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RF Energy as a Source of Ignition


This paper addresses a previously uninvestigated phenomenon that is capable of causing an airliner with a volatile chamber full of fuel vapor to explode, leaving no evidence of the cause. It describes a historical tragedy that was caused by an electromagnetic wave induced voltage in a jet’s ordinance cabling. The technology of High Power Microwave (HPM) is then explored in terms of theory and possible links to the explosion of TWA 800.

It is not the author’s contention that TWA 800’s crash was caused by an HPM attack. However, as the investigation lengthens without any identifiable cause for the center fuel tank ignition, less common sources of ignition must be considered. The author’s contention is that this expanded population of potential causes must include HPM until electromagnetically induced arcing can be eliminated as the source.

Logically, the cause for any incident will focus first on the most likely of origins. This is the level at which most accidents will be solved. When, due to lack of evidence or conflicting evidence, the population of the highest probability fails to produce a supportable accident scenario, the next level of analysis, into less common and more theoretical areas of investigation must be explored.

In the case of TWA 800, even this level of analysis has failed to produce a supportable explanation. Although the explosion of the plane’s center fuel tank is inarguable, there has been an absence of evidence showing the source of ignition. 95% of the plane has been recovered from the ocean floor, with some of it being salvaged by actually scraping the ocean floor. Presumably, if a missile or other projectile had caused the explosion, pieces of the weapon would have been found in the wreckage. No missile remnants have been found, nor unexplainable explosives, nor any other source of explosion except the vapor in the fuel tank.

This justifies mining to the next level of analysis, into phenomena that may not have been experienced in the commercial aviation community before and that may seem unlikely but are nevertheless physically possible. HPM is such a phenomenon. This paper is meant to contribute to the technical investigation into the cause of TWA 800 and is meant to raise another possibility for consideration, not a definite answer.

The Forrestal

On July 29, 1967 the USS Forrestal (CVA 59) was cruising in the Gulf of Tonkin off the coast of North Vietnam when it experienced the worst carrier fire since World War II. The Forrestal had several A-4 Skyhawk jets on deck, fully fueled and armed with a variety of air-to-air and air-to-ground ordinance. A Zuni rocket was accidentally launched. The missile hit a parked A-4, igniting its drop tank. The resulting fire burned for 13 hours, claimed 134 servicemen, caused $72 million in damage and required 7 months to repair the ship.

Subsequent investigation showed that the missile launch was caused by perturbation of electronic systems being subjected to a powerful electromagnetic field. One of the missile cables apparently had an improperly mounted shielded connector. When a shipboard radar illuminated it, RF voltages were developed in the degraded connector, resulting in a Zuni rocket being fired across the deck.

Of course, increasing the range between the source of the energy and the potential victim can easily reduce the risk of similar electronic upsets. This is a primary reason for locating airport radar antennas in relatively remote areas of airfields. By calculating the maximum effective radiated power of an emitter, an appropriate minimum safe range can be defined. The critical piece of information in this derivation is maximum power from the antenna.

As high power microwave (HPM) technology advances, this RF susceptibility can be exploited by subjecting victim electronic systems to dangerous electromagnetic fields in areas assumed to be safe from such signals. An HPM weapon uses a beam of high power radio frequency (RF) pulses, similar to a radar, to irradiate a target, with the goal of coupling sufficient energy into the victim’s electronics, causing temporary upset or permanent damage.

Weapon Characteristics

HPM weapons have a far different array of limitations and attributes than a more typical arsenal. These include a much higher probability of a hit compared with projectiles, since the RF beam can illuminate the whole target and hence requires much less pointing and tracking accuracy.

Since the HPM weapon’s "ammunition" is electromagnetic energy, it does not require reloading, has a much larger "magazine" than a conventional weapon, and does not require acquisition of additional ammunition between uses. The effectiveness and speed of light velocity of the "ammunition" is unaffected by adverse weather conditions.

