Aircraft Fuel Tank Designers Eye Safety, Sustainability Improvements

Aircraft fuel tank component system vendors are designing a new generation of equipment in which weight savings, fewer parts, simpler design and less intensive maintenance appear to be the coming trends.

Honeywell Aerospace Technologies offers a new onboard inert gas generation system (Obiggs) as an aftermarket replacement kit for the Boeing 737 family’s current Obiggs. “For the [fuel tank] inerting systems, we continually look to evolve our Obiggs products,” says Nicholas Ollis, director of product management for air and thermal products. “More recent developments include design changes to the air separation module (ASM) for the Boeing 737.”

The ASM is a line-replaceable unit that goes into the higher assembly air separation unit (ASU), Ollis explains. The ASM is the fiber membrane cartridge within the ASU that provides the inerting function by separating bleed air into nitrogen-enriched and oxygen-enriched airstreams. Both the ASM and ASU are Obiggs components.

“The new Honeywell ASM, which like the original is fitted within the belly of the aircraft and upstream of the fuel tanks, has been designed with a high-durability membrane construction to meet the needs of the demanding 737 operating environment,” Ollis says. “Coupled with a new ozone destruct filter cartridge and preinstalled jacket and mounting points on the ASM, it provides lower operational costs due to reduced install time.”

Ollis also notes that an improved four-stage filter drop-in design is included as part of the 737 ASM retrofit kit. The filter reduces contaminants in the air that may enter the ASM, extending the unit’s performance over a longer time frame. “Typical filters provide a two-stage filtration process that removes particulates, oil and water-mist contamination, but the two additional stages add a carbon absorber along with the ozone destruct filter catalyst,” he says. “An improved filter membrane design also gives the unit enhanced robustness and longevity.”

Installation time for the ASM kit retrofit, Ollis says, is about 4 hr. For a standard ASM, warranties are generally 4-5 years, with on-condition filter changes running approximately every 12,000 hr. The new ASM is available with an extended seven-year warranty.

Ollis adds that for the Boeing 777-8 and 777-9, Honeywell has developed an ASM design with an optimized flow path and composite shell that improves performance, saves weight and eliminates the need for a thermal insulation blanket. Honeywell also is developing an upgraded oxygen sensor with built-in test equipment functionality to enable on-aircraft testing.

• See also: Fuel Tank Design Must Be Recast for Liquid Hydrogen

Rollin Brown, program chief engineer for sensing and control systems at Collins Aerospace, reports that the company is pursuing an alternative to the traditional capacitance fuel measurement systems, which consist of capacitive fuel probes, electrical wires and associated electronics. The new product, he explains, is a pressure-based system that uses optical pressure sensors, optical fiber harnesses and associated electronics.

Pressure-based fuel measurement/management systems offer substantial benefits for airframers and airlines, Brown says.

“For example, by replacing heavy fuel probes and harness wiring with optimized optical sensors and fiber interconnects, the weight can be reduced by up to 30% when compared with traditional capacitance systems,” he notes. “At the same time, since electrical wires are no longer needed, there’s nothing there which can conduct electricity or create a spark, considerably enhancing protection against lightning strikes and other electromagnetic environmental effects.”

Brown adds that traditional capacitance systems provide different readings based on permittivity—a property that measures a material’s opposition against an electric field. It is a fundamental parameter in electromagnetic and materials science, as well as density of different fuels.

“Pressure sensor systems are agnostic to fuel density since they measure fuel mass directly, meaning they function for all fuel types,” Brown says. “This is a capability that’s critically important as the industry transitions to sustainable aviation fuel and other alternative fuels to help reduce carbon emissions and their associated environmental footprint.”

Pressure-based systems, Brown asserts, offer improved operational maintenance advantages to airlines because they have far fewer unique parts than capacitance systems. “With a common electronic architecture for all platforms, it is applicable to all tank types where a capacitance system is installed,” he says.

For operators with mixed fleets, Brown says the common electronic architecture will “significantly simplify system maintainability while simultaneously supporting airframe production ramp rates.” Moreover, he says, because the pressure-based system is simpler to install, there is “a significant reduction in the installation labor costs for the OEM during aircraft build.”

Brown says Collins anticipates its new pressure-based system will achieve Technology Readiness Level 6 (TRL 6) this year. The company is targeting it for application to “next-generation clean-sheet aircraft and for production cut-in, ultimately making it available as a retrofit for existing aircraft as well,” he notes. “TRL 6 is a technology development milestone defined as a successful flight test of a fully functional prototype.”

Aloft AeroArchitects, a business and commercial aircraft modification company in Georgetown, Delaware, provides proprietary fully integrated auxiliary fuel systems, including the tanks, plumbing and avionics interface that integrate with the rest of the aircraft. To date, most of the company’s supplemental type certificates (STC) have been developed for aircraft that cater to VIPs, according to Wilhelm Wieland, sales director for engineering, certification and organization designation authorization. The company has developed auxiliary fuel tank system STCs for the Boeing 737 MAX and the Comac C909, among others.

Wieland explains that the tanks are constructed using high-temperature, adhesive-bonded aluminum honeycomb panels, eliminating the need for a tank bladder.

“The inner wall surface functions as the fuel tank, and the outer wall surface acts as a fuel-and-fume-proof containment shroud, while the slotted honeycomb core material forms a vented and drainable cavity between the fuel tank and the shroud,” he notes. “This cavity can be continuously purged with cabin air or bleed air and drained overboard.”

Asked about crash resistance, Wieland notes that the auxiliary fuel tanks are mounted on a support structure designed to distribute landing, maneuver and crash loads safely into the fuselage.

“The design features tank assemblies supported by longitudinal rails attached to the fuselage frame,” he says. “Stop blocks affixed to the rails secure the tanks in place. Additionally, straps are installed between the tanks and hard points on the cargo floor to help bear a portion of the forward gravitational acceleration load.”

Crashworthiness was of some concern with the 13,000-liter (3,400-gal.) rear center tank (RCT) installed under the cabin and aft of the landing gear on the Airbus A321XLR. Midway through the program, safety agencies issued conditions to upgrade the type’s crashworthiness in the event of a belly landing.

In a video presentation made available by Airbus, former A321XLR program head Gary O’Donnell reports that the belly fairing was extended and reinforced and a modification was made to the RCT in case of fuselage penetration during a belly landing. To mitigate the risk of a fire, the tank includes a rubber liner, which is designed to prevent any volume of leakage onto the tank floor.

“The objective is to make sure that the fuel does not come out of this tank in any sufficient volume to catch fire. If the fuel does catch fire, the skin at the bottom of the tank was fabricated of fiber metal laminate, which is very good at holding back the flame,” O’Donnell explains.