Somehow, errant electrons traveled through the wiring and into the dark confines of the center wing tank on the Boeing 747 flying as TWA Flight 800. There, encountering a tiny breach in the wire insulation, those electrons created a spark. That tiny bolt of lighting may have released an almost insignificant amount of energy, equivalent to dropping a dime three-eighths of an inch. But that small spark in the eternal night of the tank was enough to ignite explosive vapors.
The airplane exploded spectacularly, killing all 230 aboard in the twilight hours of July 17, 1996. Now, three years later, the most intensive, costly accident investigation in the history of U.S. aviation remains open.
The primary question remains unanswered: what was the source of energy that caused the fatal spark? One possibility is a passenger electronic device. It’s not known for sure, but a portable electronic device carried aboard by a passenger is in the lineup of suspects.
In a recent interview on the third anniversary of the TWA 800 tragedy, Dr. Bernard Loeb, head of aviation accident investigations for the National Transportation Safety Board (NTSB), outlined three possibilities that are under examination. What he has to say has profound implications for the design of avionics systems and their tolerance to interference.
"There are three ways in which you potentially can get power into the center tank and, perhaps, get enough energy to cause an ignition source, perhaps through electromagnetic interference or induction or whatever," Loeb hypothesized.
One means may be from electromagnetic interference from an external source, such as from a radar. "One of the things that is going to come out of this," Loeb said, "is that when an airplane takes off at JFK [New York City’s John F. Kennedy International Airport, the departure airfield for TWA 800] and it is near the radar, it is exposed to more than almost anything else that is going to be out there, and these planes are going in and out of there every day." Not to put too fine a point on it, but it seems that airplanes are routinely flying through the equivalent of a microwave oven.
Systems within the airplane itself are the second possibility as the source of the current that caused the spark. "When you turn on and off devices within the airplane itself, could you somehow get crossed or shorted wires, or whatever, just from that alone to produce sufficient energy?" Loeb pondered.
The third suspect is passenger electronic devices (PEDs). "We’re actually having work done on that," Loeb said. "Could somebody with a cellular telephone or laptop [computer], or one of these devices have triggered it, and it [the current] goes into the tank?"
"So we’re looking at PEDs just to see," he added. With research ongoing, Loeb naturally was circumspect about drawing too many inferences from any one of these scenarios. But he said one thing that should give pause: "It is incredible how little has been done in a hard way on PEDs."
The current policy is to restrict the use of PEDs while the aircraft is below 10,000 feet. Some carriers are permitting passengers to use their cell phones and other devices on the ground, so long as the aircraft doors are open.
Neither policy seems wholly satisfactory. One captain expressed discomfort with the practice of allowing passengers to use PEDs until the doors are shut. The emissions, he said, could give a false reading of cockpit instruments during execution of preflight checklists.
In many cases, passengers have been extremely reluctant to cease using their devices after pushback. A flight attendant recounted spotting a passenger shrinking down into his seat, trying to evade observation while continuing to use his cell phone during taxi to the takeoff position. (It seems that calls on cell phones are cheaper than on the electronically shielded air phones installed in the airplane, creating a built-in disincentive to use the seat-back equipment.)
In the air, there are reported cases of surreptitious cell phone use below 10,000 feet. One passenger, for example, made a call with his cell phone, claiming an in-flight medical emergency on the aircraft in the hopes of circumventing an air traffic control delay.
Two-way pagers appear to be a particular problem. These new devices, by the way, are not among the items of concern listed in the U.S. Federal Aviation Administration’s latest advisory circular on the subject of PEDs. That circular permits PED use except in "critical phases of flight," which is the essence of the no-use below 10,000 feet rule. The circular references a 1997 RTCA Inc. report that called for continued testing for potential interference with aircraft systems. Among the items mentioned specifically in the RTCA document: two-way pagers.
In a September 1998 case involving interference with the traffic collision avoidance system (TCAS) on a DC-9 at 7,000 feet over Wisconsin, the batteries in a passenger’s two-way pager had to be removed to solve the problem. In his report to the Aviation Safety Reporting System (ASRS), the captain stated, "This was my sixth pager... interference incident in nine months...I urge the FAA to add pagers to the list of specifically banned PEDs and to change the in-flight cabin announcement to reflect this."
In addition, the sky above 10,000 feet hardly seems to be a safe haven. In October of 1998 the captain of a 757 flying at 37,000 feet over the state of Washington experienced a rash of autopilot disconnects, on all three systems, for a period of three to four hours. The problem eventually was traced to a severely hearing-impaired passenger who was using a device with a headphone and a microprocessor in his pocket.
In his ASRS report the captain surmised, "Recalling that interference...is normally associated with wired devices not contained within the microprocessor case, and that PED interference is very site-specific, I asked the flight attendant to move the passenger forward not less than three rows, but preferably six rows." This was done and the autopilots calmed down.
However, the captain added a few more points when an ASRS official telephoned for a follow-up. "The captain has had previous experience with...passenger-operated games which are interconnected with a wiring harness. He states wiring acts as an antenna capable of radiating an RF [radio frequency] signal...the captain wonders why the aircraft manufacturer does not properly shield the aircraft electronics to prevent such interference."
Given these chilling incidents, and Loeb’s assertion regarding the "incredible" lack of hard data, one is left with this discomfiting apprehension: data-free analysis has led to an analysis-free policy. And, for sure, this question comes to mind: do the current liberal use policies regarding passenger electronic devices reflect the triumph of marketing and convenience over prudence and safety?