The primary limitation of an HPM weapon is the uncertainty in the probability of kill of a given hit. The critical parameters in this probability are the range from the weapon to target, amplitude of the emission, and vulnerability of the target at the frequency and power level of the emission.

As the Forrestal fire shows, the target vulnerability can be inadvertently increased when seemingly minor components are degraded in manufacturing or due to use. The Forrestal fire also shows that the increased susceptibility may not be known until after an investigation of a tragedy.

TWA 800

On July 17, 1996 TWA 800 exploded shortly after takeoff from New York. After months of exhaustive investigation, the cause of the crash has not been revealed. Investigators are confident that a critical question is the source of the ignition of the near empty fuel tank. One of the mysteries, if the fuel tank explosion was the initiating event, is how sufficient energy could have been introduced to the fuel-vapor mixture to cause ignition.

In an investigation that is several months old, no typical source of spark or ignition has been identified, even though over 95% of the fuel tank has been recovered and examined. Through the process of elimination, the more likely ignition sources have been ruled out. Yet there is no argument that the fuel tank exploded. Assuming that the experts have adequately analyzed the debris and that they have not withheld the results of their analysis, less common sources of ignition must be considered. Potential sources must be considered not just in terms of the probability that they caused the explosion, but also in terms of possibility.

Another clue of TWA 800 involves the extremely reliable reports of streaks of light in the sky on the evening of the explosion. Light, by definition, is an indicator of energy. Early speculation pointed towards the source of light, and therefore energy, being the engine of a missile. However, investigators claim to have found no parts of any such missile nor any other unexplained foreign object.

Public reports indicate investigators have considered the light streaks to be sufficiently important to interview witnesses multiple times in this regard. If the wreckage yields no clues to more typical sources of light, such as a missile, less probable but physically possible energy sources must be considered. This includes the class of devices called "Directed Energy Weapons," of which HPM is one member.

RF Induced Ignition

The ability of electromagnetic energy to create a spark is inarguable. Inadvertently leaving a metal "twist tie" on a package placed in a common microwave oven can present impressive evidence of this phenomenon. An even more impressive proof can be achieved by filling the oven with an explosive vapor before placing metal in the oven. Whether there will be resulting evidence of a microwave-induced spark causing the resulting explosion is arguable.

Other experiments common to scientists and technicians further prove the ability of microwave energy propagating through space to transform to other forms of energy. Tossing steel wool into the main lobe of a search radar’s antenna can produce a spectacular explosion. Fluorescent light bulbs can be lit without any wired connection at a considerable distance from a radar emitter.

The risk to commercial aircraft posed by subversive HPM attack has been documented in both open literature and classified documents. This risk has primarily focused on perturbation of electronic systems causing inexplicable loss of control of the aircraft. In such a scenario, the result is likely to be an unsurvivable crash with no apparent cause.

A previously unexplored risk posed by HPM weapons, though, is the remote ignition of an explosion. By using intrinsic attributes of the target (i.e., fuel vapors) as part of the weapon, attribution of the attack is more difficult and execution of the attack is also easier. The trade off is, of course, a reduced probability of kill compared to a more typical bomb.

A unique characteristic of an HPM attack is its lack of residual evidence. This can be particularly attractive to subversive organizations that are based in religious extremism and/or seek to avoid prosecution for the attack. Recent trends, including the bombing of Pan Am 103, are for terrorists to seek anonymity, and HPM provides a unique opportunity to satisfy that more than traditional explosives.

Military and Civilian Risks

Since HPM weaponry has grown out of the military community, commercial aviation security officials may be tempted to wait for the means of addressing the threat to come from the Department of Defense. However, the greatest risk scenario facing the commercial pilot may never be seen by a military flyer.

In most modes of transportation, including flight, the most dangerous parts of a given trip are the initiation and conclusion. This increased risk is evident in the "sterile cockpit" rule below 10,000 feet, "no wake" zones near boat ramps on lakes, the use of "harbor captains" in ports, etc. All American astronauts who have died during a mission did so in the initial phase, and the conclusion of the mission posed the greatest risk to Apollo 13. Ironically, the initiation and conclusion of a military attack flight are among the safest phases, since they occur in friendly airspace.

