WASHINGTON — "Oh . . . this can't be!"
These were the words of Arturo Valdez Prom, the captain of Aeromexico Flight 498, as he realized that something terrible was happening to the DC-9 during its approach to Los Angeles International Airport Aug. 31.
A transcript of the cockpit voice recorder, which contains only snippets of the Spanish conversation of the Aeromexico crew, was released here Thursday by the National Transportation Safety Board.
Whether Prom or his co-pilot, Hector Valencia, actually knew that they had collided with a small Piper airplane over Cerritos may never be known.
Although the tape revealed Prom's astonishment, it was of such "consistently poor quality" that investigators could glean little new information about the collision, which killed 82 people, including 15 on the ground.
James R. Cash, an air safety investigator in charge of preparing the transcript, reported that the final segments of the 30-minute tape are of "very poor quality." This, he said, was not because of the accident, but because the model of the plane's cockpit voice recorder "has a history of tape tension and recording quality problems."
The sound was so distorted and noisy that it was difficult for those transcribing the tape to understand the crew's conversation, Cash said.
"The flight crew used the cockpit overhead radio speakers for reception of air traffic control transmissions," Cash said. "This impaired intelligibility because of the close proximity of those speakers to the cockpit area microphone, the loud radio volume and the large number of transmissions in the Los Angeles area."
What little that investigators could hear indicated that Flight 498 was proceeding routinely at 11:47 a.m. PDT when the crew radioed a "good morning" to the Los Angeles Terminal Radar Approach Control facility, which told them to prepare for a final approach to Runway 25 Left at LAX.
Other than the sound of the attendant bell and a few routine instructions from the crew, nothing else from inside the cockpit is audible on the transcript until 11:51, when the captain says: "Thank you." The context of that statement is unclear.
A few seconds later, the air-to-ground communications picked up on the cockpit recording indicate that the controller advised the Aeromexico jet of a runway change, apparently because a Wings West flight had also been cleared for Runway 25 Left.
Controller: "Aeromexico 498, maintain your present speed."
Cockpit: "Roger. Aeromexico 498. Uh, what speed do you want. We're reducing to two niner . . . to one niner zero."
Controller: "OK, you can hold what you have, sir, and we have a change in plans here, stand by."
Cockpit: "All right, we'll maintain one . . . nine zero."
Seconds later, at 11:52 a.m., there is the first indication from the crew that something has happened.
"Oh (expletive deleted), this can't be!" Prom exclaimed.
There are no further words recorded in the cockpit, although the recorder did pick up the voice of the controller advising the Aeromexico jet to use Runway 24 Right. The recording ended 22 seconds after Prom's exclamation, presumably when the plane hit the ground.
The transcript was reviewed by National Transportation Safety Board investigators, Federal Aviation Administration officials, a representative from the plane's manufacturer, McDonnell Douglas, and aviation authorities from Mexico.
The safety board released a separate report of its examination of the flight data recorder, which was also recovered from the wreckage and which keeps a permanent flight record of the airplane's altitude, airspeed and heading.
The data collected by the recorder showed that the flight was proceeding smoothly as the Aeromexico jet started its descent from 10,133 feet with an airspeed of 347 knots.
About 7 1/2 minutes later, as instructed by the controller, the aircraft had descended to about 7,000 feet, with an airspeed of 228 knots. Corrected data from the recorder indicated the collision occurred at 6,589 feet.
'All Bets Were Off'
There was additional data for the 22 seconds between the collision and the impact on the ground, but Ira Furman, a spokesman for the NTSB, said these final readings--which indicate that the doomed airliner gyrated wildly--are probably unreliable.
"Once the plane was hit, all bets were off on the numbers," Furman said.
Furman said altitude readings from the data recorder were corrected by more than 500 feet after it was noted that the recording indicated that the plane was eight feet below sea level when it landed at Tijuana, the last stop before the crash. The Tijuana airport is actually about 500 feet above sea level.
The erroneous readings show only what the recorder was inscribing and do not indicate that any of the pilot's instruments were malfunctioning, Furman said. He said there may be later corrections in the airspeed readings.