An article from Air Safety Week
Issue: April 30, 2001
It worked. When it comes to inerting explosive vapors in fuel tanks, don't overlook the military's positive experience with liquid nitrogen (LN), argues a former U.S. Air Force officer. This method is not one of the options formally under consideration by a government-industry task force charged with recommending course of action to inert fuel tanks on transport-category aircraft (see ASW, April 16).
This former officer writes that the C-5A Galaxy transport, comparable in size to the B747, featured fuel tank inerting "as part of its stem to stern fire protection system." After an early crash, the entire C-5 fleet had been modified by 1977 with then state of the art optical smoke and fire detection systems, automatic Halon extinguishers in the manned areas, extra capability in the engine pylons and the LN system with fuel scrubbers, tank pressurization and wheel well protection.
"As a former C-5 AC (aircraft commander) I can tell you we LOVED our LN fuel scrubber and tank pressurization systems. Since we already required cryogenic LOX (liquid oxygen) servicing, all we really needed was a separate truck for the LN storage," he writes. "The problem was having to depend on other commands to maintain the basic truck, and I can remember that the LN servicing truck at Torrejon AB in Spain was out of service for months because mechanics could not obtain repair parts."
"We were never supposed to turn off the tank pressurization system, but we would routinely do so because we wanted to preserve our on board LN supply for critical legs going 'down range' and back to Southwest Asia or Africa, since we knew we couldn't get serviced in Spain," he added.
"On board liquid nitrogen inerting perhaps is not the solution for today's aircraft," he opined. New on-board inert gas generating system (OBIGGS) technology may be preferred, as it eliminates dependency on ground support (Indeed, Safety Board officials pointed to the OBIGGS on the F-22 fighter as the kind of example the airline industry could pursue). Of course, a modernized LN technology might be competitive, given its potential to protect a 747-size aircraft for up to three flights.
"Back in the OLD days," this Air Force pilot recalled, "LN dewars worked reliably and saved at least one airplane (at Travis AFB in the mid-1970s) from an onboard fire. That one SAVE paid for the modification of the entire C-5A fleet." By the way, in this application not only was the ullage "washed," the fuel itself was "scrubbed" by percolating nitrogen up from the bottom of the tank. Scrubbing and pressurization take care of empty, partially full and full tanks. The ARAC has rejected fuel scrubbing as not cost effective.
Based on his experience, this former Air Force officer offered two reasons for action:
"Fuel inerting is a necessity well past its due date and, like smoke detectors for Class D belly compartments, we need to get on with it and get fuel tanks inerted." "The most important action that government, industry and air carriers can take is to catch up with the fire protection that has been airborne on the C-5A for almost a quarter of a century."
The OBIGGS system described here is the system I refered to earlier.
"On board liquid nitrogen inerting perhaps is not the solution for today's aircraft," he opined. New on-board inert gas generating system (OBIGGS) technology may be preferred, as it eliminates dependency on ground support (Indeed, Safety Board officials pointed to the OBIGGS on the F-22 fighter as the kind of example the airline industry could pursue).
Here are a couple of links to the OBIGGS technology.
http://www.airliquide.com/ww/chapter.../en/obiggs.htm
http://www.airproducts.com/prism_membranes/page02.asp
[ 11 August 2001: Message edited by: Flight Safety ]