No Fire in the Hole Revisited
New developments on aircraft fuel tanks
By Fred Workley
In an article published in Aircraft Maintenance Technology in February 2000, I followed the developments of the investigation of the TWA-800 crash in July 1996, the ValuJet disaster, and various other aviation incidents and accidents. Both the Federal Aviation Administration (FAA) and the National Transportation Safety Board (NTSB) have focused on the issue of the potential for fire and explosion in aircraft fuel tanks and cargo compartments.
It's now 2003, and I am still following developments related to suppression of fire in aircraft fuel tanks. The issue is no longer just safety, but also security because of missile threats. We aren't only concerned with the center fuselage fuel tanks but also all the wing tanks.
There have been 25 fuel air explosions in aircraft since 1959. Much still needs to be learned about the explosive characteristics of Jet-A fuel, the elements required to ignite the vapors associated with fuels, the temperature and vibration characteristics associated with airplane fuel tanks, and the vapor concentrations in the tanks.
After the TWA 800 crash, the NTSB sent four recommendations for regulatory changes to the FAA, requiring the development and implementation of design or operational changes intended to eliminate, significantly reduce, or control explosive fuel-air mixtures in fuel tanks of transport category airplanes. The FAA then decided to initiate the Aviation Rulemaking Advisory Committee (ARAC) Fuel Tank Harmonization Working Group (FTHWG), to recommend new rulemaking regarding this issue. In the final 1998 ARAC report, recommendations for inerting of fuel tanks received significant attention, but technologies were not readily available at that time.
The technology now exists for full-time inerting carried on the aircraft. Also, according to Dick Hill, an explosion expert, there is an effort to investigate the hazard of copper sulfide deposits on the fuel quantity indication system (FQIS) components.
A recent article in the The New York Times touched on Boeing and its efforts to curb volatility in fuel tanks through testing on a 747.
The concern is the operation of the air-conditioners on hot days. They cool the planes' interiors while dumping their heat into the fuel in the center tank, warming it enough to turn the fuel into vapor, which can explode if a spark is present.
As mentioned in The New York Times article, the goal is to limit the oxygen level in the tank to 12 percent or less. The ideal percentage of oxygen is 9 percent. Yes, it is easier to reach a goal if you change the requirements. And, a 3 percent chance of an explosion is a lot better than 30 percent. However, why not eliminate the chance of explosion entirely if there is technology available. Furthermore, how do you know what the oxygen concentration in the ullage is unless you measure it?
There are very serious concerns regarding the currently installed technologies used for fire prevention, fire suppression, and monitoring in cargo compartments. Monitoring is of particular concern to pilots who are dissatisfied with the amount of data provided to the cockpit. Increasingly, various published data confirm that there is a potential for explosive combinations of fuel vapor and oxygen to be present in the fuel tanks of most commercial and military aircraft. Until now systems to mitigate this risk were virtually nonexistent.
An approach is now available using new methods of environmental monitoring and control. There are significant opportunities to adapt existing commercial technologies to manage the ambient environments in fuel tanks and in cargo compartments. By applying existing technologies, lead times and development expenses are reduced because fundamental research is limited to aviation issues such as temperature, pressure, and service requirements. This viable approach is inerting of the environment so that there isn't enough oxygen to sustain a fire or explosion.
The fire suppression system is based upon depriving the environment of enough oxygen to support a fire. Once ignited, a fire can only be sustained if there is both fuel and oxygen available. Fire suppression systems rely on displacement of oxygen to starve the fire. It is then logical that if the oxygen concentration can be monitored and reported to the cockpit, the pilot will have the data to make informed flight decisions. Further, if the oxygen data can be fed to the suppression system, a controlled suppression release can be used to extend the supply of the suppression gas or chemical.
It has been determined that a technology that utilizes fiberoptic sensors and a miniaturized spectrometer can be applied in aviation applications. The technology offers significant benefits in size and weight while offering the ability to measure oxygen, pressure, and temperature in an integrated system.
The system can integrate a series of fiberoptic sensors into a microprocessor-based controller that will provide data to the cockpit, fire suppression controller, and the aircraft data highway. The oxygen monitoring system will have a very low preventative maintenance requirement and will incorporate self-diagnostics and modularity to ensure ease of corrective maintenance. Accurate measurement of the oxygen levels permits the required release of an inert gas into the monitored environment. This starves the fire of oxygen, thus prohibiting further combustion.
