Class D to C Conversions: A review of cargo fire detection/suppression requirements

Class D to C Conversions A review of cargo fire detection/suppression requirements By Greg Napert November 1999 Although the FAA has been addressing improving cargo compartments over the years, several incidents, along with...

Class C - As defined at the time of initial classification in 1946, any compartment that did not fall into either Class A or B was a Class C compartment. Class C compart-ments differ from Class B compart-ments primarily in that built-in ex-tinguishing systems are required for control of fires in lieu of crewmem-ber accessibility. As with Class B compartments, smoke or fire detec-tion systems must be provided. Due to the use of a built-in extinguish-ing system and closer control of ventilating airflow, the distribution of extinguishing agent in a Class C compartment is considerably more uniform than in a Class B compart-ment. The volumes of Class C com-partments in transport category air-planes currently used in domestic service range from approximately 700 to 3,000 cubic feet. Later, two additional classes of cargo or baggage compartments were established and defined as fol-lows (Amendment 4b-6 to part 4b of the CAR effective March 5, 1952):
Class D - A compartment in which a fire would be completely contained without endangering the safety of the airplane or the occu-pants. A Class D compartment is similar to a Class C compartment in that both may be located in areas that are not readily accessible to a crewmember. As originally defined in 1952, Class D compartments were required to have smoke or fire de- Class D compartments are designed to control a fire by se-verely restricting the supply of avail-able oxygen. Because an oxygen-deprived fire might continue to smolder for the duration of a flight, the capability of the liner to resist flame penetration is especially im-portant. A note following the defin-ition of a Class D compartment stat-ed, "For compartments having a vol-ume not in excess of 500 cubic feet, an airflow of not more than 1,500 cubic feet per hour is considered acceptable. For larger compart-ments, lesser airflow may be ap-plicable." That note was interpreted to mean that a Class D compartment could not exceed 2,000 cubic feet in volume even if the leakage of air into the compartment was zero. The standards for Class D compart-ments were later amended (Amend-ment 25-60, 51 FR 18236, May 16, 1986) to specifically limit the vol-ume of those compartments to 1,000 cubic feet; however, some previously-approved airplanes in air carrier service have Class D compartments as large as 1,630 cu-bic feet. Other airplanes designed for executive transportation, and al-so used in on-demand service, have relatively small (15-25 cubic feet) Class D compartments located out-side the pressurized portions of the cabin.
Class E - A cargo compartment of an airplane used only for the car-riage of cargo (Amendment 4b-10 to part 4b of the CAR, adopted in 1959). A smoke or fire detection system is required. In lieu of pro-viding extinguishment, means must be provided to shut off the flow of ventilating air to or within a Class E compartment. In addition, proce-dures, such as depressurizing a pressurized airplane, are stipulated to minimize the amount of oxygen available in the event a fire occurs in a Class E compartment. Typical-ly, a Class E compartment is the en-tire cabin of an all-cargo airplane; however, Class E compartments may be located in other portions of the airplane. This, of course, does not preclude the installation of Class A, B, C or D compartments in all-cargo airplanes.

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