FAA rule aims to choke likelihood of fuel-tank explosions

  • 07-Aug-2008 02:27 EDT
Honeywell NGS.jpg

Honeywell’s OBIGGS technology extracts nitrogen from engine bleed air to produce a low-oxygen-content air mixture for the fuel tank.

SAE International is providing expertise to the U.S. FAA and to the aerospace industry at large regarding fuel-tank inerting, which is mandated for certain aircraft under a long-in-the-making agency rule published in the July 21 Federal Register.

In addition to having recently released AIR1903, which is a history of fuel-tank inerting systems, SAE’s AE-5 Committee is working on a new aerospace recommended practice (ARP) “to define the requirements for a fuel-tank-inerting system that could be used by the FAA as guidance on how to design and qualify [such a] system,” said Committee Chairman Sanford Fleishman.

AE-5, the lead SAE committee on fuel-tank safety, has created a working group to develop the inerting ARP, with support from SAE’s AC-9 Committee. The two committees will hold a joint meeting next spring in Dallas, according to Fleishman. FAA technical specialists are among the members of AE-5, which is focused on the development of standards in the areas of fuel, oil, and oxidizer systems, including inerting technology. The committee is composed largely of representatives from various companies’ engineering organizations. Boeing and Airbus, whose airplanes are affected by the rule, are represented.

In addition to its committee work, SAE offers several technical papers on fuel-tank inerting, including one that formed the basis of an article in the December 2002 edition of this magazine.

The rule is a response to the 1996 in-air explosion of a TWA 747 over the ocean near New York. The U.S. National Transportation Safety Board cited probable cause as ignition of a near-empty heated center wing fuel tank. Fuel vapor, not the fuel itself, was the problem. The exact ignition source for the explosion was never determined. The FAA has issued many airworthiness directives in the years since the TWA accident, focusing mainly on ignition sources such as sparks. The new rule focuses more on preventing explosive fuel vapor conditions from developing. Together, the directives and the new rule should prevent three out of the four fuel-tank aircraft disasters that otherwise would likely take place over the next 35 years, according to the FAA.

In addition to new aircraft, the rule applies to about 2700 existing Boeing and Airbus planes, for which the companies must offer retrofit solutions for each aircraft’s heated center wing tank. It is the responsibility of the airlines operating those aircraft to install the inerting systems, at their own cost.

Boeing Commercial Airplanes spokeswoman Liz Verdier said the company will offer service bulletins for retrofit per the FAA mandate. Boeing’s solution is a nitrogen-generation system (NGS) that extracts nitrogen from engine bleed air and sends it into the fuel tank to lower the concentration of oxygen.

The NGS system was developed by Honeywell, which claims its technology, called On-Board Inert Gas Generation System (OBIGGS), is the first fully FAA-certified fuel-tank inerting solution for commercial transport aircraft.

The Honeywell system is designed to provide varying amounts of nitrogen for injection into the fuel tank. The nitrogen concentration of the OBIGGS-produced air injected into the fuel tank ranges from 85 to 98%, the precise amount depending on flight conditions. The concentration of nitrogen in ambient air is 78%; for oxygen, the figure is 21%.

Boeing began deploying the Honeywell system on new 737 aircraft in June, according to Verdier. The technology will be rolled out to the 777 in the fourth quarter of this year and to the 747 and 767 in 2009 and 2010. As of the beginning of August, Boeing had delivered about 45 737s with the inerting system.

For the 787, Boeing will employ a different inerting system supplied by Hamilton Sundstrand. Verdier said the company is not revealing what type of inerting system it will use on that aircraft.

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