Proposed refinements to Boeing system
A spectrometer based gas sensor is a critical component of Boeing's system using fiberoptics to measure the percentage of oxygen in the ullage above the fuel. Gas spectrometry has historically relied on large, delicate instruments used in a laboratory environment for batch sample gas and liquid analysis.
Recent developments in the science have resulted in miniaturization of the optical bench and simplification of the associated electronics. The spectrometer used in the aircraft fuel tank monitoring system relies on an optical bench that is small enough to fit in the palm of the hand. The bench is mounted directly onto a printed circuit board (PCB) where the electrical components are soldered in place and the optics are designed to allow beam focusing in three dimensions. Once adjusted, the optics become part of a very rugged, low-cost assembly that can be assembled into a cage of multiple spectrometers.
Although the approach is to use miniaturized spectroscopy, a further development was required to enable monitoring for oxygen. Scientists determined that blue light from an LED could be quenched by diffusion of oxygen into a chemical thin film coating. The resulting signal attenuation is dramatic and clearly recognizable by state-of-the-art electronics and an embedded microprocessor.
The fiberoptic oxygen sensor uses a fluorescence method to measure the absolute concentration of oxygen. Optical fiber carries excitation light produced by a Blue LED to the thin-film coating at the probe tip. Fluorescence generated at the tip is collected by the probe and carried by the optical fiber to the high-sensitivity spectrometer. When oxygen in the gas or liquid sample diffuses into the thin-film coating, it quenches the fluorescence. The degree of quenching correlates to the level of oxygen.
The fiberoptic oxygen sensor probe is a 1-mm diameter optical fiber in a stainless-steel shroud; its distal tip is polished and coated with the oxygen-sensing material.
Another option does not use diaphragms to separate the nitrogen and oxygen from the air. There are companies that have identified technologies and methods for storing bulk inventory of inert gas in the aircraft at a minimum weight increase. The technologies, originally developed for military and space applications, are available for license into the private sector. These technologies include using dewars with vacuum jackets to reduce the dewar dimensions and increase storage time of liquid nitrogen with miniaturized cryogenic cooling units.
Cargo compartment and other inaccessible areas
The same technology can be used in other locations on the aircraft. The most recent and publicized aircraft cargo compartment and other inaccessible area fires have been the ValuJet cargo hold fire and Swiss Air with a cockpit fire inside panels. Presently, when an alarm occurs, the pilot releases fire suppressant chemicals and is now not able to determine the cargo hold status - fire, fire out, or false alarm. This results in poor flight safety decisions, which range from "emergency landing" to "continue to destination."
Some industry officials advocate the introduction of Temperature Trend Indicators (TTI's), instead of the simple warning light system now used in order to provide real-time information about the temperatures in each of the compartments controlled by fire alarms. Making an emergency descent and diversion in response to an in-flight fire alarm is itself a hazardous event. This could be avoided in many cases with an informative monitoring system and a more efficient fire suppression system.
Improved fire detection includes both early fire sensing and immunity to nuisance alarms caused by environmental conditions and hardware faults. Because of the required conversion of most Class D cargo compartments to Class C, a three-fold reduction in nuisance alarm rates will be required to maintain the status quo. The logical step beyond detection is prevention. Recent developments have made full-time inerting of cargo compartments possible. The availability of new, rugged "fabrics" that are not gas permeable, that will contain the inerting agent in the cargo compartment, make this possible.
Special Federal Aviation Regulation (SFAR-88)
The need for the proposed systems arose as a result of the investigations related to the recent accidents. To address these concerns, Special Federal Aviation Regulation (SFAR-88) was put in place. It proposes a concept called "equivalent protection" to the danger posed by ignition sources, through increased inspections of wiring and other fuel system components. This follows on the stated position of the NTSB that flammable vapors in the fuel tanks should be eliminated since the search for all potential ignition sources has not proven to be 100 percent effective.
The first area of concern is ignition sources, the second is design of future transport category aircraft, and the third is heat sources adjacent to the tank. The SFAR anticipates a safety review of maintenance actions. The SFAR requires the design approval holder to develop specific maintenance and inspection instructions to maintain design features required to preclude the existence or development of an ignition source within the fuel tank. Inerting appears to be one way to meet the proposal; that is, to revise Part 25.981 to add a requirement that fuel tank installations be designed to minimize the development of flammable vapors in fuel tanks. If the oxygen level is kept low, by inerting, then combustion cannot take place. Mitigating the effects of any ignition of fuel vapors in the fuel tanks will also meet the intent of the new rules.
Keep 'em Flying.