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Centre Fuel Tank Inerting
To date, two 737's, 737-400 HS-TDC of Thai Airways on 3 Mar 2001 and 737-300 EI-BZG operated by Philippine Airlines on 5 Nov 1990 have been destroyed on the ground due to explosions in the empty centre fuel tank. The common factor in both accidents was that the centre tank fuel pumps were running in high ambient temperatures with empty or almost empty centre fuel tanks.
Even an empty tank has some unusable fuel which in hot conditions will evaporate and create an explosive mixture with the oxygen in the air. These incidents, and 15 more on other types since 1959, caused the FAA to issue SFAR88 in June 2001 which mandates improvements to the design and maintenance of fuel tanks to reduce the chances of such explosions in the future. These improvements include the redesign of fuel pumps, FQIS, any wiring in tanks, proximity to hot air-conditioning or pneumatic systems, etc.
737s delivered since May 2004 have had centre tank fuel pumps which automatically shut off when they detect a low output pressure and there have been many other improvements to wiring and FQIS. But the biggest improvement will be centre fuel tank inerting. This is universally considered to be the safest way forward, but is very expensive and possibly impractical. The NTSB recommended many years ago to the FAA that a fuel tank inerting system be made mandatory, but the FAA have repeatedly rejected it on cost grounds.
Boeing has developed a Nitrogen Generating System (NGS) which decreases the flammability exposure of the center wing tank to a level equivalent to or less than the main wing tanks. The NGS is an onboard inert gas system that uses an air separation module (ASM) to separate oxygen and nitrogen from the air. After the two components of the air are separated, the nitrogenenriched air (NEA) is supplied to the center wing tank and the oxygenenriched air (OEA) is vented overboard. NEA is produced in sufficient quantities, during most conditions, to decrease the oxygen content to a level where the air volume (ullage) will not support combustion. The FAA Technical Center has determined that an oxygen level of 12% is sufficient to prevent ignition, this is achievable with one module on the 737 but will require up to six on the 747.
On 21 Feb 2006 the Honeywell NGS was certified by the FAA after over 1000hrs flight testing on two 737-NGs. NGS went into production at line number 2007 (Aug-2006) after an in-service evaluation program of the two aircraft. Aircraft from l/n 1935 up to the production cut over will be delivered with NGS provisions only. The NGS requires no flight or ground crew action for normal system operation and is not dispatch critical.