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. Main deck cargo compartments on freighters. 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 The assumption that any fire would be contained simply by having the compartment sealed was no longer valid and a number of accidents happened, most notably L101, vicinity Riyadh Saudi Arabia, 1980 and DC93, en-route, north west of Miami USA, 1996.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 in 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.
On 2 July 2021, during pre-departure loading of a Boeing 777-300 at Heathrow prior to passenger boarding with only the operating crew on board, a rear hold fire warning was annunciated and smoke and fumes subsequently entered the passenger cabin. The Investigation found that the source was a refrigerated container which had been subject to abnormal external impact prior to or during loading causing a short circuit in its battery pack. The refrigeration system involved was found by design to inhibit fire following a short circuit but it was noted that QRH response procedures did not apply to the circumstances.
On 28 July 2011, 50 minutes after take off from Incheon, the crew of an Asiana Boeing 747-400F declared an emergency advising a main deck fire and an intention to divert to Jeju. The effects of the rapidly escalating fire eventually made it impossible to retain control and the aircraft crashed into the sea. The Investigation concluded that the origin of the fire was two adjacent pallets towards the rear of the main deck which contained Dangerous Goods shipments including Lithium ion batteries and flammable substances and that the aircraft had broken apart in mid-air following the loss of control.
On 22 April 2013, a lower deck smoke warning occurred on an Airbus A330-300 almost 90 minutes into the cruise and over land. The warning remained on after the prescribed crew response and after an uneventful MAYDAY diversion was completed, the hold was found to be full of smoke and fire eventually broke out after all occupants had left the aircraft. The Investigation was unable to determine the fire origin but noted the success of the fire suppression system whilst the aircraft remained airborne and issues relating to the post landing response, especially communications with the fire service.
On 7 October 2013 a fire was discovered in the rear hold of an Airbus A330 shortly after it had arrived at its parking stand after an international passenger flight. The fire was eventually extinguished but only after substantial fire damage had been caused to the hold. The subsequent Investigation found that the actions of the flight crew, ground crew and airport fire service following the discovery of the fire had all been unsatisfactory. It also established that the source of the fire had been inadequately packed dangerous goods in passengers checked baggage on the just-completed flight.
On 3 September 2010, a UPS Boeing 747-400 freighter flight crew became aware of a main deck cargo fire 22 minutes after take off from Dubai. An emergency was declared and an air turn back commenced but a rapid build up of smoke on the flight deck made it increasingly difficult to see on the flight deck and to control the aircraft. An unsuccessful attempt to land at Dubai was followed by complete loss of flight control authority due to fire damage and terrain impact followed. The fire was attributed to auto-ignition of undeclared Dangerous Goods originally loaded in Hong Kong.
On 7 February 2006, towards the end of a flight to Philadelphia, the crew of a DC8-71F detected possible signs of a fire and eventually a system warning confirming that a fire may be developing in part of the main deck cargo. During the subsequent landing, thick black smoke entered the flight deck and an emergency evacuation was performed immediately after the aircraft stopped. Despite the efforts of the emergency services, the aircraft was subsequently destroyed by fire which the Investigation traced to containers which it was suspected but not proved had been loaded with goods which included lithium batteries.
On 11 May 1996, the crew of a ValuJet DC9-30 were unable to keep control of their aircraft after fire broke out. The origin of the fire was found to have been live chemical oxygen generators loaded contrary to regulations. The Investigation concluded that, whilst the root cause was poor practices at SabreTech (the maintenance contractor which handed over oxygen generators in an unsafe condition), the context for this was oversight failure at successive levels - Valujet over SabreTech and the FAA over Valujet. Failure of the FAA to require fire suppression in Class 'D' cargo holds was also cited.