The axiom "altitude is a pilot’s best friend" is certainly true when considering HPM vulnerability within a flight system. Assuming the weapon is ground based, and assuming that the military has controlled a sufficient perimeter around the approach paths, a military pilot is unlikely to get close enough to an HPM weapon to cause concern. Furthermore, the area that needs to be secured is more compact since military pilots are trained to take off and land on fields as small as aircraft carriers.

Since customer comfort is important, airliner captains use more gradual flight slopes. The ground is assumed to be free of threats to the plane’s safe flight, although it is not controlled. Therefore, while it would be very difficult to effectively position an HPM weapon against a fighter, such a deployment against an airliner would be uncontested.

Given the uncontested nature of deployment of an HPM weapon against airliners, the risk must then be analyzed in terms of probability of kill. This is highly statistical, and dependent on many factors whose quantity is unknown. For guidance, airliner susceptibility to legitimate sources of electromagnetic radiation can be studied.


Special Committee 177 (SC-177) of RTCA Inc. has investigated the effect of emissions from passenger’s hand carried Portable Electronic Devices (PEDs) on flight systems. PEDs include laptop computers, electronic games, etc.

RTCA, formerly called the Radio Technical Commission for Aeronautics, recommends standards and offers guidance to the aviation industry. Currently, most airlines in the United States and elsewhere voluntarily follow an RTCA recommendation issued on Sept. 16, 1988, that prohibits the use of PEDs during takeoff and landing. That recommendation was issued mostly to lessen any possibility of interference with aircraft avionics, but also to reduce the chance of passengers being injured by PEDs that might bounce around on a flight and to prevent passengers from being distracted from safety announcements. A new study that RTCA has been working on marks the organization's third visit to the issue of interference from portable electronics, (The first time was in the 60’s.) the SC- 177 committee has made recommendations concerning the use of PEDs in a draft report now wending its way through a lengthy approval process.

Clearly, the RTCA’s analysis is focused on the unintentional electromagnetic radiation from electronic systems that have been designed to minimize such signals. Aircraft vulnerability to these minuscule signals at relatively short range (inside the cabin) can provide insight to susceptibility to the intentional radiation at a greater distance.

The Federal Aviation Administration requires certified aircraft to be fail-safe to 10-9. There is a sliding scale that allows greater probability of failure for less catastrophic perturbations. Nuisance failures need only be safe to 10-2.

The RTCA states the probability of a PED to any one avionics system as 5 x 10-5. A direct correlation between this susceptibility and the susceptibility to a HPM weapon cannot be made from the existing data. Nevertheless, this independent analysis clearly shows a susceptibility to electromagnetic upset. This implies that the question of susceptibility is not IF airliners are vulnerable to HPM attack but HOW vulnerable they are. Given the potential catastrophic results of a successful attack, the most severe failure rate of 10-9 is appropriate. Previously described demonstrations using microwave ovens imply the statistical probability of a microwave signal generating an electrical spark is greater than this FAA imposed standard.

Terrorist Electromagnetic Energy Experimentation

Even if the potential to cause perturbation of electronics or explosion of vapors is possible by using an HPM weapon, this vulnerability is of limited interest until potential subversives acquire the interest and knowledge of the technology. Clearly, electromagnetic theory is non-trivial and the technical complexities offer significant obstacles to exploitation by subversives.

According to the Director of the Russian Federation Ministry of Defense Central Institute of Physics and Technology, Major General Vladamir Loborev, subversives have already exploited HPM technology. He claims that in 1995 Chechnyan rebels used an HPM system to defeat electronic security systems.

