Aircraft Hold Fire Risks

Description

This article describes the risks associated with fires in cargo holds. It elaborates on cargo hold design, the relevant regulatory requirements and the fire protection systems used in cargo compartments.

Cargo Compartment Classes

Several cargo compartment classes are defined based on:

  • in-flight accessibility
  • presence of fire/smoke detectors
  • presence of extinguishing systems

The following classes exist:

  • Class A. The presence of a fire would be easily discovered by a crewmember and each part of the compartment is easily accessible in flight.
  • Class B. There is a separate approved fire/smoke detector system. There is suffcient access in-flight to enable a crewmember to effectively reach any part of the compartment with the contents of a hand fre extinguisher. Ventilation within the compartment can be controlled.
  • Class C. There is a separate approved fire/smoke detector system. There is an approved built-in fire extinguishing or suppression system controllable from the flight deck. Ventilation within the compartment can be controlled so that the suppression agent can control any fire started within the compartment.
  • Class D. Compartment is inaccessible in-flight. The design relies on fire containment, by having linings designed to restrict the supply of oxygen into the compartment, without needing any fire detection and suppression systems.
  • Class E. There is a separate approved smoke/fire detection system. The flight crew can shut off the ventilating airflow to, or within, the compartment. The required crew emergency exits are accessible under any cargo loading condition. There is no fire extinguishing or suppression system.

Class D was introduced in 1952. With the development of larger aircraft in the following years, cargo compartments also grew in size. The assumption that any fire would be contained simply by having the compartment sealed was no longer valid and a number of accidents happened, most notably L101, vicinity Riyadh Saudi Arabia, 1980 and DC93, en-route, north west of Miami USA, 1996. The main reason for those was that due to the large compartment volume the fire continued long enough to penetrate the sealing linings thus releasing toxic fumes in the passenger cabin or gaining access to fresh oxygen. In response, the FAA removed the Class D cargo compartment category in 1998. All new designs of aircraft, as well as existing aircraft in-service, were to be equipped to the standards of Class C compartments, or Class E compartments for freighter aircraft. The retrofit deadline was set at March 2001.

In Europe, the situation was handled a bit differently. While Class C (for any aircraft) or Class E (for cargo aircraft) were mandated for new aircraft, retrofitting the existing fleet was considered unfeasible. Having a predicted decline in the use of aircraft with Class D compartments it was considered that the problem would eventually resolve by itself (see EASA NPA 2013-23 for details).

Protection and Maintenance

Fire protection of underfloor cargo compartments focuses on the following areas:

  • Making the cargo hold air-tight and fire-proof. Special panels are used which seal the compartment and are able to withstand burning.
  • Cargo fire detection systems. Fire detection systems are designed to alert flight crew on the cockpit within a short period of time (1 minute) of a fire starting. Based on this information, flight crew activate the suppression systems into the affected cargo compartments.
  • Cargo fire suppression systems. Halon is a very effective suppression agent which operates by chemically reacting with the radicals generated by a fire, to inhibit the reaction. To achieve this, sufficient Halon (5% concentration) needs to be released as a first shot, for a fire knock-down effect. Following this, a concentration of 3% must be continuously maintained for the rest of flight. Maintaining the concentration of Halon is crucial to the effectiveness of the system, and therefore it is essential that the cargo compartment remains air-tight.

Any damage or mis-installation of the cargo compartment lining can degrade the performance of the fire suppression system, and therefore has the potential to make a key defence against on-board fires ineffective. To ensure the lining is in adequate condition, scheduled inspections as well as inspections during loading should be performed. Any damage to the linings, as well as any other circumstances that pose risk to the air-tightness of the cargo compartment (e.g. catches not in correct position, fasteners missing or not tightened) need to be reported. Once any damage has been identified and alerted to the operator, it is the responsibility of maintenance staff to classify the damage and initiate the appropriate corrective actions.

Regulations for flight with damaged cargo hold linings are stringent, because any failure of the air-tight and/or flame-proof features of the cargo lining can lead to an uncontrolled fire on board. For this reason, operational constraints can be triggered when any damages are found to the cargo lining, particularly flying with the cargo hold empty under MEL.

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