A cockpit ground-proximity alarm warned the crew of Delta Flight 191 to "Pull up! Pull up!" moments before the jumbo jet slammed into the ground, officials said tonight.
The electronic voice alarm, which activates when a plane suddenly drops too low to the ground, was set off seconds before a tower controller at Dallas-Fort Worth International Airport spotted the plane emerging dangerously low from a thunderstorm and told the crew: "Delta, go around!"
G.H. Patrick Bursley, the National Transportation Safety Board member leading the crash investigation, said recordings show that the pilots betrayed no special concern about possible dangerous wind shears as they flew through the intense thunderstorm cell that is suspected of being a major cause of the Friday night crash.
The Lockheed L1011, carrying 163 passengers and crew, was en route from Fort Lauderdale, Fla., to Los Angeles via Dallas. Thirty-one persons survived the crash; 62 of the dead have been identified.
Bursley said tonight that not enough evidence has been gathered yet to determine whether the Delta Air Lines crew responded to the "Pull up!" alarm.
But he said the investigators are certain that by the time the tower controller issued his desperate command, the plane was doomed. "Controllability was beyond recovery," Bursley said.
The cockpit voice recorder detected "grinding sounds" of the plane breaking apart moments before the ground controller's command.
The storm suddenly gathered force and broke in violent winds and rain directly in front of the Delta jet as it headed in on its landing approach about 6 p.m.
Investigators have learned that a special airport warning system intended to alert the tower if a plane is dangerously low did not go off.
Bursley, a retired Coast Guard admiral, said the Dallas-Fort Worth system is set to ignore any plane that is lower than 394 feet high near the runway as a way of eliminating spurious signals from planes landing safely.
Flight 191 first touched the ground about 2,000 feet north of the runway, bounced into the air, hit a car on a peripheral highway, killing the driver, and fell a few hundred feet short of the runway. The tail section broke off; most of the survivors were seated there.
Bursley said the pilot of a Lear jet who made a safe landing directly ahead of the Delta plane failed to tell the control tower he had encountered severe turbulence during his landing approach. Bursley said the safety board has repeatedly urged pilots to be "more aggressive" in reporting bad weather to air controllers.
But he added that it is not certain that such a warning could have been relayed to the Delta pilot, Capt. Edward Connors, in time to help him avoid the thunderstorm.
About three miles separated the two aircraft. The Delta plane was traveling at about 150 knots, or 170 mph, meaning the elapsed time between the two planes was about one minute.
The Delta airliner started its landing approach at 210 knots, or 240 mph, but had been steadily ordered by air controllers to slow down to maintain a three-mile interval behind the slower Lear jet.
Bursley was asked if the slower speed made the Delta plane more vulnerable to a wind shear. "One hundred and fifty knots is a safe speed" for a L1011, he said, but added that any pilot prefers "as much power as he can put into it to handle his maneuvering problems."
The cockpit voice recorder and in-flight data recorder have been been recovered from the wreckage and are being analyzed in Washington.
He said preliminary findings from the data recorder indicate several fluctuations in airspeed in the final minutes of the Delta flight. As a result, investigators think that the shifts could not have been caused by throttle control changes.
The recorder information appears to strengthen the likelihood that Flight 191 encountered treacherous wind-shear conditions as it attempted to land.
Wind shear is a sudden change in wind speed or direction, often during a thunderstorm. The phenomenon sometimes causes intense downdrafts or side drafts that can push a plane downward or put it in a near stall.
On-board instruments record the speed of the plane relative to the wind, not its speed over the ground. The fluctuations found by investigators in indicated airspeed imply rapid changes in the velocity and direction of the air mass through which the plane was flying.
Ira Furman, spokesman for the safety board team, said experts think that the airspeed changes recorded by the plane's "black box" cannot be attributed solely to the Lockheed's three turbofan engines.
Officials also noted that the airport's wind-shear alert system went off about 14 minutes after Flight 191 crashed. Directors of the 11-year-old airport said they will consider installing a more advanced alert system if it is shown to be effective.