Flight Tests of Fuel Tank Safety System a Mixed Success

Air Safety Week, July 19, 2004

There is nothing like a half-dozen operational flight tests to reveal system problems or shortcomings. Such is exactly the case with recent trials of a fuel tank inerting system that Federal Aviation Administration (FAA) officials have hailed as "closing the door" on the risk of fuel tank explosions.

Flight tests of the system on an Airbus A320 reveal that it used significant quantities of engine bleed air in cruise flight. The heavy highaltitude cruise consumption of compressed air "bled off" from the engines to supply the fuel tank inerting system is not good at all for long-range fuel consumption.

The use of inlet air vents and electric air compressors might provide a more viable solution.

Details of the flight tests are in a June 2004 report issued by the FAA's Technical Center in Atlantic City, N.J. The system tested featured the hollowfiber membrane air separation technology touted by the FAA, and for which Boeing [BA] is seeking "special conditions" to install in its aircraft (see ASW, Dec. 23, 2002, and Feb. 9, 2004).

In this system, pressurized bleed air from the engines enters a heat exchanger, where it is cooled before entering canister-shaped air separation modules (ASM) containing the hollow fiber membranes. The output is nitrogen enriched air (NEA), which is piped to the center wing tank (CWT), where it dilutes the

oxygen concentration of the vapors to the point where they will not explode in the presence of an ignition source. Oxygen enriched air (OEA) is a byproduct of the gas separation process. The OEA is pumped overboard.

Up to six modules may be needed to inert the vapors in the cavernous (more than 1,800 ft.3) CWT of a B747. The CWT on the A320 is considerably smaller (289 ft.3), so a three-module system was flight tested. Neither system provides NEA to the wing tanks, the safety of which is considered adequate because they do not feature air conditioning packs immediately below that generate heat (which rises, warming the ullage).

The inerting system installed in the A320 was tested in two modes, the low-flow and the high-flow rates. In the low-flow mode, the system achieved a 15 percent oxygen concentration in the tank during the descent phase of flight - which is considered the most demanding period for an inerting system. The aim was 12 percent by volume.

The amount of engine bleed air sucked away from the engines is a critical factor in fuel efficiency (it is one reason why pilots may turn off one air conditioning pack in flight or put the air conditioning to low- flow). The FAA system, as tested by Airbus, would seem to consume (as in, waste) a considerable amount of bleed air:

"At cruise, the system is generating 4 SCFM [standard cubic feet per minute] of less than 1% NEA but was consuming almost 50 SCFM of bleed air. The increased performance of the ASM at altitude allowed excellent production of NEA, especially when considering the effect of NEA on inerting the tank is fivefold greater at 39,000 feet than at sea level. However, the system discarded more than three times the amount of bleed air."

The supply source may have to be reconsidered. A low-drag pair of inlet ducts (one on each side of the fuselage) with two in-line electrically driven air compressors might be more efficient. Indeed, the ducts could be configured with movable inlet doors to optimize (i.e., minimize) their drag profile as height and speed vary.

The electric power for the air pumps would take advantage of the fact that normally only 30 to 40 percent of each generator's electrical output capacity is being utilized in normal flight. Excessive demands on engine bleed air would have a much greater effect on engine efficiency and fuel efficiency than a slight hike in generator load.

The engine bleed air also is very hot, and needs to be cooled via the heat exchanger in the inerting system before it enters the ASMs:

"The heat exchanger accepts the hot bleed air and used a 4-inch diameter cooling bypass to cool the system flow to a temperature of 180[degrees] F or - 10[degrees] F."

By comparison, air entering a vent during cruise will be very cold and will further compress well and efficiently. This arrangement would eliminate the weight and complexity of the plumbing required to collect the bleed air from as many as four engines into a manifold, not to mention the elimination of the heat-exchanger and ducting mechanisms (to include a 187-watt clamp heater to make sure the air entering the ASMs does not drop below a target temperature). On the ground, the electric pumps would induce air through the vents and would be doing the lion's share of the compressing.

Electrically compressed ambient air may be better for another aspect mentioned in the report:

"An ASM can be sensitive to contaminants in the air source. These contaminants can decrease the effectiveness or even damage the fibers in the ASM ... [The system] has a filter to remove particulate, but how effective this type of filter is at protecting the ASM is unknown. Most hollow fiber is intolerant of hydrocarbon vapor in general and the amount of hydrocarbon vapor in the bleed air, and the filter's ability to remove it, is in question."