Fuel Tank Safety: A growing priority

Just less than 10 years ago, the tragic in-flight break up of TWA "Flight 800" set in motion the action for many amendments to regulations affecting aging aircraft, particularly fuel tanks. As a result of the Flight 800 investigation and two other fuel-system related incidents, the NTSB identified fuel tank safety as one of its top three priorities. These priorities trickle down to impact not only aircraft design and products, but also aircraft maintenance. In this article, we will look at the use of transient suppression devices (TSDs) to comply with some of the new fuel tank safety regulations and assess some of new regulations from the maintenance perspective.

In September of 2000, the NTSB issued fuel tank safety recommendations aimed at increasing protection against fuel tank flammability. The FAA initially responded to the NTSB recommendations with airworthiness directives related to fuel quantity indicating systems on the Boeing 747 and 737 classic airplanes. Thereafter, the FAA issued Special Federal Aviation Regulation No. 88 (SFAR 88 -- Apr. 19, 2001 and as amended Sept. 10, 2002) that set higher safety standards for Part 25 aircraft. These actions were aimed at eliminating sources of ignition in and near the fuel tank.

Besides setting new certification requirements for airframe manufacturers, SFAR 88 requires STC holders to perform system safety assessments if their modification in any way impacts the fuel system or comes near the critical fuel areas with electrical energy. The result of the safety assessment is an action item list. The action items result as potential discrepancies are found. The objective is to bring the fuel system and wiring near fuel areas to a higher level of safety. SFAR 88 has now been incorporated into the FAR as Amendment 102.

Safety inspection and review

From a maintenance perspective, the new fuel tank regulations require operators to inspect airplanes and review maintenance procedures and inspection instructions for deficiencies with regard to fuel tank safety. A considerable part of this effort should have been previously accomplished by the airframe manufacturer and supplied through service information to the operator. However, the manufacturer's review generally does not include the aftermarket modifications that many operators have installed. The operator's safety review includes an evaluation of configuration management of the airframes (noting any modification that may provide a source of ignition in the fuel tank area -- i.e., the impact of high voltage in-flight entertainment system wiring, new strobe lighting, fuel pump wiring, etc.) and establishing inspection procedures to verify that the safety of the installed equipment and wiring does not compromise fuel system safety. The regulations set Dec. 6, 2004 as the deadline for implementing new maintenance and inspection programs.

However, on July 30, 2004, the FAA extended the compliance deadline for operators until Dec. 6, 2008 to comply with the new maintenance procedures. The extension was to provide consistency with the FAA requirements under the "Aging Airplane Safety Rule" of Dec. 6, 2002 (requiring inspections and airframe reviews of aircraft in service for 14 years or more).

So where are we?

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.

Preventive maintenance

Preventive maintenance practices and detailed inspections may be the most effective method to reducing ignition sources around fuel systems. The FAA recently highlighted the need for improved maintenance practices when performing modifications or maintenance. The regulations now require the following notice provision in manuals:

It is imperative to prevent or significantly reduce potential contamination or debris from coming into contact with the wiring and components during all maintenance, repairs, and modifications. This begins with always being aware of potential wiring contamination, and remembering to install appropriate protection (e.g. plastic sheeting), as necessary, to cover avionics/electrical wiring and components. Furthermore, a "clean-as-you-go" attitude helps maintain the integrity of the installation. In other words, care should be taken to protect wire bundles and connectors during work, and to ensure that all shavings, debris, and contamination are cleaned up after work is completed.

While this may be common sense, my colleagues and I can provide you with many stories that have been reported of items and equipment being left on, inside, and around the airplane.

When performing maintenance on airplanes with TSDs installed, the typical avionics rules apply. Most importantly, be careful to protect the connector pins. If the pins get bent or do not make contact, the equipment that the TSD is protecting may not operate properly. While the equipment may not work quite right, the nice feature about the TSD is that it always works. As a passive, fail-safe device, the TSD will always protect the wiring and the fuel tank. The failure mode is an open circuit. Therefore, the protection is always effective.

Finally, anytime there is an inspection, check the wiring harnesses and connectors around the fuel tank for broken wires or compromised wiring insulation. Look for leaking or seeping fuel. Connector seals can often be a source for fuel leaks. Maintenance is the first line of preventive measures -- how often have you ever seen a captain or first officer look into the wheel well at a fuel tank wall?

We expect that the aging aircraft rules will someday require added protection to aircraft wiring. The common circuit breaker is no longer as trustworthy as we might expect. While maintenance practices will supplement the enhanced safety level brought on with new airframe devices, maintenance will be the key element to keeping these airplanes safe, particularly the classic fleet.

Additional ReSources

Butler National Corp.
Olathe, KS
(913) 780-9595