Airlines told to make fuel tank changes
FAA order will affect 3,800 planesThe Associated Press
Updated: 10:45 a.m. ET Feb. 17, 2004WASHINGTON - The government will order airlines to install a system to reduce the chance of fuel tank explosions like the one that downed a TWA Boeing 747 in 1996, a Federal Aviation Administration official said Tuesday.
The official, who spoke on condition of anonymity ahead of a news conference Tuesday, said existing planes will be retrofitted with the device and new planes will have them as standard equipment.
The FAA wants to speed the regulatory process, but itï¿½s unclear when the final rule will take effect because a cost-benefit analysis still has to be conducted, the official said.
The order will affect large passenger jets ï¿½ about 3,800 Boeing and Airbus aircraft operated by domestic airlines.
TWA Flight 800 crashed off the coast of Long Island, N.Y., on July 17, 1996, killing all 230 people aboard. The National Transportation Safety Board blamed the accident on an explosion, saying vapors in a partly empty fuel tank probably were ignited by a spark in the wiring.
The accident prompted FAA scientists to step up research aimed at eliminating potential ignition sources for such explosions and reducing the flammability of vapors in fuel tanks.
The device they came up with pumps nitrogen-enriched air into fuel tanks, reducing the oxygen in fuel vapors and lessening the chance of an explosion.
In 2001, a government-industry task force concluded it would be too expensive ï¿½ up to $20 billion ï¿½ to retrofit airliners with the equipment necessary to pump nonflammable nitrogen into fuel tanks. The FAA estimates the cost between $600 million and $700 million, the official said.
quote:Wow I didn't catch this at first. Pretty appalling. I don't even have the energy to get into why this statement is horrible but I think it speaks for itself. Let's just hope that nobody else's kid (there were many children aboard TWA 800) is killed while they are mulling over the cost.
From Air Safety Week:
Fuel Tank Inerting Requirement Announced
All transport category aircraft with heated center wing tanks (CWT) will be required to feature systems to eliminate any flammable vapors that may accumulate in the ullage, or the space between the top of the liquid fuel level and the top/sides of the tank. A heated CWT is one with air conditioning packs located underneath the tanks. The packs generate heat, which rises, warming the tanks and leading to the formation of flammable vapors therein. The vapor can be ignited by a tiny spark associated with fuel pumps and other electrical components located inside the tank. The Feb. 17 press conference announcement of this coming mandate was intended to put the industry on notice, and manufacturers in particular, that the Federal Aviation Administration (FAA) intends to require what could be a significant increase in fuel system safety.
A notice of proposed rulemaking (NPRM) to this effect will be issued later this year. If no action is taken, another CWT explosion of the type that destroyed the TWA B747 in 1996 and the Thai Airways International B737 in 2001 could occur, said FAA Administrator Marion Blakey. If inerting the CWT prevents just one accident, the fleetwide action will be more than justified, she declared.
The elimination of flammable vapors in fuel tanks is on the National Transportation Safety Board's (NTSB) "Most Wanted" list of aviation safety improvements. The FAA's action does not eliminate flammable vapors entirely, but does mark a significant step forward on the road to this ultimate goal. In response to the FAA's announced action, NTSB Chairman Ellen Engleman-Connors commended the FAA "for taking a two-pronged approach to the problem by recognizing that ignition source prevention alone cannot protect transport airplanes from this potential danger, [and] that the issue of fuel flammability had to be confronted."
"The safety board will analyze the FAA's action to determine the extent that it satisfies one of our 'Most Wanted' safety recommendations, and looks forward to this technology being incorporated into all commercial air carrier aircraft," Engleman-Connors said. That statement implies inerting for all fuel tanks, not just the heated center tanks. A plain reading of the NTSB recommendation certainly suggests inerting for all tanks, all the time. In other words, zero percent exposure, to include wing tanks.
The FAA appears to be focused on center tanks only and appears willing to accept an upper limit of 12 percent oxygen in the ullage, as opposed to a more conservative limit of 9-10 percent. The relaxed standard is based on a scenario involving ignition sources that originate inside the tank (e.g., overheated pumps, electrical arcing, etc.). The 12 percent oxygen standard will not necessarily protect against high-energy and/or high-temperature objects (e.g, engine blades) puncturing the tank ullage from outside. As such, the favored inerting technology provides protection for some hazards but by no means the full range of threats that could cause a tank explosion.
