A new National Transportation Safety Board report on the Delta Air Lines crash that killed 137 people at Dallas Aug. 2 shows that the jumbo jetliner encountered a severe microburst wind shear as it approached the runway, then crashed short.
Further, according to aviation sources, significant evidence has been developed to show that crew members could have flown through the shear if they had applied full engine power and used different techniques. However, the sources cautioned, it is one thing to state a theoretical possibility, another to understand what the crew did and why they did it.
Microburst wind shear -- a sudden shift of wind speed and direction -- has been suspected as playing a role in the accident almost from the moment it happened in a violent rain squall, but yesterday's report is the first official statement pinpointing wind shear.
The safety board will hold a public hearing in Dallas next week to hear testimony about whether the Lockheed L1011 could have survived the wind shear. Representatives of the manufacturer are expected to state that the shear was "flyable." Delta and pilot groups are expected to hold the opposite view.
The safety board will present its own experts and will examine the question of what the crew knew about the weather and what the plane's flight instruments were indicating. Of particular interest is an instrument called a flight director, which tells pilots the appropriate aircraft angle and airspeed settings for specific situations. There has been some concern among pilot groups that the standard flight director commands for a landing would be inadequate to handle a severe wind shear.
Yesterday's report, one of many factual documents the board's technical committees have prepared, says Delta Flight 191 entered a strong downdraft as it descended. "During entry of the downflow, the headwind increased from about 10 to a maximum of 27 knots in approximately five seconds," the report says. "The headwind then sheared to an approximate 40-knot tailwind over the following 26 seconds."
That combination would amount to a loss of 67 knots of airspeed in 26 seconds -- more than enough to stop a plane from flying -- if additional engine power were not applied. The digital flight recorder shows that the plane actually lost 60 knots of airspeed in 17 seconds.
The cockpit voice recorder, released earlier, shows that it took the crew several seconds to decide to go to full throttle, and by then it was too late to recover control of the aircraft. The plane barely grazed the ground, then flew or bounced into a freeway, striking a truck. It continued across the freeway and into some water tanks, where it burst open. Twenty-eight people in the rear of the plane survived.
Other documents in the report show that the plane pitched violently up and down as it passed through the shear, with a nose angle as high as 16 degrees and as low as minus eight degrees. Board specialists said there is no evidence that lightning struck the plane, despite several witness reports of lightning in the area.