When Haueter’s air traffic investigators checked radar tapes for other planes in the vicinity of Flight 427, they found that the closest was a Delta Air Lines 727 that was 4.2 miles ahead. That was more than a mile beyond the FAA’s minimum spacing, so there was plenty of room between them. But when the air traffic group mapped the radar tracks of both planes, investigators discovered that the instant at which Flight 427 crossed the Delta jet’s wake was the exact moment when things started to go wrong.
Big jets left wakes in the sky just as cruise ships left them in the water. Wakes were powerful spinning tubes of air the size of sewer pipes that kept spinning after a plane was miles away. They were invisible, but definitely noticeable. When a plane encountered a wake, it felt like a strong bump or it jostled the plane slightly to one side. Pilots could easily recover, but they might have to roll their wings level or adjust their heading.
Wake turbulence experts from NASA studied Flight 427’s radar data and said there was plenty of evidence of a wake encounter. The calm weather on September 8 had been ideal for long-lasting wakes. The flight data recorder showed that the USAir plane’s indicated airspeed jumped from 190 to 195 knots in one second. A big 737 cannot speed up that quickly, but that kind of change is often caused by an encounter with wake turbulence, which alters the flow into the airspeed sensors. But the NASA experts doubted that the wakes could have caused the accident. They might have rolled the plane slightly, but they weren’t strong enough to flip the 737 upside down.
One of Haueter’s bosses, Bud Laynor, was especially interested in the effects of a wake. As test after test cleared the plane’s rudder valve, Laynor became more and more intrigued by the possibility that the wake had severely jostled Flight 427. But Haueter thought that was impossible.
“If that’s so,” he told Laynor, “why aren’t we crashing them every day at National Airport?”
Still, Haueter acknowledged that it was possible that the wake had triggered some other kind of malfunction. The bumps may have activated the yaw damper, a device that made tiny adjustments to the rudder to make a flight smoother. If there was some kind of jam in the hydraulic valve, the rudder might have suddenly turned to one side—a problem known as a hardover.
Unfortunately, the government and the airline industry knew relatively little about wake turbulence. Engineers knew that wakes could be strong, but they didn’t know how strong or how planes would behave when they crossed wakes at different angles. So Laynor began to push for a special flight test. He wanted to fly a 737 behind a 727 to re-create the conditions before the crash. Special equipment would measure the effects on the plane and show whether the wake could have flipped Flight 427 out of the sky.
The wreckage had been in the Pittsburgh hangar for seven weeks, the odor growing worse each day. It was a musty mixture of jet fuel, Clorox, and God-knew-what-else. The plane seemed to be decaying.
Dave Supplee, the USAir mechanic from Tampa, had been summoned back to the creaky hangar to rebuild the front sections of Ship 513. That wasn’t unusual in a crash investigation, to put wreckage back together and look for patterns. But most accidents left large pieces that could easily be identified. That was not the case with Flight 427. There were thousands and thousands of pieces that were blackened, scarred, dented, flattened, and chipped. Putting them together was akin to working the world’s most difficult jigsaw puzzle after the family dog had chewed up the pieces and they had been set on fire.
At Boeing’s insistence, Haueter agreed that his structures team would reassemble the wheel wells, rudder cables, the leading edges of the wings, the floor beams, and the forward pressure bulkhead (the area just behind the nose of the plane). The group would examine the wreckage for evidence of birds, explosions, cracks, or fires. Haueter was still considering the possibility of a sudden failure of the rudder cables, a collapse of the floor beams or an explosion in the wheel well, a critical area where the landing gear came close to vital hydraulic lines, pumps, and cables.
Much of the USAir wreckage had been laid out on the hangar floor in the rough shape of the plane, but thousands of small, unidentified pieces had been tossed into three giant Dumpsters. The contents were poured onto the floor, and Supplee and other team members waded in.
“We’re going Dumpster diving!” they exclaimed.
For two days they rummaged through the piles, looking for pieces they could recognize. Most were no bigger than a business card and were so badly charred that they looked like burned peanut brittle. The team started with the floor beams but found only 15 to 20 percent of them. So Supplee and his colleagues dove into the pile again.
Full-size blueprints had been spread out under Plexiglas so the team could locate each piece. Once Supplee found a piece that looked familiar, he searched for its location on the blueprint. He got a huge feeling of accomplishment when he was able to find the right spot. But the task was maddening. He kept asking, “Where’s the rest of the plane?”
After several days, everyone realized the effort was impossible. There were huge gaps at the front of the plane. The wreckage had been so badly burned or so disfigured by the impact that there was no point in continuing. Supplee was discouraged. It seemed as if they had accomplished so little.
Still, the wreckage provided a few answers. Notches on the floor beams showed that the rudder cables had cut into them, which meant the cables were taut at impact and had not broken in midair. The burn pattern on the reassembled wreckage was random, which indicated that there was no fire or explosion until the plane hit the ground.
The partial reassembly of the front sections of the plane also allowed the structures team to look for evidence of birds. It was a long shot. Bird blood or feathers could have been washed away by the Clorox solution. But it was worth a check, especially around the slats, the movable panels on the leading edge of the wings. Boeing officials continued to suggest that a bird might have hit the slats right at the hinge. That could have caused the slat to pop up, making the plane roll to the left.
The bird scenario was one of a dwindling number of theories. Haueter was also interested in the gurgling sound that passenger Andrew McKenna had reported on the plane’s previous flight from Charlotte to Chicago. The NTSB got an early explanation of the sound a few days after the crash, from a USAir pilot who sat in the cockpit on that flight and said his knee was leaning on a PA microphone button. But the issue had popped up again because of theories involving the electronics and equipment bay, where the computers and gyros were kept. There had been a few other incidents where water from the lavatory toilet had leaked into the bay and short-circuited the electronic hardware. The phenomenon was known as “blue water contamination,” a nice way of referring to toilet water.
USAir had not been much help in pursuing the suspicions about the gurgling sound. The safety board had asked for a complete list of passenger names and phone numbers for the Charlotte-Chicago flight McKenna was on, but the airline had provided only thirty of them. To add to the mystery, Gerald Fox, USAir’s O’Hare maintenance chief, did not tell investigators about the call he received about the strange noise until three months after the crash. The woman who reported the sound to Fox had never been found, nor had several other people who talked about the sound at Gate F6.
After dozens of phone calls, Haueter’s investigators could track down only thirteen passengers, and only two of those had heard an unusual noise—McKenna and a woman who was sitting a few rows behind him. The woman did not think the sound was unusual, and she accepted the flight attendant’s explanation that it came from the PA system. Haueter decided to put the gurgling inquiry to rest. The pilot’s explanation about the noise was reasonable, and it seemed unlikely that the sound was important. If there had been leakage of blue water that caused a short circuit, it probably would not have made a sound and could not have moved the rudder as far as estimates said it had gone.