D.W. Bracket, in his book, Holy Terror examines the cult Aum Shinri Kyo, who used gas to attack Tokyo subways March 20, 1995. While the use of sarin gas has received most of the publicity surrounding the cult, Bracket claims that they were also trained in use of electromagnetic energy. Bracket claims that those who tried to leave the Aum faced execution from microwave radiation.

Inadvertent HPM Exposure

As the Forrestal incident shows, a highly complex sequence of coincidences can combine to unintentionally expose systems to HPM. Some may even argue that the probability of such a combination of normally incidental mistakes, failures and/or unconsidered circumstances is greater than intentional attack.

The Forrestal fire started from close range exposure of a system to a single radar. In other environments, however, a system could be inadvertently exposed to numerous systems in a short period of time.

Technicians and engineers maintaining military surveillance and fire control radar typically use targets of opportunities to evaluate the performance of their systems. This includes analysis of fixed target returns, such as mountains and other known ground clutter, and tracking air traffic in the area. In these tests, it is common for fire control radar to "lock onto" airborne targets of opportunity to prove the system's ability to track targets and measure their position.

This practice is generally safe, since there is never an intent of launching weapons. Just as in the Forrestal tragedy, however, a combination of events could become catastrophic. Since fire control radar are typically continuous wave and trained on a target, instead of sweeping an area with a pulsed waveform, the amount of average power illuminating a target is much greater than in acquisition radar. Under the proper conditions, the result can be similar to the demonstration of placing a twist tie in a microwave oven.

Such conditions include those experienced by TWA 800. They include an aircraft at low altitude and multiple fire control radars in the immediate vicinity. Since the 747 was taking off near an area of military exercises, it is feasible that it could have been a "target of opportunity" for one or more tracking radars. The low altitude and close down range proximity would have combined to allow a much stronger field strength than normal.

Another example of a RF risk scenario is an airliner entering airspace that is near an area of military exercises. Generally, operations that are sufficiently complex to include the use of tracking radar and other sources of extraordinarily intense RF signals occur isolated from populated areas. This often restricts such maneuvers to desert or ocean environments.

If an airliner flies too close to powerful RF emitters, it can be subjected to dangerously intense electromagnetic fields. Since the existence of military exercises is unchangeable, the risk they pose to airliners requires a statistical analysis defining a "safe" exposure level.

The FAA cites 200 volts per meter as High Intensity Radio Frequency (HIRF). Levels below that are assumed to be safe. Much more powerful fields may be encountered, however, near remote military activity with multiple emitters or if locked onto with a single fire control radar.

According to USA Today, the electrical components inside 747 center fuel tanks are limited by Boeing design to less than .02 millijoules of energy.

For purposes of risk analysis, a single RF source emitting pulses 10 microseconds (m S) wide is assumed. This is a reasonable pulse width for long-range search radar, which is a likely source of unintentional HPM illumination.

A 10-microsecond pulse creating an electric field of 200 V/m will result in more than one millijoule of energy. The degree of risk this energy poses depends on both design and circumstance. Important factors such as how much margin for safety is built into Boeing’s specification of .02 millijoules and foreseeable coupling paths of the energy can be analyzed. However it is more difficult to predict the effect of faulty components (such as in the Forrestal incident) or unanticipated coupling paths.

At the very least, it is safe to say that an airliner with a fuel tank full of explosive vapors is at greater risk of explosion when exposed to the electromagnetic fields associated with some types of military training exercises.

Government Consideration

Simple description of a problem provides limited societal value unless accompanied by ideas for addressing the problem. Since any solution must be achievable within the constraints of the cognizant organization, it is useful to explore the current security development goals, guidelines and assets of the Federal Aviation Administration.

The Department of Defense has proven the feasibility of uncooperative HPM detection. Sensors have been fielded that vary in size, cost and capability over a very wide range. The Air Force’s Phillips Laboratory has also funded research into modification of these sensors into systems that can protect commercial aircraft for a cost of pennies per passenger. This research takes advantage of tens of millions of dollars the DoD has invested in defining the capability of HPM weapons, as well as detection characteristics and methods.

