Nitrogen Generating System
Following
two Boeing 737CL explosion investigations in Asia (and others including the
B747 TWA 800 midair explosion), a protection was developed by Boeing to
minimize explosive vapors in the center tank. The 737 explosions were caused by
trapped fuel high temperatures due to radiant heat from the Packs under the
tank which formed highly explosive vapors. The fuel was ignited by the center
tank fuel pumps which were still running with an
empty center tank. Early days center tank fuel pumps did not had an automatic
shut off with LOW PRESSURE as the newer modified ones that shut down after ±15
seconds of LOW PRESSURE. This is also the reason that someone has to be on the
flight deck when a center tank pump is running as by the FCOM, the book does
not cover explicit modifications to each aircraft.
This
protective device (NGS) divides Nitrogen from Oxygen by a separation module and
leaves Nitrogen enriched air (NEA) in the center tank to a level which will not
support combustion. The oxygen level is decreased by the NGS to ±12% which is
sufficient to prevent ignition.
The
NGS has only an indication available in the right main wheel well next to the
APU fire control panel, so it has no visible clew for crews of its operation
during flight.
Indications are:
• OPERATIONAL (green)
• DEGRADED (blue)
• INOPERATIVE (amber)
• OPERATIONAL (green)
• DEGRADED (blue)
• INOPERATIVE (amber)
The
nitrogen generation system gets bleed air from the left side of the pneumatic
manifold where after its cooled, driven through the separation module and
directed to a flow valve into the center tank. The NGS operates automatically
only in flight and shuts down in the next conditions:
