Published: April 1999

Concern over the potential for catastrophic in-flight fires in commercial airplane cargo compartments has focused attention on cargo compartments that depend on oxygen deprivation to prevent and suppress combustion (Class D compartments). Though the risk of fire in a cargo compartment is statistically very low, the U.S. Federal Aviation Administration (FAA) has issued a rule change to require airplanes registered in the United States to convert all Class D cargo compartments to Class C or Class E compartments by installing a smoke-detection system (Class E), fire-suppression system, or both (Class C), depending on whether the airplane is a passenger or freighter configuration. Boeing has been supplying the information and installation hardware that operators need to meet the rule change deadline of March 18, 2001.

For many years, commercial airplanes have relied on oxygen deprivation to control the risk of cargo-compartment fires below the main passenger cabin. However, a cargo fire in a commercial airplane that resulted in hull loss has caused the perception of cargo-fire risk to grow. In response, the U.S. Federal Aviation Administration (FAA) issued a rule change on March 18, 1998, that mandates the conversion of Class D compartments to Class C or Class E compartments by installing smoke-detection systems, fire-suppression systems, or both.

Boeing is positioned to help commercial airplane operators meet the rule change deadline of March 18, 2001. The Boeing support plan is based on

1. Historical and recent use of Class D compartments.
2. Current criteria for Class C compartments.
3. Current criteria for Class E compartments.
4. Technical assistance available to operators.
5. Tested and validated design.

(Boeing has developed retrofit kits for operators of Boeing-designed commercial airplanes and the Douglas DC-10. Operators of other Douglas-designed airplanes may contact their local Boeing Field Service representative or regional FAA office for information on purchasing retrofit kits from various suppliers. These kits are available for the DC-8, DC-9, MD-80, and MD-90. Operators may also contact their regional FAA office for information on approved suppliers for these kits.)

Historical and Recent Use of Class D Compartments

Class D compartments, which are minimally ventilated cargo holds that are 1,000 ft³ (28.3 m³) or less, have been used in smaller commercial airplanes that fly short to medium routes. These compartments contain primarily passenger luggage and present a low ignition hazard. Reduced oxygen at cruising altitudes further lowers the likelihood of sustained combustion in the cargo compartment.

However, with the growth of air cargo as a revenue source, the mix of items and materials carried in cargo compartments has changed. Commercial products, industrial materials, and airplane components en route to airline maintenance stations have joined baggage below the passenger cabin. Though the compartments are designed and required to be free of ignition sources, the risk that a sequence of events involving human error could lead to a fire is perceived to have increased.

Current Criteria for Class C Compartments

Boeing- and Douglas-designed commercial airplanes that have Class C compartments are shown in table 1. These compartments contain both the smoke-detection and fire-suppression equipment required by the new FAA rule.

Smoke detection.
Current specifications for Class C compartments require that the smoke-detection system alert the flight crew within 60 sec from the time smoke first appears in the compartment.

Fire suppression.
Current specifications for the fire-suppression system in each Class C compartment require a minimum initial concentration of 5 percent Halon throughout the compartment to suppress any combustion to controllable levels. Thereafter, the system must sustain a minimum concentration of 3 percent Halon for 60 min to prevent re-ignition or spreading of the combustion. For airplanes certified for extended-range twin-engine operations (ETOPS), the fire-suppression system must be able to sustain a 3 percent concentration of Halon within the compartment for a maximum of 180 min (fig. 1).

Current Criteria for Class E Compartments

Boeing-designed commercial airplanes that have Class E compartments are shown in table 1. These compartments contain the smoke-detection equipment required by the new FAA rule for in-service airplanes. Current specifications for Class E compartments require that the smoke-detection system alert the flight crew within 60 sec from the time smoke first appears in the compartment.

Technical Assistance Available to Operators

Boeing and several airplane system suppliers have developed retrofit solutions to help operators comply with the new FAA rule. Boeing offers these solutions in two main forms:

• Service bulletins.
• Retrofit kits.

Service bulletins.
Boeing has prepared service bulletins for all affected Boeing-designed commercial airplanes, including the 727-100/-200 and all 737s except the 737-900. Operators have the option to obtain a supplemental type certificate for their own designs manufactured under contract to a supplier.

In anticipation of operator needs, Boeing completed the development and engineering for service bulletins for all affected Boeing-designed commercial airplanes in September 1997. Experience developing smoke-detection and fire-suppression systems for other Boeing models significantly benefited the systems' design.

The service bulletins give operators information to route wires and tubing, mount Halon bottles, and locate circuit breakers and the flight-deck control panel in accordance with their own airplane configurations (fig. 2). Operators may request a master change if they would like Boeing to develop customised service bulletins to reflect the unique configurations of their airplanes.

The service bulletins include

• Complete installation details for detectors, suppression nozzles, cargo electronic units fire suppression bottles, and flight deck control panel.
• Wire and tube routing information.
• Detailed information on connectors and fittings, type and size of wiring and tubing, tube bending and splicing criteria, and wire separation criteria and wiring schematics.
• Ground rules governing areas not covered in detailed installation procedures.
• List of approved smoke detector and fire bottle part numbers and suppliers.

Retrofit kits.
Boeing also offers for purchase a retrofit kit that can be used either by operators performing the installation at their own maintenance centres or by commercial modification centres that install systems for operators. Boeing demonstrated a retrofit system on a 737-400 airplane as part of the certification process. Elements of these installations were applied to the development of the fleet retrofit kits.

Tested and Validated Design

When beginning the effort to provide operators with the information and hardware necessary to comply with the FAA rule change, Boeing considered

• Design.
• Operation.
• Production.
• Retrofit.

Design.
The smoke-detection retrofit design for the 727 and 737 airplanes is a dual-loop photoelectric system (fig. 3) that has been demonstrated to detect smoke from any location in the forward or aft cargo compartment within the specified 60-sec limit. The dual-loop system reduces the incidence of false reports by requiring that at least one sensor in each loop detect smoke simultaneously before the annunciator on the flight deck is activated. If power loss prevents either loop from functioning, the system will automatically reconfigure to transmit a warning upon detection of smoke by the sole remaining detection loop. Failure of a single detector in either loop will leave the remaining detectors in the loop functional and capable of detecting smoke anywhere in either cargo compartment within the 60-sec limit.

Operation.
Flight-deck indicators and controls for the system are located on the centre-aisle stand for the 737 and the overhead panel for the 727. The service bulletins will allow operators to install the control panel in alternate locations on the flight deck.

The indication and control panel includes the following:

• Detector select switch.
• Cargo fire test switch.
• Detector fault indicator.
• Master fire warning light and bell.
• Cargo fire warning lights and armed switches.
• Extinguisher test lights.

Detector select switch.
The detector select switch is a three-position rotary switch marked A, B, and NORM. The flight crew uses the switch to command FWD or AFT single-loop (A or B) operation or normal dual-loop (NORM) operation of the detection system.

Cargo fire test switch.
The TEST switch is a pushbutton that verifies the operation of all detectors in both cargo compartments and the suppression system. When the system is functioning normally, depressing the switch causes the FIRE WARN light and the red FWD and AFT indicators to illuminate, the warning bell to sound until the test button is released, the extinguisher test light to illuminate, and the cargo fire bottle DISCH lights to illuminate.

Detector fault indicator.
A faulty detector will cause the amber DETECTOR FAULT indicator to illuminate and the FWD or AFT indicator to fail to light, depending on which compartment contains the faulty detector. The flight crew can determine whether the A or B loop contains the fault by selecting position A or B on the detector select switch. When the properly functioning loop is selected, the DETECTOR FAULT indicator will not light, and the FWD and AFT indicators will both illuminate under test. The DETECTOR FAULT indicator will also illuminate if the selected loop loses power after the test switch is depressed. The airplane can be flown under master minimum equipment list procedures with the single-loop position selected for either the forward or aft compartments.

The airplane operations manual will require the flight crew to perform these tests before the first flight each day to prevent detector faults from being unrecognised for more than one day.

