For more than a year, the inquiry into the fall of TWA 800 has addressed three questions: whether mechanical trouble can be ruled out, whether a bomb inside the plane can be ruled out, whether a missile or other high-velocity object (such as a meteorite) can be ruled out. But there is a fourth possibility that has been ignored and that needs to be raised in the inquiry.
To a civilian, the phrase “electromagnetic interference” may at first sound puzzling, even though every commercial flight begins with the instruction to passengers to turn off during takeoff all computers, headsets, radios, and telephones. The power radiated by these objects is tiny. But their emissions can travel out of the cabin windows to the antennas on the outer body of the plane; therefore the FAA regulation requiring airlines to prohibit passenger use of such objects has remained firmly in place. and can have much more serious consequences for airborne planes. Because ten military planes and ships were in the vicinity of TWA 800 that night, we need to ask the airmen and sailors on the planes and ships to describe with precision the pieces of equipment that were in use.
Electromagnetic interference may come from inside the plane or from outside it. What makes the internal sources a matter for concern is that they are so close to the systems they might affect; what makes the external sources a matter for concern is that, despite their distance, their power level can be very high. Although the internal and external overlap in their effects (see the box on page 62), in general the external sources involve much higher power levels, even after traveling some distance; hence they may have more serious effects. Called HIRFs—which sometimes stands for High Intensity Radiated Fields and other times High Intensity Radio Frequency—the external signals come either from huge ground transmitters such as radio, radar, and television antennas, or airborne transmitters such as high-powered radar and radio on military planes.
The distinction between “ground” and “airborne” transmitters is sometimes instead referred to as a distinction between “fixed” and “intermittent” transmitters, words that somewhat obscure the possible effects of military equipment but have the advantage of making clear why the airborne sources of HIRFs may be harder for pilots to avoid. Because a ground transmitter is “fixed,” its location is marked on most aviation maps and can be avoided by the pilot. If a pilot on a particular flight has an electrical problem and, upon reviewing it, discovers the plane was at that moment flying in the area of a powerful ground transmitter, there is a possible concrete cause to investigate. An airborne, intermittent transmitter, in contrast, cannot be as easily avoided; nor, if an anomalous electrical situation emerges, is there usually any way to know what military craft were nearby and what particular transmitters were in use.
Beginning in 1989 …