Fire suppression in cargo components

Fire Suppression in Cargo Compartments

By Fred Workley

May-June 1998

Fred Workley

The FAA has finalized its Revised Standards for Cargo or Baggage Compartments in Transport Category Aircraft. This was effective Feb. 17, 1998, for 14 CFR Parts 25 and 121. At the same time, the FAA requested additional comments from Part 135 operators. The FAA on or before May 18, 1998, must have received comments on the proposed changes to cargo/baggage compartments for Part 135 certificate holders.

The effect of this final rule is to change the primary means of preventing uncontrolled fires in Class D compartments in passenger-carrying aircraft from containment to suppression. The necessity to revise the standards was prompted by uncontrolled fires in cargo or baggage compartments that have caused accidents and loss of life, and also by the increased danger of explosion and fire caused by aerosol cans in passenger luggage. The aerosol cans now manufactured use a mixture of propane, butane, and isobutane for propellant. These aerosol cans are widely used and are found in a high percentage of checked baggage.

Tests conducted by the FAA Technical Center found that this type of aerosol can explodes in burning luggage with such force that the compartment liners are ruptured. With the liner no longer intact, an unlimited supply of oxygen is available to support the fire that can now spread throughout the aircraft. However, in a Class C compartment in which an extinguishing agent has been released, the aerosol propellant does not ignite and explode.

The goal of requiring a "fire suppression system" is not the same as a "fire extinguishing system." The suppression system is to suppress the fire until it can be completely extinguished by ground personnel following a safe landing. To achieve this goal, the FAA proposed in Notice 97-10 to amend Part 25 to eliminate Class D compartments altogether. Compartments in passenger-carrying airplanes that can no longer be approved as Class D compartments must be retrofitted to meet the standards of Class C compartments. Compartments in all-cargo airplanes that can no longer be approved as Class D compartments must be shown to meet the standards of Class E compartments in lieu of those for Class C compartments.

A brief overview of the different classes of cargo or baggage compartments will help in understanding the required changes.

Class A - A compartment in which the presence of a fire would be easily discovered by a crewmember while at his or her station, and where all compartments are easily accessible in flight. No compartment liner is required.

Class B - A compartment with a separate, approved smoke or fire detection system to give warning to the pilot or flight engineer station and with sufficient access in flight to enable a crewmember to effectively reach any part of the compartment with a hand fire extinguisher. A liner meeting the flame penetration standards of 25.855 and Part III of Appendix F of Part 25 must be provided to prevent the fire from spreading.

Class C - Class C compartments differ from Class B compartments primarily in that built-in extinguishing systems are required for control of fires in lieu of crewmember accessibility. Smoke or fire detection systems must be provided. An approved liner is required.

Class D - This is the class that must be changed. In lieu of providing smoke or fire detection and extinguishment, Class D compartments were designed to control a fire by severely restricting the supply of available oxygen. The capability of the liner to resist flame penetration is especially important. As discussed, exploding aerosol cans can rupture the liner so that the supply of oxygen can no longer be restricted.

Class E - A cargo compartment used only for the carriage of cargo. Typically, a Class E compartment is the entire cabin of an all-cargo airplane. A smoke or fire detection system is required. In lieu of providing extinguishment, means must be provided to shut off the flow of ventilating air to or within a Class E compartment. In addition, procedures such as depressurizing a pressurized airplane are stipulated in the event that a fire occurs (something you can't do on a passenger-carrying airplane).

The overhead storage areas and certain other areas in the cabins of passenger-carrying airplanes are considered "stowage" compartments rather than cargo or baggage compartments. They are not required to meet any of the above standards.

The standards for liners are found in Part III of the Appendix F of Part 25. In addition, Section 121.314(a) permits the ceiling and sidewall panels to be constructed of materials that meet the test requirements of Part III of Appendix F, or of glass fiber reinforced resin. Section 121.314(a) also permits the continued use of ceiling and sidewall panels constructed of aluminum, provided they were approved prior to March 20, 1989.

Section 25.858, adopted Sept. 11, 1980, (Amendment 25-54, 45 FR 60173) requires the smoke or fire detection systems of Class B, C, and E compartments to provide visual indication to the flightcrew within one minute of the start of a fire. Since the installation of detection systems alone would provide only a small incremental increase in safety, both detection and suppression systems must be installed in Class C compartments.