Following the TWA 800 explosion, Congress and the President provided the FAA an increased research and development budget for security system enhancements. In spite of this increased funding, the ability to leverage DoD R&D, the risk posed by HPM weapons, the increased use of microwave energy by terrorist organizations and the FAA’s public interest of fielding improved detection technology, the FAA has shown little interest in addressing this risk.

The present status is comparable to the requirement of smoke detectors in cargo holds. Before May 11, 1995 the technology to prevent crashes like that of ValuJet 592 existed, was inexpensive and the need had been defined. The FAA rejected the requirement of smoke detectors, however, until an airliner full of Americans had died.

Until the actual cause of the explosion aboard TWA 800 is identified, HPM cannot be eliminated as one of the possibilities. The FAA has shown a tendency to ignore known safety issues until they can be identified as the cause of a crash. Ironically, since HPM effects often leave no residual evidence, there may never be a crash that can directly be attributed to microwave attack irrespective of how many planes crash as the result of such attacks.

Aside from TWA 800, there have been two other unsurvivable and unsolved crashes that fit a scenario published in a classified paper describing the risk of airliners to HPM terrorism. The scenarios were published in 1990, and survived expert peer review without dissension. The crashes occurred in 1991 and 1994, and both coincided with periods of greatly increased Iraqi hostility towards American interests.

Whether or not airliners have already been attacked with HPM weapons is arguable. Their susceptibility to such attack, however, has not been denied by any recognized expert in the field of High Power Microwave technology.

The technology to protect travelers and apprehend subversives attempting to exploit the vulnerability exists, and can be implemented at a low cost. The greatest obstacle to fielding this protection is the organization charged with ensuring the flying public’s safety.

For additional information contact:

A.E. Pevler
Texas Engineering Solutions
P.O. Box 1342
Coppell, TX  75019





"A New Threat to Aircraft Survivability: Radio Frequency Directed Energy Weapons (RF DEW)," John T. Tatum, U.S. Army Research Laboratory; Fall 1995 Aircraft Survivability Newsletter

"Air Force High Power Microwave Technology Program," Dr. William L. Baker, Air Force Phillips Laboratory; Fall 1995 Aircraft Survivability Newsletter

"RF Weapons in the Hands of Terrorists – Threats and Countermeasures," A.E. Pevler; 5th National Conference on High Power Microwave Technology; June 1990 (SECRET)

"RF Terrorism – A Menace of the 90’s," A.E. Pevler; limited distribution white paper; January, 1992

High Power Microwave Systems and Effects; Clayborne D. Taylor, Ph.D., Mississippi State College of Engineering and D.V. Giri, Ph.D., Pro-Tech;

"Effects from High Power Microwave Illumination," C.D. Taylor and N.H. Younan, Microwave Journal June 1992

"TWA investigators track traces of soot, sparks," USA TODAY, November 13, 1996

"NTSB proposal calls for measures to guard against explosions," Christine Negroni ,Cable News Network, November 15, 1996

Holy Terror, D.W. Bracket

"Threat Assessment," John O’Neill, Federal Bureau of Investigation Office of Counterterrorism, 2nd Explosives Detection Technology Symposium and Aviation Security Technology Conference, November 1996.

"High-Power Microwaves: An Overview with a Focus on Cerenkov Devices," J.A. Swengle, Ph.D., Lawrence Livermore National Laboratory; AMEREM ’96 International Conference on "The World of Electromagnetics."

"HPM Terrorism," Major General Vladimir M. Loborev, Russian Federation Ministry of Defense Central Institute of Physics and Technology; AMEREM ’96 International Conference on "The World of Electromagnetics."

Collision Course: The Truth About Airline Safety; Ralph Nader and Wesley J. Smith

"Do Portable Electronics Endanger Flights? The Evidence Mounts"; Tekla S. Perry and Linda Geppert, Institute of Electrical and Electronics Engineers; Spectrum, September, 1996.