The crash of Pan Am Flight 759 earlier this month has lent new urgency to efforts to detect "wind shear," treacherous shifts in wind speed and direction. They have been implicated in at least seven major accidents since 1973 and may have brought the Pan Am jet down in a New Orleans suburb.
The National Transportation Safety Board, leading the crash investigation, is reevaluating wind shear warning equipment the Federal Aviation Administration has installed at great expense at about 60 U.S. airports. And new thought is being given to old questions about how pilots decide if the weather is good enough to fly.
The July 9 crash, which killed 154 people, came shortly after National Science Foundation researchers began flying through turbulence near Denver and tracking it on radar in a $4 million, 2 1/2-year study of wind shear. Their work is being watched closely by the NTSB.
Meteorologists feel they have begun in only the last five years to understand the frightening swirls of powerful and quick-changing headwinds, tailwinds and downdrafts that pilots have reported for years near thunderstorms and, at times, in seemingly harmless weather.
The breakthrough came after an Eastern Airlines Boeing 727 trying to land at New York's JFK Airport in 1975 during a storm crashed short of the runway, killing 113 people. Meteorologists studying that accident identified a previously unknown weather phenomenon, a sudden downdraft since dubbed a "downburst" or "microburst."
A form of wind shear, microbursts can be several hundred yards or several miles across, meteorologists believe. They can last as few as three or as many as 15 minutes.
A downburst occurs when a mass of air cools and begins to sink, gaining speed. As the mass nears the ground, it spreads out, much the way water from a garden hose splatters in all directions when it hits the ground.
A jet flying through a downburst at low altitude will first meet a strong headwind, slowing the plane's speed in relation to the ground and giving it extra lift. Near the center of the column, the headwind will diminish and, as the plane emerges from the other side, the plane will suddenly hit a tailwind.
This is the danger point. The plane's airspeed, the speed at which air is rushing past it, suddenly drops, impairing lift. The plane starts sinking, with the ground only a hundred feet or so below. "You lose your ability to fly," says Dr. John McCarthy of the National Center for Atmospheric Research, which is participating in the Colorado study.
Investigators have concluded this is what caused Eastern Flight 66 to crash in 1975. Though final conclusions on why Pan Am's Flight 759 crashed are still months way, the most common bet among aviation experts is that the jet fell victim to the same meteorological trap. It took off with thunderstorms nearby and slammed into single-story homes in the suburban town of Kenner 30 seconds later.
After the Eastern crash, airlines began giving pilots simulator training on dealing with wind shear. They are taught to add thrust at the first sign of trouble. The FAA began installing computerized detection systems that compare wind speed and direction at different points around airports and set off alerts in the tower if differences over 15 knots are found.
About 60 airports, including New Orleans International (Moisant) Airport where 759 took off, now have the $200,000 Low Level Wind Shear Alert Systems. But they have the limitation of not actually detecting microbursts, just indicating the potential for them.
The New Orleans system began operating just before the accident and the tower controllers broadcast alerts to pilots. Pilots say they regard such messages not as prohibitions against takeoff but as another piece of information to be used in deciding whether to go or wait.
The fact that wind shear was detected repeatedly, that another jetliner, a Republic Airlines DC9, ran into dangerous winds as it took off just before Pan Am and that Pan Am still took off and crashed has led investigators to take a detailed look at equipment and procedures in this area, board spokesman Barbara Dixon said.
The Air Line Pilots Association is considering whether to recommend that the FAA establish a scale of intensity for wind shear. "What we're looking for is more detailed information than what's being given now," said an association spokesman who asked not to be named.
The equipment now in place was intended to be temporary and, despite some concern in the industry that it could turn out to be permanent, the FAA says it is working on new, better systems."We had a tremendous head of steam up on this before the accident," a senior FAA official said.
Yesterday, as part of these efforts, the FAA published standards that cockpit warning systems will have to meet if airlines install them. The devices now are not required or in general use. The FAA official said the devices are designed mainly for shear encountered by planes preparing to land. Thus they would not have helped Flight 759, if that caused the crash.
The Colorado study group, meanwhile, is testing six types of wind shear equipment, McCarthy said, including ground-based radar and cockpit equipment that compares airspeed and speed over the ground and scans the air ahead with laser beams.
Like all foul weather accidents, the crash of Flight 759 has led people to ask why planes fly in storms at all. Pilots respond that the public demands that flights go in all weather, and protests only when a plane goes down. Pilots also maintain that weather that looks dangerous is often safe to fly through.
The goal is to to identify the danger spots, like microbursts, and go nowhere near them.