FAA finally takes action on fuel inerting

 - January 27, 2011, 10:12 AM
The 14th anniversary of TWA 800 came and went this past July 17. Except for family members of the victims, few remember anniversaries of tragedies after the 10th year has passed. The media and general public might lose interest, but for those working to prevent a future disaster, the memory of the Boeing 747 midair explosion remains vivid and concerns about preventing a similar future disaster remain strong. While developing and mandating a fix has been agonizingly slow, here’s hoping that implementing it across aircraft types will not be.

To recap for those who may not remember, TWA 800 took off from JFK International Airport en route to Paris on a hot summer night. Not long after departure, the aircraft exploded off the south shore of Long Island, N.Y., killing all 230 people on board. Many initially believed the cause of the accident to be an explosive device of some kind, but after more than a year of intensive investigation, including the painstaking reconstruction of much of the aircraft fuselage, the NTSB determined the probable cause of the accident to be an explosion in the center wing fuel tank.

The Safety Board ultimately determined that the likely sequence that led to the explosion was as follows: the aircraft had sat for hours under the hot July sun on the ramp at JFK; in addition, the auxiliary power unit had been running for much of that time, providing air conditioning inside the fuselage but creating high temperatures beneath the center wing fuel tank. The heat of the day combined with the heat from the air-conditioning system likely caused higher-than-normal concentrations of fuel vapors to build in the tank. After takeoff, an electrical charge outside the tank found its way into the tank, creating a spark that ignited the fuel vapors in the tank. A catastrophic explosion followed, with tragic results.

Accidents are usually caused by a chain of events and the links in the chain are often not unforeseeable, especially in hindsight. For example, while aircraft are manufactured to prevent high-voltage electricity from entering the fuel tank, normal deterioration of the wiring bundles in an aircraft more than 25 years old caused the insulation to flake off. The bare wires crossed in the bundle, putting high voltage through the fuel quantity indication system, and ultimately into the fuel tank itself. Many aircraft flying then and now are more than 25 years old with wiring vulnerable to similar, age-related problems. An FAA study group convened after the accident found as much.

Preventing Future Explosions

For those who doubt that a center fuel tank could explode again, at least two such explosions have occurred both before and after TWA 800. Two that I am familiar with involved Boeing aircraft and both, fortunately, occurred on the ground. The first occurred before TWA 800, in May 1990; in that accident a Boeing 737 with 120 passengers on board had just pushed back from the gate at Manila Ninoy Aquino Airport in the Philippines when an explosion tore through the center fuel tank. Although eight people died, it is certain that all aboard would have been killed if the explosion had not occurred on the ground. The aircraft was destroyed.

The second instance occurred in May 2006 and involved a Boeing 727 freighter on the ground in Bangalore, India. The left wing fuel tank exploded while the aircraft was on the ground. None of the crew was injured but the aircraft was destroyed. (While all three aircraft accidents I am familiar with involved Boeing aircraft, the same potential for explosion exists in other aircraft, including Airbus aircraft.)

So what has been done to prevent future explosions and why is it taking so nerve-wrackingly long? Inspections across all aircraft types were performed. But inspections go only so far when there are hundreds of miles of wiring in an aircraft and the inspections depend on the visual acuity and thoroughness of the human inspector. So long as vapors can accumulate inside a fuel tank, concerns remain that a chain of events could cause a spark to ignite a fuel tank again.

We have long known that the only surefire way to prevent a fire in the fuel tank is to prevent the deadly vapors from forming in the first place by replacing the oxygen in the air of the tank with an inert gas that does not support combustion. While the answer was there, no practical system existed to do this until the FAA itself finally developed a lightweight system to provide nitrogen to inert the fuel tank.

While it is certainly commendable that the FAA took on this challenge itself, why it took 14 years to accomplish is unfathomable. I am old enough to remember President John F. Kennedy’s 1961 challenge to NASA to put a man on the moon in eight years. And that deadline was met. Was it really easier to put a man on the moon in 1969 than to create a fuel-inerting system post-1996? Was this a typical lack of bureaucratic will at the FAA and NTSB? Was it the industry’s failure to acknowledge the potential significance of the problem? Was it a lack of resources? Or was it all of the above? I don’t know.

So finally in 2008, almost 12 years to the day after TWA 800’s final flight, the FAA issued a final rule aimed at reducing the risk of fuel-tank explosions in certain transport-category aircraft. The rules affect production of new models and the retrofitting of some in-service aircraft. Cargo aircraft were excluded from the rules. While the rule is not perfect, at least implementation of fuel-inerting systems has begun.  

Regardless of the reasons for past foot-dragging, the challenge now is to install the system in applicable aircraft as expeditiously as possible. (While the rule does not include cargo aircraft, operators should consider voluntarily retrofitting their aircraft.) It would indeed be tragic if our collective failure to remember the cause of TWA 800’s explosion allowed another similar tragedy to occur.



The system is not cost effective as I understand. It is bulky, expensive possibly not compatible to all aircraft.

FAA is kind of looking the other way now that so much time has gone by and most of the upper management if gone that made all the promises to the families.

My concern here is how is it safe to pump 190 degree nitrogen contaminated with exhaust by products into fuel tanks.

The Flight 800 is still an unknown cause. The other fuel tank explosions were not really investigated properly as to the cause. However pilot error left a fuel pump running for hours on one aircraft and a terrorist bomb blew up the other

So when is the last time a center tank blew up on a 747?

I believe this inerting is not the answer.



Where are exhaust gases entering the inert nitrogen injection system? I am honestly curious as to where this information comes from. It would appear to me to be a major oversight if the inert gas--the whole point of the system--were to be so simply contaminated through normal operation.

On your explanation of the flights John mentioned, I must wholly disagree. The NTSB has documentation on both of the aircraft he mentioned and neither of them is as you describe. Philippine Air Flight 143 was not pilot error or a bomb, it was caused by flashpoint-heated fuel vapor in an unused-for-two-months center tank igniting, the ignition likely caused by poorly-implemented retrofits of logo lighting by the airline after Boeing delivered the aircraft.

Source: http://aviation-safety.net/database/record.php?id=19900511-1

The Transmile cargo 727 was likewise not pilot error or a bomb, it was caused by faulty wiring passing through a wing tank leading to the fuel tank boost pump.

Source: http://aviation-safety.net/database/record.php?id=20060504-0

As for Flight 800, I would hardly consider it 'unknown cause'. The NTSB's final report states a clear probable cause based on all available evidence--the same one reported here and widely accepted as accurate--which is a far cry from 'unknown'.

Source: http://www.ntsb.gov/investigations/summary/AAR0003.html