Fuel Tank Inerting
Modeling In-Flight Fuel Tank Inerting
To develop the capability of determining the effect an inerting system would have on a specific single bay fuel tank, a scale model of an A320 CWT was built from plywood and installed in the Fire Safety Branch Environmental Chamber to determine if simple scale replicas could accurately model the inerting behavior of a full-size aircraft CWT. The scale tank was inerted with a scaled NEA flow that modeled the inerting system behavior installed on the A320 flight test aircraft in terms of NEA oxygen concentration. Additionally, an analytical calculation model was developed, based on perfect mixing, that can calculate the resulting oxygen concentration of any single bay tank given an inerting system performance over time. The results of these two modeling methods, using measured system performance from flight tests, were compared with the results of measured time varying ullage oxygen concentration in the A320 center wing tank.
Download A320 Modeling Paper
Tests were also performed with a 24% scale model of a mulitple-bay Boeing 747SP center wing tank to validate the existing assumptions for inerting of a compartmentalized tank in flight. The scale model was installed in the Hughes Technical Center Environmental Chamber and instrumented to evaluate inert gas distribution under a variety of flight and ground conditions as well as different deposit and venting scenarios. The model was equipped with thermocouples, and oxygen analysis ports that were monitored and recorded continuously by a data acquisition system during each test. The model was inerted with a scaled NEA flow rate and appropriate purity designed to simulate the performance of a commercial transport airplane OBIGGS.
Additionally, a multiple bay fuel tank inerting analytical model, based on the single bay perfect mixing model, was developed and applied to study the effect of system performance and flight cycle on the change in oxygen concentration within the compartmentalized commercial transport airplane fuel tank. This model can be used with primitive system performance models being developed to further study fuel tank inerting, including potential trade studies to determine the effect of increased system performance, changing system flow mode "tuning," and different system operational methodologies. The model uses lessons learned from previously developed analytical models focused on calculating the inert gas distribution in a compartmentalized tank during ground inerting (see AIAA paper 2002-3032 ) as well as a new distribution calculation methodology.
A paper comparing the different modeling methods with aquired flight
test data was written and presented at the AIAA 35th Annual Fluid Dynamics
Conference. This data was also compared to a computational fluid dynamics
model developed independantly.
Download 747 Modeling Paper
Onboard Inerting System Evaluation
The FAA simplified fuel tank inerting system was installed in the cargo bay of an A320 and tested in flight to validate the concept. The system was operate in a degraded mode (less ASMs) to better size the system for the A320, and used to inert the aircraft CWT during simulated typical flight operations. The results of the flight test illustrated the feasibility of the FAA inerting system and validated many existing assumptions about in-flight fuel tank inerting. ASM performance data was also acquired.
Download the Final Report (DOT/FAA/AR-03/58)
To understand the inerting systems capabilities and limitations further,
additional flight testing of the system was performed on a 747 operated
by NASA for the purposes of transporting a space shuttle orbiter from one
location to another. Flight testing was performed to examine the limitations
of a two flow system with a single deposit to inert a compartmentalized
tank as in the case of the 747 classic aircraft. System performance was
studied further and inert gas distribution was measured for several different
flight scenarios to study commercial transport fuel tank inerting in depth
and help define future system requirements and capabilities. The data will
be used to validate existing FAA fuel tank inerting models and also to develop
a rudimentary model of the OBIGGS performance. Additionally, fuel tank flammability
measurements were made to gauge the ability of the inerting system to reduce
the overall flammability exposure of a commercial transport CWT.