Two significant factors lie behind the FAA's announcement. The first is that it will make retrofit mandatory of inerting technology like the Boeing [BA] flammability reduction system (FRS), for which special conditions have been proposed regarding its performance (see ASW, Feb. 9 and Feb. 16). The retrofit program would impact some 2,700 in-service Boeing and more than 1,000 in-service Airbus jetliners with heated center tanks. The ullage in these tanks is estimated to be flammable up to 30 percent of the time on Boeing jets, and reportedly up to 15 percent of the time on Airbus jets.
The retrofit program, details of which will be contained in the NPRM, could allow operators up to seven years to complete the retrofit. With the first retrofit installations occurring as early as 2005, the last airplane could undergo CWT modification by 2012, roughly 16 years after the TWA jet blew up. The retrofit program could run to $700 million, or about $200,000 per aircraft. However, these costs are highly uncertain. The kits alone could cost this much per aircraft. Installation and maintenance could push the cost higher. However, with the CWT inerted, operators would be relieved of the existing requirement (and the associated cost) of carrying extra fuel as a short-term stratagem to keep the pumps immersed in fuel (by which means an ignition source would not trigger an explosion).
The second significant aspect of the FAA announcement informs manufacturers that new airliners now in design must have inerting systems installed for heated center tanks. An FAA official said the announcement was intended to advise manufacturers that they should begin planning to meet this requirement. As such, the announcement affects Boeing's new 7E7 jetliner now in development, which could enter service in 2008. The requirement also impacts Airbus' new double-deck super jumbo, the A380, which is expected to enter service in 2006.
Depending upon the number of air separation modules required to inert the CWT, the 7E7 system could weigh 200-300 pounds. For the larger Airbus jet, the system may weigh 500 pounds or more - although that depends upon the type of technology selected for installation.
The two manufacturers could skirt the requirement entirely by placing the air conditioning tanks elsewhere - not under the CWT. Indeed, in its investigation of the TWA disaster, the NTSBcriticized the practice of placing heat-generating equipment under the CWT, noting that on other aircraft the air conditioning packs are located elsewhere - such as well forward in the under-fuselage, as in the MD-11, for example.
If the two giant manufacturers elect to place the packs elsewhere, they will not be required to inert the CWT.
Wing tanks, whose vapors are flammable about 3 percent of the time, will not require inerting. The FAA's goal is to reduce CWT exposure to flammable vapors down to equivalency with the wing tanks. It should be noted that in neither case, CWT or wing tanks, will the exposure meet the test of an extremely remote catastrophic failure, the vaunted one in a billion standard (1 x 10-9). However, inerting will improve the safety level of Boeing center tanks by a factor of 10, and do so during the period of time from taxi to takeoff to climb when the tanks are most vulnerable to the deadly combination of explosive vapors and an errant ignition source.
The Threat From Objects Penetrating Tank Ullage
From "The Potential for Fuel Tank Fire and Hydrodynamic Ram From Uncontained Aircraft Engine Debris." (Extracts):
"Uncontained engine failures in commercial aircraft produce high-speed, hot fragments that can impact and penetrate fuel tanks."
"Fragment impact ... can produce a fuel spray in the ullage that effectively increases the vapor pressure of the fuel, making it flammable."
"Vibration can produce a spray (fine droplets or mists suspended in the ullage space) ... if the ignition source occurs within the spray region ... heat from the ignition source evaporates some of the droplets and thus makes an initially-lean region flammable."
"For a fragment that enters the ullage [from an uncontained engine failure], both the composition of the fuel/air mixture and the temperatures are sufficient to ignite a fire within the fuel tank in the presence of mechanical agitation (either by aircraft or fragment motion)."
"Our results indicate that because the velocities of the fragments are so high, a fragment still within the tank that crosses to the ullage will have a sufficient temperature to ignite the vapor."
"For typical conditions with Jet-A [fuel] and a blade fragment from the turbine section impacting the ullage, we predict ignition for a blade temperature exceeding about 1000ï¿½ K [1,340ï¿½ F], which is above the operating temperature of titanium and well below that of Inconel blades. Also, the predicted threshold temperature for ignition increases with altitude."
"[Regarding the] 1985 accident in Manchester, UK. A Boeing 737 .... suffered an uncontained failure of the left engine ... A fuel tank access panel on the lower surface of the left wing immediately outboard of the engine was punctured, producing a large hole in the base of the main fuel tank. ...the wing suffered additional damage caused by an explosive overpressure within the fuel tank when the fuel vapor in the tank was ignited by some unexplained means."
Source: Report No. DOT/FAA/AR-96/95, see www.tc.faa.gov/its/worldpac/techrpt/ar96-95.pdf
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