Improved Fire Protection Planned for Air Force C-5s
Modification of the Air Force C-5 fleet with a fire suppression system (FSS) began recently, according to the C-5 system program office at WrightPatterson Air Force Base, Ohio.
The FSS, developed and designed by the Fuel and Air Division of Parker-Hannifin, Irvine, Calif., will enhance the protection offered to the crew and aircraft in case of fire or explosion. All C-5s are scheduled to be modified by May 1977 at an estimated cost, including installation and spare parts of $26 million.
The FSS consists of three separate subsystems, one fuel tank inerting and two fire fighting.
Oxygen percentage cut
Fuel tank inerting is the major portion of the modification. The inerting system maintains the oxygen concentration in the ullage, or empty space in the fuel tanks, below that required to support a flame front or combustion. The ullage concentration will normally be less than 4 percent, compared to 21 percent in the atmosphere. The inerting subsystem has two major functions: pressurization and scrubbing.
The pressurization function maintains a small positive pressure in the tanks to prohibit outside air from entering the ullage. The existing C-5 vent system has been modified with vent valves and regulators to maintain the proper tank pressure. Liquid nitrogen stored aboard the aircraft is passed through heat exchangers to convert the liquid to gas for use in the pressurization system. The system operates automatically as fuel is fed to the engines and when the aircraft descends.
The scrubbing function bubbles gaseous nitrogen through the fuel to remove dissolved oxygen at a controlled rate so that the oxygen concentration in the ullage remains below 8 percent during climb.
Valves in the aircraft vent system prevent outside air from entering the tanks, thus keeping stable mixtures of gases in the fuel tanks. Gaseous nitrogen is furnished by the pressurization function to replace the fuel as it is used and to maintain fuel tank pressure as the aircraft decreases altitude.
Once the aircraft is serviced with liquid nitrogen, the fuel tank inerting subsystem operates automatically— even with the aircraft unattended and all power turned off.
One of the fire fighting subsystems being installed uses liquid nitrogen in unmanned areas of the aircraft—leading edge of the wing, leading edge of the engine pylons, under the cargo floor, wheel wells, wing root dry bays and power transfer unit compartments.
Cargo hold protected
The other fire fighting subsystem uses Halon 1301 (bromotrifluoromethane) in the cargo compartment, center wing section, and the avionics and guidance equipment bay.
The Halon 1301 is contained in 20 bottles, 19 with 70 pounds of 1301 each, and one in the avionics area with 10 pounds. Liquid nitrogen is stored in two tanks under the wing-to-fuselage fairings, each containing 750 pounds.
The FSS has already undergone extensive ground and laboratory testing at Parker-Hannifin and LockheedGeorgia Company. Service evaluation testing was flown on three C-5s which logged a combined total of 1000 flight test hours.
According to Lt. Col. W. T. Wetzel, Aeronautical Systems Division program manager, the subsystems have operated well during flight testing, which focused on reliability, maintainability, operational suitability and effectiveness, and adequacy of the fire protection training program for pilots and flight engineers.