Airport Alert System To Spot Wind Bursts Is Successful in Test
By RICHARD WITKIN
Published: August 7, 1988
A new system that could go far toward conquering a persistent threat to aviation safety was put spectacularly to the test one day last month when three loud beeps summoned the Denver airport tower to action.
Using radar to detect abrupt changes in wind speed or direction and a computer to alert air traffic controllers automatically to the dangers of such wind shifts, the system was in the early days of a two-month trial. Shortly after 4 P.M. on July 11 it flashed data to the controller that wind conditions were threatening planes near the airport.
In the next seven minutes the controller radioed five United Airlines jetliners descending to two westbound runways that they were approaching a one of aviation's deadliest hazards, one pilots refer to as wind shear. At least 30 accidents or incidents in this country have been attributed to it since the mid-1960's, and the death toll is well over 500. Disbelief in the Cockpit
The Denver controller's warnings became more urgent, and the last one told a crew to expect a loss of 80 knots in the lift-generating speed of the airflow over the big jet's wings. That is the second-highest loss in lift ever recorded.
''You say 80 knots?'' a pilot asked, disbelief in his tone.
''He's correct and we can confirm it,'' came a voice from another plane, apparently one of the other United jets.
All five jetliners broke off from their landing attempts at Denver, which is a United Airlines hub, and circled to give the troublesome wind time to dissipate. The planes all landed safely. A ground-based system of wind speed indicators used by Denver and many other airports to detect wind shears had not picked up the threat, which was brought on by conditions beyond that system's boundaries.
The successful demonstration of the upgraded system, based on advanced Doppler radar detection, elated officials involved in the test. The Federal Aviation Administration has set a goal of 100 of the new systems at major airports around the country; if that goal is met, each unit is expected to cost $5.5 million. Detecting and Quick Warning
The system is the first to identify the wind's abrupt change automatically and alert controllers. The controllers then immediately relay warnings to pilots. In preliminary phases of the warning program, detection systems depended on an expert analyzing readings from several instruments. The speed and precision of that analysis could make the difference between a crash and a close call.
The test, which ends Aug. 31, is meant to clear the way for awarding of contracts in October for 47 production versions of the new detection system, which will ultimately replace 16 interim systems. The system being tested was developed by federally financed researchers; the manufacturer of the final version will be chosen as a result of bidding. The interim systems are expected to start going into operation in 1991; officials said they expect to start putting the full systems into operation in 1992.
Government and industry experts offer varying views whether any of the United planes might have crashed without the warning system. But they agree that the equipment more than proved its worth. Some said it might have averted a disaster. Views of the Pilots
''We've now convinced ourselves that the automatic system works,'' said John McCarthy, manager of the a group from the National Center for Atmospheric Research that is working on the program. ''The F.A.A. said demonstrate that the automation feature does the job and, with one month gone, we're halfway there.'' Lincoln Laboratories of the Massachusetts Institute of Technology is also working in the program.
Patrick Clyne, a DC-10 captain who is chairman of the weather committee of the Air Line Pilots Association, said it is likely that without the alerts ''we would have had an accident.''
Capt. D. A. Simmon, United's director of flight safety, said the planes, several hundred feet about the ground, were high enough for the crews to have avoided a crash. ''But if someone had got into some of that wind shear below 100 to 200 feet without the advance warning, the outcome becomes questionable,'' he said.
Many airports use ground-based systems of wind meters scattered around the field; such a system was in place at Denver, but not activated July 11 when the Doppler radar set off alarms. The wind shear had passed outside the bounds of the ground system.
An abrupt change in wind speed or direction or both are not normally serious for planes cruising between airports, but that change has proved deadly close to the ground for planes landing or taking off.
The worst danger near airports comes from a narrow but powerful downward rush from a cumulus cloud that fans out in all directions when it reaches the ground. This can abruptly curtail the normal lift-producing flow of air over a plane's wings and lead to disaster.
First, a pilot encounters excess air moving over the wings as the horizontal flow spreads toward the plane; the headwind provides extra lift. Quickly, the plane passes directly under the down-draft and proceeds into an area where the horizontal spread of air is in the same direction in which the plane is flying. The headwind has turned into a tailwind. The excess airflow over the wings has gone.
Suddenly the airflow is abnormally low, bringing a loss of lift, and that loss, coupled with the push of the down-draft, can cause a crash if the plane is low.
The search for solutions has been going on for decades. It intensified after a 1975 Eastern Airlines crash in New York killed 112 people. It has been spurred by several other fatal crashes, notably the loss of a Pan American jet near New Orleans in 1982 that killed 153 and the Delta Air Lines crash at the Dallas-Fort Worth International Airport in 1985 that took 137 lives. How the Radar Functions
The new system in Denver uses Doppler radar to detect the movement of the air masses that form a wind shear. The motion of the reflecting surface, in this case the stormy air struck by radar beams, changes the wavelength of the returning signal just as a train's motion changes the perceived pitch of its whistle. This Doppler effect enables the radar to record the relative motion of an object toward or away from it.
While the Doppler equipment is generally viewed as the most promising development to date, the Government and industry do not consider it the end of the wind problem. It is a major addition to a variety of other programs that include intensive standardized training of pilots in emergency procedures, the low-level alert system that uses 6 to 12 wind-speed meters installed around an airport, and systems aboard planes that detect wind shear, but only after the plane has actually entered the troubled air.
The airborne system then shows the pilot the best nose-up attitude for escape. The Doppler system can warn before the plane enters the hazardous air and in many cases avert it.
One limitation of the Doppler system, like the low-level system, is that because of limited budgets, hundreds of airports will not install it. A big advantage it has over the low-level system is that it can cover a much wider area and scan to high altitudes, assuring much earlier warnings. Wind shears, generally only one-half to two miles wide, can evade detection by on-the-ground speed meters.
Diagram of the monitor type used at Denver airport to test wind shear (NYT) (pg. 28)