The changes from Class D to Class C compartments to Part 121 will require compliance within three years, that is, March 19, 2001. Every Class D compartment, regardless of volume, must meet the standards of a Class C compartment unless the aircraft is an all-cargo carrier, in which case it may meet the standards of a Class E compartment.

The easiest and cheapest time to make the conversion would be during a C-check when many areas of the plane are easily accessible. Of course, after installation these systems must be maintained and inspected.

This will include:

1) leaks check
2) visual inspection of the system
3) sensor test
4) hydrostatic check of the fire bottles

The first three checks could be accomplished at each C-check. A hydrostatic check will involve removing and replacing the fire bottle and will occur every five years. The fire bottle, containing the inerting agent, will be returned to the provider where it will be recharged and checked for leaks. This inerting agent could be carbon dioxide or nitrogen.

The Montreal Protocol of 1987 prohibits the manufacture or import of new halon as of Jan. 1, 1994. However, there is no restriction on the use of existing supplies of halon. The Environmental Protection Agency (EPA), which is responsible for the regulation of halons, advised the FAA in a letter of May 8, 1997, that it does not intend to ban the use of halon in installed fire suppression systems for the life of the airplanes. Further, it can support the use of stockpiled halons to retrofit aircraft holds, and it can support these policies in international negotiations related to aircraft or environmental matters. This support from EPA came in the political climate following the Valujet crash. This support is conditional on airline and aircraft industry support of on-going efforts to develop alternatives to halon for use in future aircraft. Nitrogen doesn't seem practical because of the large tank size required to support the range of some aircraft.

The Aviation Rulemaking Committee of the FAA has a Fuel Tank Inerting Task Group #3. This group is looking into the fuel tank explosion on TWA 800 center tank and in Manila, an aircraft incident involving the explosion of a wing tank. They have heard presentations on using carbon dioxide as an alternative to Halon. The same technology, as inerting in fuel tanks, can be used in cargo compartments. Cargo compartments and lower deck fires or explosions could be combated with a two step approach.

Step One: An alarm system can be installed using state-of-the-art oxygen analyzers. When smoke or a flame is detected, the crew will be alerted and automatically CO2 will be metered into the compartment in just enough quantity to change the oxygen level so it cannot support combustion and the fire will go out immediately. The advantage of utilizing CO2 is that the gas will penetrate baggage and cargo containers. CO2 is also drawn to the heart of the fire with the oxygen as it is being consumed in the fire. CO2 causes less clean up.

Step Two: After discharge of CO2, the system continues to monitor the cargo compartment and keeps the oxygen levels at a level that would not support combustion. This is now possible through technology-transfer that is now available for utilization on aircraft by using liquid CO2 released through a metering system.

As well as mandating changes to Class D cargo compartments, the FAA is proposing new reporting requirements. The new reporting proposed is in two parts. First, Part 121 operators would report to the FAA on a quarterly basis the serial numbers of the aircraft in that holder's fleet in which all Class D compartments have been retrofitted to meet Class C or E requirements. The operators would also report the serial numbers of aircraft that have Class D compartments yet to be retrofitted. Second, the FAA could inform the public of the operator's progress in achieving compliance.

The FAA wants to increase the safety of aircraft. It also wants to make the public aware of its efforts. Whether the comparison of the progress of different 121 airline operators is truly helpful to passengers is questionable. However, in contrast to the number of fatalities from fires originating in Class D compartments, there have been no deaths from fires in Class C compartments.

Nonetheless, Part 135 operators have argued that their operations are so different from Part 121 operations that they should be exempted from making any changes to their fleet's cargo compartments. No final decision has been made. The FAA has requested additional comments. These comments must be received on or before May 18, 1998. You can do this electronically to refer to Docket 28937. Comments may be mailed in triplicate to: Federal Aviation Administration, Office of Chief Counsel, ATTN: Rules Docket (AGC 200), Docket No. 28937, 800 Independence Ave. SW, Washington, D.C. 20591. For more information, contact Gary Killion at the Transport Airplane Directorate, (425) 227-2114 or