For new airplanes and new certifications (i.e., STCs) related to fuel systems, the new safety regulations concerning design are in place. Additionally, the FAA has required operators to incorporate new design features provided by the airframe manufacturers. These new features generally resulted from the manufacturers' system safety analyses. In cooperation with the FAA, airframe manufacturers implemented the changes through airworthiness directives (ADs). These ADs range from connector replacements and wiring harness changes to regular inspections of fuel system components. From the operator level, more detailed fuel-related maintenance and inspection procedures will eventually be mandated. The paperwork effort will undoubtedly increase.
Sooner than later, operators will also see additional fuel system and wiring protection equipment. The buzzwords are "energy suppression", "GFI," and "inerting" -- applications of new devices to reduce the risk of fires and fuel tank ignition.
Transient suppression devices
Suppression is most commonly accomplished by a transient suppression device (TSD) or sometimes called a TSU (U stands for unit). TSDs are simply high-tech electrical surge protectors -- minimizing the energy that may enter into a fuel tank. Butler National manufactures a TSD that is entirely passive -- no electrical energy is required for operation of the TSD.
FQIS (fuel quantity indicating systems) have been noted as one of the most suspect systems when evaluating means for high levels of electrical energy to enter the fuel tank. FQIS systems use electrical energy to measure change in the fuel volume. The FQIS probes, even in the late-model digital systems, are really electrical capacitors. The FQIS computer sends an electrical signal to probes and then measures the capacitance values across the probes. Electrical energy goes directly into the tanks.
Operators install the TSD to protect the FQIS wires entering the airframe fuel tanks. Other than FQIS wiring, there are very few other wires that directly enter and transmit electrical energy into the fuel in the tank. TSDs generally mount on the fuel tank wall between the existing tank-wall connector and the FQIS plug. TSDs are one method of compliance to the FQIS airworthiness directives for the 737 and 747 FQIS systems that resulted from the TWA Flight 800 investigation. The TSDs inhibit the ability for excess energy to enter the fuel tank along the FQIS wiring. Many airlines around the world have installed TSDs.
TSDs offer operators a cost-effective method for making fuel tanks safer. A TSD has a potential operator price of from $18,000 to $100,000 depending upon the number of tanks (and tank wiring). An advantage of the Butler National TSD is the minimal installation downtime. Customers have reported system installation times of two hours on the Boeing 737. The TSD is certainly a more cost-effective and easy-to-install solution compared to the inerting systems presently being investigated.
GFI and inerting devices
A second device being promoted in the aging aircraft marketplace is the GFI or ground fault interrupter. The GFI generally operates like the GFI circuit you might find in the kitchen or bathroom of a new home. The GFI interrupts the current flow in a circuit when excessive transient signals are detected. GFI devices range from complex monitoring circuits (monitoring both voltage and current) to basic current sensing circuits. With the focus on preventing electrical fires, GFI devices appear to be a viable option for protecting wiring in both new and aging aircraft.
The third device that may soon have a presence in the commercial aviation market is a fuel tank inerting system. The inerting system is designed to remove or dilute the oxygen in a fuel tank with nitrogen to a concentration that will prevent ignition. In the past, inerting systems have been heavy and expensive -- two elements counter to commercial aviation. Over the past several years, the FAA has performed extensive research into inerting designs. Considerable progress has been made toward the development of a practical system. However, the proposed price for inerting remains significant. In light of the economic challenges facing all of the airlines today, mandating aftermarket installations of inerting systems would cripple airlines. Furthermore, fuel fumes and ignition sources may be found outside the tank (i.e., through leaky connectors) -- areas where inerting does not help.
The NTSB concluded that flammable vapors in the center fuel tank under the passenger deck exploded, tearing Flight 800 apart.
No Fire in the Hole - Revisited New regulations affecting aircraft fuel tanks By Fred Workley July 2001 Editor’s note: Workley’s original "No Fire in the Hole" article...