Download the Final Report (DOT/FAA/AR-04/41)
Commercial Transport OBIGGS Development
In an effort to gage the practicality of using HFM technology to develop an onboard inerting system for a commercial aircraft, the FAA has designed an Onboard Inert Gas Generation System to inert the CWT of a 747SP during normal operations. This system is based on the simplified concept presented by Ivor Thomas ( download discussion of concept here ) which capitalized on existing knowledge from joint committee work. The system was sized to allow for the 747 CWT to land inert (less then 12%) given the aircraft was at cruise conditions with a very low oxygen concentration. This system was designed, constructed, and installed on the 747SP Ground Test Article with the help of aviation oriented companies, and was ground tested and analyzed to evaluate such factors as performance, cost of installation and operation, weight, and reliability. Additionally, the system was installed in the cargo bay of an A320 and tested in flight to validate the simplified FAA concept and evaluate the system performance.
Download paper on OBIGGS development
Fuel Load Effects on Inert Ullage
Although some research has been done to prove the concept of GBI for commercial airplane fuel tanks, very little data exists which quantifies the effect of a fuel load on an inert ullage space. Fuel has a greater affinity for oxygen then nitrogen and can have a profound effect on an adjacent inert ullage if the fuel load is relatively large and ullage is relatively small. Work was done to verify the effects of altitude, temperature, and stimulation on gases evolving from fuel into the adjacent ullage space. A 17 ft3 fuel tank was built to allow for scale testing of an inert ullage space with different adjacent fuel loads and inerting conditions, as well as different altitudes and temperatures, in the Hughes Technical Center Environmental Chamber . The fuel tank can be inerted with nitrogen. The tank is fitted with a manifold in the bottom to allow for gases to be passed through the fuel load in question. This can be used to aireate the fuel with pumped ambient air to ensure the maximum amount of oxygen has been dissolved in the fuel. It can also be used to mix the ullage gas and the fuel to bring the system quickly to equilibrium. Oxygen concentration, ullage temperature, fuel temperature, and altitude can be continuously monitored in the tank ullage space. The results of the experiments illustrated that air escaping from fuel has a relatively small effect on the ullage oxygen concentration provided fuel loads are small and not intentionally stimulated. Results from the GBI flight test demonstration were compared with simulations in the altitude chamber to illustrate the ability to model fuel effects on an inert ullage during simulated flight events. A report detailing this work is pending review.
Download the Final Report (DOT/FAA/AR-05/25)
To validate the previous research on commercial transport airplane fuel tank inerting, the FAA performed a series of tests using a Boeing 747 SP test article. The Center wing tank was instrumented with thermocouples and sample gas probes to allow for the measurement of inerting and flammability parameters during testing. The focus of the testing was to validate the existing assumptions for inerting a large, relatively complex geometric space with a ground based NEA supply, continuing the work performed on the 737 demonstration testing. The test article had a co-located Industrial NEA generator that could inert the tank in less then 20 minutes. The CWT was modified with a simple NEA deposit manifold to allow for inerting of the tank under a host typical operating conditions. This allowed for the development and demonstration of a simple, effective and certifiable GBI NEA distribution plumbing for the center wing tank of the 747 SP. The testing also identified potential problems with NEA mixing associated with the GBI methodology and determined potential fixes for these problems. The test article has also been used to measure typical operational resource parameters (bleed air volumes, pack bay temperatures, etc.) to allow for the design and development of an on-board inert gas generation system (OBIGGS).
Download a Review of all GBI Research
Scale Model Ground-Based Fuel Tank Inerting
Tests were performed in a 24% scale model of a Boeing 747SP center wing tank to validate the existing assumptions for inerting complex geometric spaces on the ground developed from previous experiments and to facilitate the design of an efficient inerting gas delivery system for the full-scale ground-based inerting test article. The model was equipped with a variable NEA distribution system, thermocouples, and oxygen analyzers that were monitored and recorded continuously by a data acquisition system during each test. For each test, the model was inerted in different configurations with different flow rates and bay distributions. The collected data was non-dimensionalized in terms flow rate and tank size to allow for comparisons between tests. The tank was inerted in a balanced manner, placing the approximate volumetric amount in each compartment based on the volume of the compartment, and was also inerted in an uneven manner. Uneven distribution of inerting gas allows for a delay in the mixing process and if deposited in the proper location in the tank, can lead to an improvement in inerting efficiency. It appears that a single discharge located furthest from the outside vent will effectively inert a multiple bay tank with half the vent system blocked using a minimum quantity of NEA.
