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Avionics make the skies safer

Avionics make the skies safer

Would you feel comfortable boarding a plane knowing a crash will occur once every week to ten days? According to NASA Administrator Dan Goldin, that's the prospect travelers will face if air-travel safety is not improved. Goldin cites studies that project traffic tripling in 20 years. But while the worldwide accident rate of about 1.5 accidents per million departures is minuscule, "If the rate of air traffic triples and we don't dramatically improve the safety rate, we're headed for major problems," since the rate has been flat for 20 years.

In February, President Clinton and Vice President Al Gore's White House Commission on Aviation Safety challenged NASA, the Federal Aviation Administration (FAA), and the Department of Defense (DOD) to reduce the accident rate by a factor of five in 10 years, and by 10 in 20 years. But where to begin?

Stuart Matthews, president of the Flight Safety Foundation, notes data show "as many as 80% of all accidents have a human-error component. About 40% of all accidents are controlled flight into terrain (CFIT), and they cause well over half of all aviation fatalities. About 50% of all accidents happen during approach and landing," he adds. Finally in the past five years, accidents caused by loss of control have challenged CFIT as the leading cause of fatalities.

In CFIT, a crew flies under control, and unaware, to an impact--often on approach and landing. Of 156 CFIT accidents between 1988 through 1994, for 108 aircraft where data were available, 81 were not equipped with a radar-based ground proximity warning system (GPWS). Loss-of-control accidents are often weather related: Disorientation induced by lack of visibility, for instance, can result in an unusual attitude--one that the crew may have little experience in handling.

In the works. Here is a sampling of technology designed to make air travel safer. Simulator-training programs will give pilots practice in avoiding loss of control. Weather-induced accidents should be curtailed with wider use of turbulence-radar and head-up displays (HUDs) giving eyes-out-of-the-cockpit data presentations. HUDs are being coupled with infrared-sensors to penetrate darkness and millimeter-wave radar to pierce rain and fog.

Other enhancements to flight safety can be realized by outfitting aircraft worldwide with collision avoidance systems currently required in the US. These devices automatically exchange position and direction information between nearby aircraft and provide avoidance maneuver instructions. Such a system on a Kazhak cargo plane could have prevented it from colliding with a Saudi 747 over India last year.

New large, multifunction liquid-crystal displays will filter information presented to pilots to only that needed for the current flight phase or requested by the crew. And the National Transportation Safety Board has asked the FAA to require that airliners have a visual indication of lift governing wing angle of attack. Such basic instrumentation is not required on commercial transports--akin to driving a car by odometer and clock to determine your speed.

Going global. Widespread satellite communications and Global Positioning System (GPS) navigation will allow "free flight" operations: Pilots will fly the best routes for weather avoidance and fuel efficiency, and not overload airway routes. Local airport GPS receivers will provide precision approach guidance for airliners.

Probably the greatest gain in crew situational awareness designed to curtail CFIT will come from enhanced GPWS. Current GPWS use radar altimeter data to determine when straight-ahead sink rate is excessive. But only sensing straight down, these systems cannot provide adequate warning of a steep rise, such as a cliff, or accommodate turning paths. And they are "desensitized" when flaps and landing gear are extended close to the ground to eliminate false alerts.

EGPWS designed by AlliedSignal Aerospace (Redmond, WA), incorporates not only altitude information but a worldwide terrain height data base that features runway locations. Aircraft altitude, position, and direction are combined by system algorithms to project a three-dimensional path--including curved paths needed to land on runways in mountainous areas. Aural warnings are typically available a minute or more from impact, as opposed to, at most, 30 sec with traditional GPWS.

A map display shows significantly higher areas in red, other higher terrain in yellow, and ground to 2,000 ft below the airplane in green. The more "solid" each color, the greater the potential threat. The EGPWS can also provide alarms for wind shear, when ground speed drops abruptly, and approaches short of a runway or to a "non-existent" runway, the most common CFIT accidents.

In the wake of one of its 757s having a CFIT crash into a mountain near Cali, Columbia, in 1995, American Airlines has ordered the AlliedSignal system for its fleet, as has United Airlines. Alaska Airlines, flying into rugged terrain with much precipitation, is installing EGPWS on its 25 737-400s. Captain Jim Applegate notes, "It's working out beautifully. It is a great enhancement to aviation safety, simple to operate, and very intuitive . Before, you had to use a paper map and try to figure out where you were." With noticeable relief, he adds, "Now the system orients you as to where the aircraft is in relation to terrain."


GPS: Where in the world are we?

The Global Positioning System (GPS) was originally developed by the Department of Defense (DOD) to provide U.S. and allied forces with precise position information anywhere on Earth. Not only can GPS determine locations of troops, ships, and aircraft but it can guide weapons such as cruise missiles. Now available for civilian use, GPS provides 156m vertical and 100m horizontal accuracy in the presence of the DOD Selective Availability (SA) induced error signal. This thwarts potential adversaries use of the system. With SA turned off, these numbers become 32m vertical and 20m horizontal.

A GPS receiver works by taking ranging codes from the satellites visible to its antenna from the 24 satellite constellation orbiting the Earth. Satellite distances are determined by measuring the signal transit time from each, and by knowing their broadcast locations, determining the antenna location. The more satellites in view, the more accurate the position.

To aid aircraft landing, differential GPS (DGPS) has been developed by several manufacturers. They place one or more GPS receivers at known surveyed locations with respect to a runway or runways. The receivers compare location information derived from GPS signals to their true location, allowing determination of the accuracy, availability, and integrity of the GPS signals. Any difference includes errors due to atmospheric delay effects as well as SA. The system broadcasts the measured errors to an approaching aircraft's GPS receiver, which receives signals from the same group of GPS satellites. On board the aircraft, the system corrects for the common error effects for an accurate navigation solution (within 2m vertically and 1.5m horizontally) to touchdown at the numbers painted on the runway end.

A single DGPS system, such as the SLS-2000 from Honeywell (Phoenix, AZ) and Pelorus (Calgary, Alberta, Canada) can serve runways at airports within a radius of 22 to 33 miles. Thus, precision approaches, including curved paths and those to parallel runways, can be used at facilities without conventional instrument landing systems.

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