Manufacturers and modifiers of about 7,000 U.S.-registered aircraft will have until December 2002 to review fuel tank systems and recommend safety enhancements, such as stepped-up inspections, and operators will have another 18 months to put new procedures into practice, a sweeping regulation to be published Monday says.

The Federal Aviation Administration (FAA) rule will require airplane makers and designers of supplemental type certificate (STC) modifications that affect fuel tank systems to figure out how to eliminate ignition sources from the systems, both through design changes and maintenance requirements.

Operators will be required to put new airworthiness instructions developed by the manufacturers and any mandated modifications - which would likely come through aircraft-specific airworthiness directives (ADs) - into their maintenance programs.

Finally, makers of yet-to-be-certified aircraft and fuel systems modifications must ensure that certain failures - such as frayed wires - don't create ignition sources, and that their designs minimize flammable vapors or include tanks that would withstand "catastrophic damage" if an explosion does happen. They also must ensure operators have adequate maintenance instructions to keep fuel systems safe.

Companies are expected to spend $165.1 million by 2013 on the rule, FAA projects. Operators will bear the bulk of the costs - about $132.5 million - for maintenance program changes, the inspections they call for to determine fuel tank conditions, record keeping, and other requirements. Type certificate holders will spend about $27.1 million, and STC holders, $5.4 million.

FAA is developing two advisory circulars (ACs), Fuel Tank Ignition Source Guidelines and Fuel Tank Flammability Minimization, to provide guidance on how to meet the new requirements, the rule says. AviationNow.com understands the ACs could be issued as early as next week.

The biggest changes between the draft rule, published in October 1999, and the final rule are the compliance times. FAA proposed giving designers 12 months to review their fuel systems and develop recommendations, and six months for operators to get the new continued airworthiness instructions into their maintenance programs. The new rule gives the designers another six months, for a total of 18 months from the rule's June 6 effective date, and doubles the compliance window for operators to three years.

The extensions narrowed the gap between what FAA proposed and what many in the industry wanted. Joint comments on the proposal filed by five large aerospace associations lobbied for a 54-month compliance time, noting that FAA took 22 months just to release the proposal after making a commitment to do so in 1997.

FAA estimates the rule will affect 6,871 U.S.-registered aircraft - including 683 turboprops - flown by 143 airlines and 76 private operators. Design analyses will be required by 46 manufacturers, airlines, and repair stations that hold 173 fuel tank system STCs. Holders of another 325 non-fuel tank STCs will have to review their certificates to see if they could affect fuel systems - such as by having wires installed near fuel system components - and make safety-enhancing recommendations if they do.

The agency believes that "between two and four" new aircraft designs will be introduced during the next decade that will have to comply with the more stringent fuel tank design standards. As many as four new STCs will per year are expected to be granted that will be subject to the new requirements, the rule projects.

The rule applies to the same general group of aircraft proposed in the draft version -- turbine-powered aircraft certified after Jan. 1, 1958, to carry 30 more seats or a payload of at least 7,500 lbs.

The regulations weren't expanded to include smaller aircraft because "it is not clear at this time that the possible benefits...would be commensurate with the costs involved," the rule says. The agency says it will do an economic analysis to see if similar requirements should be mandated for smaller planes.

Lockheed and Mitsubishi wanted FAA to exclude L-188 Electras and YS-11s, respectively, arguing that the cost of compliance for the few venerable models still in service would be impractical. FAA denied the requests, but said operators of the aircraft could petition the agency for exemptions that could include "a lesser degree of fuel tank system evaluation."

The new requirements are part of a multi-pronged FAA effort to improve non-structural system safety in aftermath of the July 1996 in-flight explosion of TWA Flight 800, a 25-year-old Boeing 747-100. Monday's rule and some 40 finalized or proposed, aircraft-specific ADs target reducing ignition sources and keeping fuel tanks cooler through design and operational changes. The agency is researching ways to remove oxygen from fuel tanks, such as by pumping nitrogen into them. Changes to wiring systems, such as providing more space between bundles, have also been mandated, and more are being considered.

Besides the TWA 747, two other accidents have been blamed on fuel tank explosions likely triggered by a combination of flammable fuel vapors and an aircraft system failure that formed an ignition source. In May 1990, a Philippine Airlines 737-300 exploded on pushback at Nimoy Aquino Int'l Airport in Manila. Two months ago, on March 3, a Thai Airways 737-400 exploded (pictured) while sitting at a gate at Don Muang International Airport, Bangkok. That incident led FAA to issue a rule on running 737 center tank pumps with little or no fuel in the tank.

In all three cases, the plane's center wing tank exploded, but investigators have been unable to confirm the ignition sources.