Download the Final Report (DOT/FAA/AR-02/51)
B-737 Ground / Flight Testing
A series of aircraft flight and ground tests were performed by the Federal Aviation Administration and the Boeing Company to evaluate the effectiveness of ground-based inerting (GBI) as a means of reducing the flammability of fuel tanks in the commercial transport fleet. Boeing made available a Boeing 737 for modification and testing. A nitrogen-enriched air (NEA) distribution manifold, designed, built, and installed by Boeing, allowed for deposit of the ground-based NEA into the center wing tank (CWT). The fuel tank was instrumented with gas sample tubing and thermocouples to allow for a measurement of fuel tank inerting and heating during the testing. The FAA developed an in-flight gas sampling system, integrated with eight oxygen analyzers, to continuously monitor the ullage oxygen concentration at eight different locations. Other data such as fuel load, air speed, altitude, and similar flight parameters were made available from the aircraft data bus. A series of ten tests were performed (five flight, five ground) under different ground and flight conditions to demonstrate the ability of GBI to reduce fuel tank flammability. It was demonstrated under the most hazardous condition-an empty center wing tank-that GBI would remain effective for a large portion of the flight, or until aircraft descent. However, it was also shown that the dual venting configuration of some Boeing airplanes would have to be modified to prevent loss of inerting at certain ground and flight cross flow conditions.
Download the Final Report (DOT/FAA/AR-01/63)
Ullage Washing Experiments
To help determine the inerting gas requirements for commercial transport fuel tanks a series of experiments were performed to determine the quantity and purity of nitrogen enriched air (NEA) required to inert a vented aircraft fuel tank. NEA generated by a hollow fiber membrane gas separation system was used to inert a laboratory fuel tank with a single vent on top designed to simulate an transport category airplane fuel tank. The tank ullage space could be heated as well as cooled and fuel could be heated in the bottom of the fuel tank to provide varying hydrocarbon concentrations within the ullage space. Several inerting runs were performed with varying NEA gas purities and flow rates. The data was nondimensionalized in terms of NEA purity, volume flow rate, and fuel tank size to provide a universal inerting curve. This curve was compared with an exact solution for fuel tank inerting as well as a model of ullage washing developed by the FAA Chief Scientist and Technical Advisor for fuel system design. There was excellent agreement between the data and both the model and exact solution, demonstrating that the laboratory fuel tank was well-mixed during the inerting process.
Download the Final Report (DOT/FAA/AR-01-6)
GBI Cost Analysis
One recommendation of the 1998 ARAC fuel tank harmonization working group was for the FAA to better estimate the cost of ground-based inerting or GBI. GBI is the process of inerting commercial transport aircraft fuel tanks during ground operations when the perceived threat of explosion is greatest, and allowing the protection to slowly disperse over time. A cost analysis of ground-based fuel tank inerting for the commercial fleet was performed by a group of industry experts led by a Federal Aviation Administration (FAA) representative. The cost analysis considered the cost of implementing and performing GBI for all US departures carrying more than 19 passengers. The cost of GBI for only departures of airplanes with heated center wing tanks (HCWTs) was also determined. Airplanes that have the air conditioning equipment, or packs, located below the center wing fuel tanks are considered to have heated center wing tanks. This analysis considered all nonrecurring and recurring costs of GBI at all major U.S. airports over 10 years, with a 3-year start-up period. The cost of modifying the aircraft to allow for GBI was not considered in this analysis. The calculated cost of implementing GBI throughout all U.S. airports was essentially consistent-actually less costly-than the estimates of the 1998 ARAC working group. For example, if GBI was restricted to aircraft with HCWTs, the 13-year cost was approximately 800-million dollars.
Download the Final Report (DOT/FAA/AR-00/19)
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