet per minute descent--a decent which would take them directly into the mountains. On the display, the only difference between 3.2 degrees and 3,200 feet/min was in a decimal point: the display read "3.2" or "32" depending on which mode was set. Similar confusion led to the near crash of another A320 two years prior near San Diego. This plane was saved only be the presence of an on-board Ground Proximity Warning Device which alerted pilots to the fast approaching terrain.

Following the accident at Strasbourg, Airbus redesigned the descent mode display so that degrees would be displayed with a decimal (e.g., 3.2) and vertical speed would be shown as a four digit number (e.g., 3200). It was a simple enough change, but one made only after a great loss of life.

Human interaction with computerized flight systems is viewed by many as the "Achilles heel" in modern commercial flight. While pilots as the ultimate end-users must bear the responsibility for the systems under their control, it is the designers who must shoulder the burden for taking into account the human factors likely to cause confusion. How information is displayed has a tremendous impact on how it is interpreted. Indeed, graphical displays and glass cockpits are here to stay--but so too are human pilots. Just as pilots must learn to work with each other, so too must crew and computer communicate effectively. Better displays, pilot training, and improved designs which take into account the human factor can only make for a safer, smoother flight.