National Weather Service radar was "painting" a moderate thunderstorm cell over the end of the runway as Pan American Flight 759 lifted off from New Orleans July 9, but that information never reached the cockpit crew of the flight that ended a minute later and killed 153 people.
That thunderstorm of apparently moderate intensity and others around it are widely regarded as having spawned a weather phenomenon that doomed the airplane. The existence of the radar picture and the absence of any mention about it in forecast information are among facts released yesterday by the National Transportation Safety Board as its investigation continues into the nation's second worst plane crash.
The board also released a transcript of the cockpit voice recording. Many of the sounds and words were obliterated by the thumping of the windshield wipers, but it is clear that Capt. Kenneth L. McCullers and First Officer Donald A. Pierce -- who was flying the plane -- recognized their plight.
"Come on back, you're sinking, Don," McCullers said in the last words noted on the transcript, doubtless a reference to loss of altitude.
Thirteen seconds later, a device called the ground proximity warning system sounded. A horn blared a repetitive whoop, and a computerized voice commanded, "Pull up, pull up."
The Boeing 727-235 never climbed high enough to be seen on air traffic control radar, which usually registers first between 50 and 200 feet above the ground.
The plane hit a tree 53 feet above ground about half a mile from the end of the runway, then flew for another four seconds before plunging into a residential neighborhood in suburban Kenner, La. Eight of the fatalities were on the ground.
The investigation, which will continue with a public hearing in New Orleans Sept. 13, is focusing increasingly on whether accurate, timely weather information is reaching the cockpits and, if so, whether pilots are paying appropriate heed.
The thunderstorms in New Orleans are suspected of having created wind shears, where bands of winds traveling in different and sometimes opposite directions lay over each other. A headwind will improve airplane performance and climbing ability, and a tailwind will retard them; if a plane passes suddenly from a headwind to a tailwind at low altitude, it may not be able to recover.
But stormy weather appeared to be more a topic of conversation than a concern with McCullers and Pierce as they prepared to take off. As the catering truck was pulling away from the plane after loading, McCullers told the truck driver that "I figured the rain would be here before now from the looks of the radar when we came in . . . ."
Later, when shifting wind conditions around the airport caused a preceding jetliner to request take off from a perpendicular runway, McCullers commented, "Now we might have to turn around and come back."
When a wind shear alert was broadcast by the tower, McCullers and Pierce continued without comment a review of procedures to follow if the takeoff attempt was to be aborted.
If the crew members had a worry, it appears to have been about the weight of the aircraft. With every seat and fuel tank filled for the flight to Las Vegas, the plane weighed 170,100 pounds, 1,100 pounds below the legal limit for that airplane under those conditions.
"Any more than 1 knot of tailwind," Pierce said at one point, "and we wouldn't be legal for 15," apparently a reference to a flight control setting that indicates the plane was on the edge of the weight limit.
McCullers ordered a "no packs" takeoff, which means that the air conditioning and cabin pressurization systems were to be turned off during takeoff and climb so as not to rob power from the three jet engines. Several wind shear alerts were broadcast and heard in the cockpit, including one five minutes before takeoff: "We have, ah, low-level wind shear alerts all quadrants. Appears the frontal passing overhead right now; we're right in the middle of everything."
Twenty seconds later, McCullers advised Pierce, "Let your airspeed build up on takeoff," referring to a technique that can be used to give a plane extra thrust in such shears.
The tower's wind shear alert, however, did not contain specific information about the speed and direction of the shear, as is specified in the air traffic control manual. Pilot groups have complained that wind shear alerts occur so frequently that a "cry wolf" problem may have developed among crew members.
Information unavailable at the airport, however, was being recorded by weather service radar at nearby Slidell, La. At the time of takeoff and at the end of the runway, according to the board's report, radar was showing a "Level 2 intensity radar echo," indicating a thunderstorm cell. Three other thunderstorm cells were registered within four miles of the airport, one at Level 2 and two at Level 3.
Thunderstorms are classed at six levels, with Level 2 considered moderate, with light to moderate turbulence. Level 6 is severe, with large hail, lightning, extensive wind gusts and turbulence.
Level 2 storms are not uncommon and do not require weather service alerts, according to a service source. However, pilots generally seek to avoid flying into any thunderstorm cell, no matter what its reported intensity.
About the time the plane crashed, the Houston regional air traffic center called the New Orleans tower and advised that "from here it looks nasty . . . . Our forecaster says it could shut you down."
The New Orleans controller responded, "I'm in the tower right now, looking at this stuff . . . . We're looking at something in the tower here; we don't quite know what it is at this point; we"ll get back to you."
"What's it look like, a tornado?" Houston asked.
It was black smoke rising off the end of Flight 759's runway.
Just before the takeoff, McCullers had gone on the public address system for the routine message to the passengers. "After takeoff," he added, "we'll be maneuvering around, circumnavigating some, ah, some little thundershowers out there, so we would like to ask you folks to please remain in your seats."