Master fire warning light and bell.
Upon detection of smoke, a fire warning bell will sound, the cargo fire warning light on the control panel will illuminate, and the appropriate red location indicator will specify if the smoke is detected in the forward or aft compartment.

Cargo fire warning lights and armed switches.
The cargo fire warning lights illuminate red when smoke is detected in the forward or aft compartment. Pressing the illuminated FWD or AFT indicator causes the upper half of the indicator to illuminate ARMED in white. This indicates that the system is ready to discharge suppressant into the indicated compartment. Depressing the guarded DISCH pushbutton switch releases the suppressant into the selected compartment. Once Halon has been discharged to the respective cargo compartment, the control panel DISCHARGE light will illuminate red.

Extinguisher test lights.
A bottle in the air-conditioning mix bay between the forward and aft cargo compartments contains Halon 1301. The Halon bottle can discharge to either compartment, but once activated, the system cannot be reversed to the other compartment. A sensor detects Halon bottle pressure and activates the Halon low-pressure indicator on the flight deck when the bottle discharges or loses pressure.

The TEST switch checks the condition of the discharge nozzles in both compartments. The FWD and AFT indicators illuminate green when the system is functioning normally. Airplanes may not be dispatched with a loaded cargo compartment if the fire-suppression system exhibits any faults.

Production.
Boeing began installing smoke-detection and fire-suppression systems into 737 production airplanes at line position 3078 for the 737-300/-400/-500 models and at line position 91 for 737-600/ -700/-800 models. The production deliveries also include 195-min ETOPS configurations. (The 195 min allows for 180-min ETOPS diversion followed by 15 min for missed approach and emergency passenger evacuation.)

Retrofit.
To support operators in retrofitting their airplanes, Boeing has developed modification kits, including revised documentation and the respective service bulletins. The Boeing portion of the kits will include all hardware for installing the detection and suppression systems, including suppression tubes, wire bundles, support brackets, detector pans, suppression pans and nozzles, and all standard-type hardware. An additional buyer-furnished equipment (BFE) hardware kit will also be required. The retrofit system uses the same components as the production system. Table 2 identifies the respective model, service bulletin, and anticipated weight increase associated with the retrofit.

The retrofit 737-300/-400/-500 service bulletin package will include multiple configurations, including auxiliary fuel cell­configured and ETOPS-equipped airplanes. Auxiliary fuel cell configurations include aft cargo compartment single Boeing fuel cells, single Rogerson fuel cells, and dual Rogerson fuel cells. These unique configurations will be identified as separate groups within the service bulletin. ETOPS-equipped airplanes will have a second 33-lb Halon bottle installed on the opposite side of the mix bay.

Boeing has identified five different suppliers for the BFE kits and has qualified hardware from each supplier. Four suppliers have been qualified to provide smoke detectors (fig. 4) for Boeing-designed airplanes. All four detectors are considered interchangeable and intermixable. Additionally, Boeing has qualified two Halon bottle sources. The cargo electronics units and flight-deck control panel are considered single-source items. Boeing suggests that operators contact individual manufacturers for BFE kits.

To ensure that the Boeing-supplied and BFE kits meet installation requirements, Boeing has completed service bulletin validations on the 737-200/-300/-400/ -500. The 727-200 validation is expected to be completed in early 1999. The company is in the process of determining the method of validation when the 737-600/ -700/-800 bulletins become available.

Summary Boeing has developed a comprehensive plan to assist operators in meeting a new FAA rule to convert Class D cargo compartments to Class C or Class E compartments. To help operators comply with the March 18, 2001, deadline to install smoke-detection systems, fire-suppression systems, or both in existing Class D compartments, Boeing has completed the necessary steps to support the in-service fleet of Boeing-designed commercial airplanes. Those steps include the certification efforts, system design, service bulletin validations, and kit development to make Class D compartments compliant with Class C or Class E specifications.

Thomas Flynn
Engineer
Service Engineering
Boeing Commercial Airplanes Group

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