A Bump in the Sky

By Jill Smolowe

When USAir Flight 427 plunged from the sky on Sept. 8, none of the 127 passengers or five crew members survived to help explain what might have triggered the 6,000-ft. nose dive. Nor have investigators found evidence of wing, rudder or engine failure in the charred rubble of the 737 jet. That leaves little to explain the tragedy except a "bump" -- a sudden airspeed increase detected by the plane's flight-data recorder. Wind has been ruled out, since only a 7-m.p.h. breeze was evident that evening. And earlier reports of the cry "Traffic!" on the cockpit voice recorder have proved false. So what was it? Some aviation experts speculate that the bump in the sky may have been caused by the air turbulence created by the jet that preceded Flight 427 into Pittsburgh International Airport.

While still unproved, the hypothesis is stirring a debate about an aeronautical phenomenon called wake vortex. That dry bit of technical jargon refers to the rotating, high-energy tornadoes that spiral behind and downward from the wing tips of an aircraft. Such turbulence behaves much like the wake of a ship: the heavier the vessel's displacement weight, the more violent and long lasting the disturbance. In air, as on water, if a craft trails this whirling vortex too closely, it can be buffeted brutally. For more than a decade the National Transportation Safety Board, which investigates accidents, has exhorted the Federal Aviation Administration to be more aggressive in studying, monitoring and regulating the way following aircraft navigate wake vortex. Now, even if such turbulence fails to account for Flight 427's crash, the Safety Board has trained sufficient attention on wake vortex to prod the FAA into action.

Wake vortex began to emerge as the prime suspect early this month, after a Safety Board member told reporters that the NTSB was trying to determine the effect the bump had on Flight 427's controls and crew. Last week the board was more cautious. "This is an ongoing investigation," said spokesman Mike Benson. "No probable cause has emerged yet."

The Safety Board is under considerable pressure to offer a plausible explanation for Flight 427's demise. Aviation experts -- not to mention airline passengers -- hate a mystery. Since 1967, the board has succeeded in finding a probable cause for all but three air disasters. On average, such investigations take a year. The rush in this instance owes much to the magnitude of the human toll, the largest in the U.S. since 1987, when a Northwest Airlines crash claimed 156 lives. The tragedy also involved a Boeing 737, the most common of all passenger jetliners. Moreover, there is an eerie resemblance between the September catastrophe and the March 1991 crash of United Airlines Flight 585 near Colorado Springs, Colorado. In both instances the 737s banked abruptly, rolled belly-up, then plummeted vertically. The cause of Flight 585's crash has never been established.

For now, the evidence supporting the wake-vortex theory is thin. As Flight 427 approached the airport, it was following a Delta Airlines 727, a heavier Boeing plane that generates a slightly stronger wake. Flight 427 trailed the other jet by 4.1 nautical miles, well within the FAA regulation that requires two planes of such weights to maintain a separation of 3 nautical miles. If the 727 wake did jostle the 737 sufficiently to contribute to the latter's plunge, it would be a first. While 727s were the lead craft in seven of the 52 wake-vortex encounters documented by the NTSB from 1983 through 1993, all of those incidents -- some merely unsettling, some disastrous -- involved much lighter trailing aircraft.

The Safety Board's most recent warning about wake vortex, issued in February, concentrates on the turbulence stirred by the heavier 757, whose wake has upset or downed seven planes -- among them a 737. The NTSB called upon the FAA to reclassify the 757 so that other craft must follow at greater distances during takeoffs and landings. The FAA has yet to act. Canada, however, upped the classification of the 757 from "large" to "heavy" earlier this year; Britain made a similar change last year by carving out a new category to accommodate the 757.

Air-safety watchdogs have been frustrated by the FAA's slow response to their repeated calls over the years for rules requiring pilots to report wake- vortex incidents more thoroughly. "The FAA has got to develop a sense of urgency where wake-vortex phenomena are concerned," says Jerome Lederer, founder of the Flight Safety Foundation in Arlington, Virginia. At the same time, however, the FAA has been urged by cash-strapped airlines to reduce the separation distances between landing airplanes so that carriers can turn the planes around faster to make more flights.

Long paralyzed by these competing demands, the FAA is at last responding to safety concerns. Last month the agency established a special office that will devise a system to catalog and analyze turbulence data. Prodded by other organizations in the flying community, pilots have begun reporting about five wake-vortex incidents a month. Participants predict that a more complete network, which is expected to be operating by next February, will catalog quite a few more.

With reporting by Jerry Hannifin/Boise, John Moody/New York and Edwin M. Reingold/Los Angeles