NGS
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Joined: Sep 2002
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From: Europe
NGS
My company recently installed NGS (Nitrogen Generation System) in one of our 744F, but issued very limited info to pilots.
I would appreciate any info or link towards detailed information regarding purpose and function of this system.
Thanx
I would appreciate any info or link towards detailed information regarding purpose and function of this system.
Thanx
Joined: Jun 2005
Posts: 954
Likes: 5
From: USA
The purpose would seem to be fuel tank inerting. Following the TWA 800 investigation, the US NTSB issued safety recommendations which included fuel tank inerting as a method of preventing similar future occurances.
The basic theory behind this is that empty or only partially filled fuel tanks may provide an environment where, under certain circumstances, a volitile vaporous air/jet fuel mixture may be formed and possibly be ignited by electrical arcing or sparking inside the tank.
The electrical potential difference between two surfaces which is necessary for arcing may be provided by a number of sources including powered electrical circuits, sympathetic induction or static electricity generated as the natural result of aircraft operation.
The gap between two conductive surfaces necessary for arcing may be provided by corrosion, loose connections, worn or damaged wire insulation to name a few.
All that is needed for ignition is a spark hot enough to ignite the volitile vaporous mixture. Corrosion of bonding wire attachment points and worn electrical wire insulation are most common in older aircraft and may facilitate arcing if enough electrical potential exists to jump the air gap. If the wrong combination of sufficient electrical potential, suitable air gap and and a fuel/air mixture of proper proportion exist in the same place at the same time, BOOM. Works just like a sort of low energy spark plug in a big low pressure cylinder. Yet the potential explosive power is substantial under just the right (wrong) circumstances.
That's a synopsis of the official theory of how TWA 800 blew up and has also been cited as possibly being causal in a couple of other aircraft fuel tank explosions over the years.
Conventional fuel tank venting systems simply allow air from outside the tanks to enter the tanks and replace the volume of fuel as it is pumped out. Some, like the Falcon series of aircraft, use engine bleed air to pressurize the tanks and provide a positive head pressure. The oxygen in the air is a necessary component for combustion in the vaporous air/fuel mixture in the tank.
Now if the air is instead replaced with an inert gas such as nitrogen, then the the vaporous mixture will not be volitile and support ignition or combustion. No explosion even if an otherwise sufficient ignition source such as a strong electrical arc is provided. One side of the fire triangle has been eliminated. This solution is generally considered to be more failsafe and less costly than attempting to remove the ignition source by inspecting and/or replacing all possibly faulty electrical and bonding components in all fuel tanks. Elimination of the combustable liquid jet fuel would work a little TOO well!
With the consequences of a fuel tank explosion being so dire, a plan for elimination of the threat has some supporters. On the other hand, the rather high mean time and number of operations between fuel tank explosions appears to have dulled any sense of urgency on the part of either industry or regulators to rush toward a final solution. Coupled with the fact that there is no hard proof (just the best theory given the known facts) that this is actually what caused the TWA 800 tragedy and the significant expenses associated with any fleetwide retrofit scheme, this comes as no great shock to me.
As to the particulars of an actual fuel tank inerting system in use today, I haven't a clue. The USAF are the only organization I know of who have had it for any length of time. There may well be others. Perhaps someone might comment on the weight and volumetric capacity of an actual in-use inerting system.
Best regards,
Westhawk
The basic theory behind this is that empty or only partially filled fuel tanks may provide an environment where, under certain circumstances, a volitile vaporous air/jet fuel mixture may be formed and possibly be ignited by electrical arcing or sparking inside the tank.
The electrical potential difference between two surfaces which is necessary for arcing may be provided by a number of sources including powered electrical circuits, sympathetic induction or static electricity generated as the natural result of aircraft operation.
The gap between two conductive surfaces necessary for arcing may be provided by corrosion, loose connections, worn or damaged wire insulation to name a few.
All that is needed for ignition is a spark hot enough to ignite the volitile vaporous mixture. Corrosion of bonding wire attachment points and worn electrical wire insulation are most common in older aircraft and may facilitate arcing if enough electrical potential exists to jump the air gap. If the wrong combination of sufficient electrical potential, suitable air gap and and a fuel/air mixture of proper proportion exist in the same place at the same time, BOOM. Works just like a sort of low energy spark plug in a big low pressure cylinder. Yet the potential explosive power is substantial under just the right (wrong) circumstances.
That's a synopsis of the official theory of how TWA 800 blew up and has also been cited as possibly being causal in a couple of other aircraft fuel tank explosions over the years.
Conventional fuel tank venting systems simply allow air from outside the tanks to enter the tanks and replace the volume of fuel as it is pumped out. Some, like the Falcon series of aircraft, use engine bleed air to pressurize the tanks and provide a positive head pressure. The oxygen in the air is a necessary component for combustion in the vaporous air/fuel mixture in the tank.
Now if the air is instead replaced with an inert gas such as nitrogen, then the the vaporous mixture will not be volitile and support ignition or combustion. No explosion even if an otherwise sufficient ignition source such as a strong electrical arc is provided. One side of the fire triangle has been eliminated. This solution is generally considered to be more failsafe and less costly than attempting to remove the ignition source by inspecting and/or replacing all possibly faulty electrical and bonding components in all fuel tanks. Elimination of the combustable liquid jet fuel would work a little TOO well!
With the consequences of a fuel tank explosion being so dire, a plan for elimination of the threat has some supporters. On the other hand, the rather high mean time and number of operations between fuel tank explosions appears to have dulled any sense of urgency on the part of either industry or regulators to rush toward a final solution. Coupled with the fact that there is no hard proof (just the best theory given the known facts) that this is actually what caused the TWA 800 tragedy and the significant expenses associated with any fleetwide retrofit scheme, this comes as no great shock to me.
As to the particulars of an actual fuel tank inerting system in use today, I haven't a clue. The USAF are the only organization I know of who have had it for any length of time. There may well be others. Perhaps someone might comment on the weight and volumetric capacity of an actual in-use inerting system.
Best regards,
Westhawk
