Aviation Safety Issues Head ‘Most Wanted’ List of Improvements

Stronger call for fuel tank inerting could pit safety board against regulators

Aviation issues head the National Transportation Safety Board‘s (NTSB) list of “Most Wanted” safety improvements. The safety board is giving greater priority to fuel tank inerting, putting added pressure on an airline industry reeling from continuing operating losses and unprepared to absorb the cost of new safety initiatives.

“This is not the time to introduce a new program that costs money,” said Jim Stewart, senior representative for air safety and regulatory affairs of the Air Line Pilots Association (ALPA). Speaking at ALPA’s annual safety forum last week, Stewart said that “safety and efficiency are not mutually exclusive.” Moreover, Marion Blakey, head of the Federal Aviation Administration (FAA), told pilots at the ALPA forum that “all signs are good” regarding the performance of a Boeing [NYSE: BA] system as a viable means of inerting the center wing tanks of its aircraft.

After a three-month review of the effectiveness of its “Most Wanted” list, the NTSB decided that continuing the list is a constructive way of focusing attention on needed safety initiatives, even if some of the items have languished on the list for many years. Frustrated with the list’s evident lack of impact, NTSB Chairman Ellen Engleman directed a special 30-day review of the efficacy of the “Most Wanted” program, holding the 2003 edition of the list in abeyance pending completion of that review (see ASW, May 12).

Priorities adjusted

With that review completed, the board issued an expanded list August 19 that draws greater attention to auto and truck safety issues, to include drunk drivers, reflecting Engleman’s concern that the board must focus more attention on where the most deaths are occurring. Nonetheless, long-standing items of concern in the aviation arena have been given added emphasis in the 2003 listing.

Fuel system safety has been moved from the number five to the number two position on the list. In addition, the safety board has sharpened its call for fuel tank inerting, pointing out this year the number of deaths from fuel tank explosions, the development of deployable inerting technology, and the continuing risk of ignition sources in fuel tanks from such things as heat sources in flawed fuel pumps. In addition, the safety board has called for the installation of inerting technology not only in new aircraft, but in the existing fleet.

The threat posed by terrorist attacks on airliners, while not mentioned by the NTSB, provides an added impetus for inerting the fuel tanks. Recall that at a recent fire protection workshop, an official from the National Aeronautics and Space Administration (NASA) said that the threat posed by man portable air defense systems (MANPADS) suggests that not only the center wing tank but the wing tanks on airliners also need to be inerted (see ASW, Aug. 4).

What’s taking shape is a huge debate about how best to respond to both safety and security challenges. A missile countermeasure system could cost $2 million to $3 million per airplane to install, and would weigh some 300 pounds (see ASW, Aug. 18). Boeing’s fuel tank inerting system weighs about 200 pounds and the kit would cost about $200,000 for a widebody.

One system defends, one system limits damage (not just against missiles, but against sparks and whatnot that can ignite vapors).

For David Schmieder, a senior research engineer at the Georgia Tech Research Institute, limiting the damage caused by a missile might be better than trying to stop it from hitting the plane altogether: “These sorts of missiles may be too small to take out a passenger jet. They home on the engine’s heat, and an airliner can lose an engine and keep flying.”

The problem comes from the secondary effects – the shrapnel from the missile’s exploding warhead could puncture fuel and hydraulic lines, or the blast could ignite flammable vapors in the wing tank, located right over the engine pylons, Schmieder explained.

Toughening components and inerting the fuel tanks are two approaches for hardening the airplane against the blast effects of a missile warhead.

In a telephone interview, Schmieder divided the problem into two parts. Missile defenses come under the heading of susceptibility reduction, or reducing the prospect of being hit by a weapon. Vulnerability reduction, he explained, “is what you do to limit the damage once susceptibility prevention has failed.”

Recall that the industry is considering as much as five feet of separation between vital electrical, hydraulic and other systems to mitigate the effects of terrorist bombs exploding inside the airplane (see ASW, July 29, 2002).

Given the missile threat, Schmieder said, “there are now reasons for inerting.”

How much is enough?

Whether the Boeing system, hailed by Blakey, will satisfy the safety board, or provide an adequate response to the missile threat, is another matter.

The Boeing system will limit the presence of flammable vapors to about 3 percent of the flight time, as measured from pushback to engine shutdown after the flight. It provides this protection only for the center wing tank. The three- percent exposure window is likely during descent, the most demanding period for an inerting system to generate sufficient nitrogen-enriched air (NEA) to keep the tank inert. It is also a period of time when pilots are least likely to be activating fuel pumps. Recall that a Boeing inerting system patented in 1983 stored extra NEA during cruise flight, when the system could produce more than needed to keep the tanks inert, and dispensed the stored gas during descent to keep the tanks completely safe. The pressurized vessel to store the NEA added weight. The system Boeing is now touting saves weight by discarding the storage bottle featured in its earlier design (see ASW, Dec. 23, 2002).

The safety board has called for the elimination, not reduction, of explosive fuel/air vapors in fuel tanks. That is understood to mean the tanks are inerted throughout the flight, from “gate to gate.” The industry hopes to reduce the presence of explosive vapors in center wing tanks from 30 percent of the flight to about 7 percent of the flight time. This approach would bring the vulnerability of center tanks into line with that of wing tanks, where the exposure to explosive vapors is about 7 percent of the time.

The safety board has questioned any such exposure periods, saying the 7 percent and 30 percent figures are industry averages, and the exposure periods could vary considerably by airplane and operating region. Besides, the board has said, wing tanks have exploded, so limiting center tanks to the same exposure period of wing tanks may not be good enough. The thrust of the safety board’s position appears to be zero exposure time, and for all fuel tanks, not just center wing tanks.

Boeing hopes to obtain supplemental type certification (STC) for its inerting system in 2004. If the FAA grants its application, the safety board will then be faced with two choices: (1) endorsing a system whose performance does not meet the desired level of protection specified in the “Most Wanted” language, or (2) accepting deployment of a limited system as the best that could be gotten under present financial conditions. >> Schmieder, e-mail [email protected] <<

Sharpening Its Case

A comparison of the 2003 to the 2002 versions of the safety board’s call for improved fuel system safety (extracts, changes in bold):

Explosive Fuel/Air Mixtures in Fuel Tanks on Transport Category Aircraft*

Center wing fuel tank explosions have resulted in 346 fatalities. Operations transport category airplanes with flammable fuel/air mixtures in fuel tanks presents an avoidable risk of explosion.

A fuel tank design and certification philosophy that relies solely on the elimination of all ignition sources, while accepting the existence of fuel tank flammability, is fundamentally flawed because experience has demonstrated that all possible ignition sources cannot be predicted and reliably eliminated.

In the fall of 2002 there were two developments from the FAA. In November, the FAA issued an emergency order requiring operators of Boeing 737, 747, and 757 aircraft to keep enough fuel in the center wing tank to keep the fuel pumps submerged. This order followed a number of findings of overheated fuel pumps that might pose an explosion hazard. Also in the fall of 2002, the FAA publicized promising research that demonstrated a fuel tank inerting system for new and existing aircraft was practical and effective.

The Board urges the FAA to require fuel tank inerting in all new and existing transport category aircraft.

*”Fuel/Air” has been added to the title to more accurately describe the mixtures being addressed in the recommendations.

Source: http://www.ntsb.gov/Recs/mostwanted/explosive_tanks.htm