Hurtling down the runway at more than 100 miles an hour, the Northwest Airlines jet took an unusually long time to lift off. And when it finally became airborne, its tail nearly struck the concrete.
Then the plane struggled to gain altitude, banking left and right, according to witnesses. Seconds later, after a wing struck a light pole in an airport rental car lot, the DC-9’s fate was sealed.
It rolled left, struck two other obstructions and finally slammed down on a highway outside the airport, disintegrating and bursting into flames as it slid along the pavement.
The pilots were among the 154 people aboard the twin-jet who died in the accident at Detroit Metropolitan Airport on Aug. 16, 1987. But what they said minutes before the plane took off–or, more accurately, what they didn’t say–helped authorities pinpoint what caused the accident.
Investigators from the National Transportation Safety Board discovered a fatal blunder after listening to the plane’s cockpit voice recorder, a small metal box that was extracted from the blackened wreckage.
Possibly distracted by communications with air traffic controllers, perhaps preoccupied with concerns about approaching bad weather and running behind schedule, the pilots failed to complete a pre-takeoff checklist.
Incredibly, they forgot to set the plane’s flaps, the panels on the wings that provide lift critical to get off the ground and climb.
Now safety board experts are seeking to harvest clues from a cockpit voice recorder pulled Sunday from the Florida Everglades where another DC-9 crashed. Despite intense impact forces that obliterated the plane and killed all 110 on board on May 11, experts now analyzing the device in a Washington, D.C., lab are hopeful it will help them understand what happened in the final minutes of ValuJet Airlines Flight 592.
The cockpit voice recorder, or CVR, has a long history of providing crucial information to investigators whose mission is to determine the cause of a crash to prevent a similar tragedy in the future.
It is one of two so-called “black boxes”–though they typically are painted bright orange to stand out in the debris of a shattered aircraft–that are aboard every big passenger airplane in the U.S.
The other device, also an important investigative tool, is a flight data recorder. It tracks altitude, heading, speed and other operating characteristics of the plane during flight.
Cockpit voice recorders were mandated on big passenger planes by the federal government in 1967. Though technological advances have spawned a new generation of digital recorder now being installed on planes coming off the production line, most now in service feature one-quarter inch recording tape similar to the kind found in most audio cassettes. The recorder operates on a 30-minute loop, erasing the oldest material as it records the newest.
A plane’s recorder is connected to the microphones of the pilot, co-pilot and, if there is a third cockpit crew member, the flight engineer. The system also includes a “cockpit area microphone,” located in the overhead panel between the seats of the pilot and co-pilot on DC-9s, that is designed to pick up sounds–from engine noises to remarks by anyone in the cockpit not wearing a headset mike.
The recorder, hooked up to the microphones by wires, measures about 4 inches wide, 6 inches high and 12 inches long. Encased in stainless steel and insulation, it usually is located in the aft portion of the plane. That’s because the tail section typically is farthest from fuel tanks and, since crashes often are nose-first, it tends to have the best chance of remaining intact after an accident.
CVRs must be able to survive incredible stress under Federal Aviation Administration standards that have become increasingly stringent since the devices were first required three decades ago.
In testing, a recorder must be able to survive:
– 2,000 degree heat (the hottest that jet fuel can burn) for 30 minutes.
– 500 degrees of heat for 10 hours.
– Pressure equivalent to being under 20,000 feet of water for 30 days.
– The corrosive effects of saltwater for 30 days.
– The weight of 5,000 pounds for five minutes.
– The crush of a 500-pound weight, with a protruding pin, that is dropped from a height of 10 feet.
– The force of 3,400 Gs–3,400 times the press of gravity–or the rough equivalent of attaining a speed of 400 miles an hour and coming to a dead stop in a distance of 18 inches.
Older recorders, like the one recovered from the 27-year-old ValuJet plane, were built to somewhat less stringent standards, said Greg Francois, product manager for recorders at AlliedSignal Commercial Avionics Systems, a leading CVR manufacturer. For example, they were required to withstand a crash force of only 1,000 Gs.
Still, even the older devices are remarkably crashworthy.
Safety board officials say that more than 90 percent have been in good enough shape to yield information in post-accident investigations, even though tapes in some cases have had to be extracted from their mangled cases with tin snips and crowbars.
The recorder on the ValuJet plane was found underwater about 100 yards southeast of the crater where the nose of the jet hit. Packed in water to prevent rusting of its oxide coating, it was flown to the safety board’s Washington headquarters in a sealed cooler Sunday for cleaning, drying, copying and analysis.
Officials said the tape was in good condition, but its sound quality was poor and difficult to decipher.
That could be the result of wind noise and other ambient sounds that sometimes make analysis difficult, or it could stem from the age of the recorder or tape, experts said.
Nevertheless, the tape already has confirmed earlier indications of fire.
“It appears that the cockpit door opened,” Robert Francis, the safety board’s vice chairman, reported Monday after initial analysis. “There were verbal indications from the cockpit there was fire in the passenger cabin. There was also an indication from the cabin that there were problems obtaining oxygen.”
Safety board experts, who are expected to spend days deciphering the tape’s contents, have an array of analog and digital filters that permit them to isolate and analyze sounds.
The new generation of CVR, introduced about three years ago, promises greater clarity and dependability than its tape predecessor. Sounds are stored on computer chips and there are no moving parts to wear out.
“The recording quality is going to be as good 10 years from now as the day it ships from our factory, and you don’t have to touch it,” Francois said.
Meanwhile, the National Transportation Safety Board is pressing for replacement of the 30-minute recording requirement now in force with a two-hour standard to further enhance the CVR’s value as an investigative tool.
