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. Last Updated: 07/27/2016

TWA Crash Prompts Changes to 747s




WASHINGTON -- The U.S. Federal Aviation Administration has ordered an immediate change in fuel pumps in Boeing 747s and has also proposed a change in the wiring of older 747s, saying that tests conducted after the crash of TWA Flight 800 pointed to both as potential causes of a midair explosion. Although "there is no evidence from the accident airplane that leads us to conclude that either one of these is the cause of the accident," said Thomas McSweeny, director of the FAA's aircraft certification service, the agency believed it was "prudent" to make the changes.


The National Transportation Safety Board, another federal agency investigating the crash that killed all 230 people aboard the TWA airliner in July 1996, has concluded that Flight 800 was destroyed when the center wing tank, containing only a few gallons of fuel but filled with explosive vapors, somehow ignited.


Sixteen months after the crash, exhaustive inquiries by the safety board and the FBI have yet to pinpoint what actually caused the fumes to ignite shortly after the plane took off from New York City's John F. Kennedy International Airport en route to Paris.


The NTSB has not recommended the changes announced Wednesday. In fact, it has been pushing for a far more ambitious action -- inserting inert gas into the empty space in fuel tanks to prevent an explosion even if a spark is present.


But because the gas process is so expensive, the airlines have objected. And the FAA, which has often been criticized as being too close an ally of the airlines it regulates, has resisted the recommendation.


The FAA announced its action less than two weeks before the NTSB is scheduled to begin hearings in Baltimore into the cause of the crash. The FAA has now put itself on record before those hearings as having taken some corrective action, if not the one that the NTSB wanted.


In a statement Wednesday, NTSB Chairman James E. Hall said, "As always, the NTSB applauds any safety initiative." And, referring to the proposed change in aircraft wiring, he added, "we look forward to reviewing the FAA's proposed rule.''


The NTSB's approach would cover all planes with belly tanks, not just Boeing 747s.


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Wednesday's FAA action is the second significant development in the TWA investigation in just over a week. On Nov. 18, the FBI suspended its criminal investigation into the crash, saying it had found no evidence that a bomb or missile destroyed the plane. That announcement effectively left mechanical failure as the only plausible explanation.


In a conference call with reporters, McSweeny said his agency decided to issue the order regarding the fuel pumps after tests showed that silicone seals used in parts of one of the fuel pumps could dissolve on contact with jet fuel, which, in addition to powering the plane, is used to cool and lubricate the pump motor.


If the silicone disintegrates, fuel could squirt into the wheel well behind the center tank where it could ignite, McSweeny said.


The pump in question is called a "scavenge pump," which drains the last few gallons from the center tank. The scavenge pump from the TWA plane is part of the 4 percent of the airplane that searchers have not recovered.


In a statement, Boeing also said that the silicone had been added to some fuel pumps after their initial installation, and that the company had already issued a "service bulletin" to its customers "to inspect and correct all scavenge pump connectors on affected 747s."


The FAA gave American owners of 747s 90 days to replace any pumps that use the silicone seal. There are 196 Boeing 747s registered in the United States that could have such pumps and 970 worldwide, the agency said, and it characterized inspecting or replacing the pump as a simple job that could be done when the aircraft undergoes routine maintenance.


The second change sought by the FAA would be far more complicated and will be put out for a 90-day public comment period. The agency wants airlines to replace wiring in a place where wires enter the tank, known as the "fuel quantity indication system."


Investigators have long theorized that a spark was created in this system, where fuel probes send signals back to the cockpit to tell the crew how much fuel is left.


The system consists of a small pipe inside a large pipe, with a ring-shaped gap in between. The pipes are fastened vertically in the fuel tanks, like a drinking straw pointed straight down to the bottom of a glass of water.


Using very small electrical currents, the probe senses how high the fuel rises in the space between the pipes. But, McSweeny said, lab tests had shown that if the wiring to the fuel probes was bundled with high-voltage wiring for several feet, and if the high-voltage wiring carried an electrical current that was suddenly cut off, an electrical surge in the fuel system wiring could occur.


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Engineers have always known that such electromagnetic interference could induce a surge or a spike, but they were surprised by the size of the surge -- up to 800 volts in lab tests. McSweeny said a surge of 800 volts still would not be enough to create a spark in the gap in the fuel probes.


But in trying to assemble a plausible chain of events to explain the destruction of Flight 800, investigators have also found that 747s in service can have little pieces of metal in the fuel tanks, and these can become lodged in the gaps of the fuel probes, where they would help create a spark at lower voltages. Potential sources of such contamination, McSweeny said, were stray bits of wire or steel wool. Copper sulfite deposits could also form in that area, he said.


The FAA's proposed solution is to have airlines install components that would suppress electrical surges, or rewire their planes to separate the fuel probe wiring from other wires. The proposed order affects 747-100s, the kind that crashed, plus 747-200s and 747-300s, but not the models currently under production. In the newer planes, the wiring was routed separately.


The rule, if adopted, would affect 167 aircraft registered in the United States out of 650 worldwide. It would cost dollars 13,200 per aircraft, the FAA said. It is unclear who would bear that cost.


NTSB officials also believe, however, that an electrical surge might cause a spark even without metal contamination, if there was a flaw in the wiring where it is attached to the fuel probes. One of the problems for the NTSB, though, is that no probes were recovered intact from the wreckage, making the role of the fuel probe wiring in the catastrophe unclear.


In its statement, Boeing said that it had conducted the testing to produce electrical surges. The surges generated, however, were "far below what would be required to present any hazard to the airplane," even if contamination was present, the statement said.


"Earlier laboratory testing using energy levels far in excess of what is available on the airplane did demonstrate the ability to create an arc in a fuel probe that had been purposely contaminated with debris," the statement said. Boeing said it would do additional tests before filing comments on the proposed rule.


The FAA's announcement Wednesday demonstrates an unusual facet of air safety: what happens when an old-model plane that engineers thought they understood perfectly is put under the microscope after a crash. Just as with the investigation of an unsolved crash near Pittsburgh three years ago involving a Boeing 737, investigators have turned up potential problems. On the 737 it was the rudder; here it is the fuel system.