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 public pressure following the May, 1996 incident involving the Class D compartment of a McDonnell Douglas DC-9 operated by Valujet Airlines, have pressured the FAA to force issue a requirement to upgrade Class D compartments to Class C.

According to the FAA, the Air Transport Association of America (ATA) followed on the heels of the Valujet accident by joining Vice President Gore, and formally announced that its membership would voluntarily install fire or smoke detection systems in Class D compartments. The ATA is a trade organization that represents the major US airlines. Details of the ATA plan — including an implementation schedule — were presented to FAA officials on January 31, 1997. The announcement, which affects approximately 2,700 airplanes operated by 21 ATA members, might appear to make the detection portion of this rulemaking moot; however, the FAA considers the installation of both detection and suppression systems in these compartments to be essential. As a result, the FAA published it's final rule on February 10, 1998.

The rule (14 CFR Parts 25 and 121 Revised Standards for Cargo or Baggage Compartments in Transport Category Airplanes; Final Rule) establishes a mandatory modification of the smoke detection and fire suppression characteristics of all Class D cargo compartments to that of either a Class C cargo compartment on passenger/combi aircraft, or Class E cargo compartment on cargo only aircraft (See Class distinction P. 52).
This mandate has a three-year compliance period, beginning March 19, 1998, and requires quarterly progress reports by operators during the three year implementation period.

What does this mean to you?
This upgrade involves installing detection and extinguishing systems.

Several companies have been working diligently over the last couple of years to develop STCs to upgrade these baggage compartments.

These STCs generally involve the installation of an extinguishing system — to include plumbing, nozzles, and fire bottles — and a separate detection system which typically includes smoke and/or heat detectors, and a cockpit mounted annunciator and control panel.

Several companies in the industry to include Barfield, Inc., Securaplane Technologies, and Walter-Kidde, have aggressively pursued Supplemental Type Certificates (STCs) since 1998 for the express purpose of addressing the D to C conversion. And although their systems include the two basic requirements — detection and suppression — that's where their similarities end.

Options for compliance
Components used in the systems being offered range from commercial detection units made by Cerberus, Kidde, and Hochiki to Halon extinguishers manufactured by companies like Kidde and Pacific Scientific Corporation.

The following are some examples of STCs that are available to the aviation community:

Further, Securaplane's approach to reducing false alarms by using smoke detection and temperature detection. Maintenance of Securaplane's system consists of testing the smoke sensors every 3,000 to 4,000 flight hours using artificial smoke, and cleaning the smoke chamber with compressed air and replacing batteries every three years. The company says it will take 8 to 10 minutes per sensor to remove and replace batteries and blow clean the easily accessible smoke detector chamber.

Avoiding false alarms
There is one factor which all of the parties selling these systems agree on — be sure that the system you are considering has a reputation for high quality.

When you consider the cost of landing at an alternate airport, deploying emergency slides, recharging and inspecting the suppression system, and paying for damages, it doesn't make sense to compromise on the quality of the system you invest in.

An Explanation of Class Distinctions

According to the FAA, during the early post-World War II period, it was recognized that timely detec-tion of a fire by a crewmember of the airplane while at his or her sta-tion and prompt control of the fire when detected were necessary for protection of the airplane from a fire originating in a cargo or baggage compartment.

Because the requirements for de-tection and extinguishment varied depending on the type and location of the compartment, a classification system was established. Three class-es of cargo or baggage compart-ments were initially established and defined in 1946 (Amendment 04-1 to part 04 of the Civil Air Regula-tions (CAR) effective November 1, 1946) as follows:

Class A - A compartment in which the presence of a fire would be easily discovered by a crewmem-ber while at his or her station, and of which all parts are easily acces-sible in flight. This is typically a small compartment used for crew luggage, and located in the cockpit where a fire would be readily de-tected and extinguished by a crewmember. Due to the small size and location of the compartment, and the relatively brief time needed to detect and extinguish a fire, a lin-er is not required to prevent the fire from spreading to other parts of the airplane or protect adjacent struc-ture.

Class B - A compartment with a separate, approved smoke or fire detection system to give warning at the pilot or flight engineer station and with sufficient access in flight to enable a crewmember to effec-tively reach any part of the com-partment with a hand fire extin-guisher. Smoke or fire detection sys-tems must provide indication of a fire to the flightcrew. Because it has a smoke or fire detection system, a Class B compartment may be locat-ed in an area remote from any crewmember's station. Due to the potentially larger size of the com-partment and the greater time in-terval likely to occur before a fire would be extinguished, a liner meeting the flame penetration stan-dards of ¤ 25.855 and Part I of Ap- pendix F of Part 25 must be pro-vided to prevent the fire from spreading to other areas of the air-plane and to protect adjacent struc-ture. As originally defined in 1946, there was also to be sufficient ac-cess to enable the crewmember to move all contents of a Class B com-partment by hand; however, that re-quirement was subsequently delet-ed. Although Class B compartments are typically the large cargo por-tions of the cabins of airplanes car-rying a combination of passengers and cargo (frequently referred to as "combi" airplanes), there are also Class B compartments that are rela-tively small baggage compartments located within the pressurized por-tions of airplanes designed for ex-ecutive transportation.

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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