Safety Features of Cargo Compartment Lights


In addition to illuminating the cargo compartment during ground handling and maintenance operations, the ceiling lights in commercial airplane cargo compartments are part of an important safety system. These light assemblies are a component of the cargo lining and are designed to contribute to the smoke- and fire-containment requirements of various aviation regulatory agencies. However, if a light lens assembly is damaged, missing, or modified, the bulb could become a fire ignition source. In addition, if the light lens were missing on some newer commercial airplane models, the cargo compartment would no longer meet its certification requirements for fire containment.

The ceiling lights in the cargo compartments of Boeing- and Douglas-designed airplanes are intended to be rugged, lightweight, and reliable sources of illumination for ground and maintenance crews. These lights on many newer airplane models, including the 737-600/-700/-800/-900, 747-400, 757, 767, and 777, are also built for easy maintenance. When a bulb burns out, it can be replaced inexpensively and quickly without removing the light assembly. The "push and turn" action to replace a bulb is even faster and easier than for a standard household lightbulb. Ensuring the safe and proper operation of these lights, regardless of airplane model, includes understanding the following:

  1. Safety features.
  2. Role in fireworthiness.
  3. Recommendations and precautions.

1. Safety Features
To protect the bulb from damage during cargo loading and to protect baggage from the heat of a bare incandescent or halogen bulb (in accordance with U.S. Federal Aviation Regulation (FAR) 25.855 2(g)), cargo lights are covered by lenses made of tempered glass or other durable material. The light assemblies are qualification tested to ensure that the temperature of an article in constant contact with any portion of the light will not exceed 140° to 170°F (60° to 77°C). This temperature is well below the ignition point of airborne baggage and cargo.

Another safety feature on many newer airplane models prevents the cargo lights from operating in flight. Power to the lighting circuits is not available unless the cargo compartment door is completely open.

An additional safety feature has been included on most 757 and 767 airplanes and all 737-600/-700/-800/-900 and 777 airplanes (figs. 1 and 2). When the lens is removed from the cargo light assembly, the power to the bulb in that assembly is interrupted by a safety switch on the unit. However, if the cargo light lens breaks or is removed and the lens housing is reinstalled on the light assembly without the lens, the safety switch is activated, and power will be available to the bulb.

It is also possible to circumvent the safety switch when the lens housing is missing if an object is pushed into the fastener hole used to hold the lens housing onto the light assembly. If this occurs and baggage is packed too close to the light, the bare bulb may eventually heat the baggage to its ignition point and cause a cargo fire (fig. 3). Boeing has reviewed photographs that show how this latter situation may have contributed to a cargo fire as recently as August 1998. In this incident, the airplane sat on the ramp for an extended time with the cargo door open and with baggage packed against a bare bulb.

2. Role in Fireworthiness
Cargo lights also serve an important safety function. They are integral to the lower hold cargo compartment ceiling liners, which must conform to smoke- and fire-containment requirements. As a result, the lights become part of the containment system. Certification requirements for newer airplane models have become increasingly more stringent, including passing tests that simulate the worst possible conditions for a cargo fire.

Current tests require cargo ceiling liner material and ceiling-mounted furnishings to withstand a 1,500°F (816°C) wall of flame for 5 min without permitting any flames to reach the airplane structure. Additionally, the ceiling liners and furnishings must limit the temperature of items above the lining to 400°F (205°C) or less for a distance of 4 in above the liner. For a few of the models certified to the latest fireworthiness levels, the light lens must be in place in order for the light assembly to contain the flames for the duration of the test. These levels are set forth by U.S. Federal Aviation Administration (FAA) FARs. Table 1 summarizes the FAA certification requirements for many Boeing- and Douglas-designed airplanes. It also indicates whether the light lens must be in place to meet fire-containment certification requirements.

3. Recommendations and Precautions
To ensure that the cargo furnishings in the lower holds of airplanes remain in compliance with certification type designs, Boeing issued an In-Service Activity Report (ISR 98-08-3337-00) on the 757. Boeing also recommends the following cargo light precautions:

SUMMARY
The primary purpose of cargo compartment ceiling lights in Boeing- and Douglas- designed commercial airplanes is to provide adequate lighting for ground handling and maintenance operations. These lights also serve in an important safety capacity by functioning as part of the smoke- and fire-containment system in the lower hold of airplanes. Properly maintaining cargo light assemblies and not overriding their built-in safety features can help maintenance personnel prevent cargo fires that otherwise may result from exposure of baggage to lit bare bulbs.

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Table 1
Light Lens Requirement for Lower Lobe Cargo Compartments
Model Lower lobe compartment classification U.S. Federal Aviation Regulation (FAR) Part 25.855 or noted Civil Aviation Requirements (CAR) certification basis Lens required for fire containment? Comments
707 Class Da CAR Amendment 4B-3 No  
727-100/-200 Class D CAR Amendment 4B-11 No  
737-100/-200 Class D Amendment 25-15 No  
737-300/-400/-500 Class D/Cb Amendment 32
(plus FAR 121 Amendment 202)
No  
737-600/-700/-800 Class D/Cb Amendment 72/Amendment 25-60 No Fiberglass light housing provides flame penetration protection.
747-100/-200 Class C Amendment 25-15 No  
747-400 Class C Amendment 32
(plus FAR 121 Amendment 202)
No  
747-400F Class C Amendment 25-60 Yes 25.855 certification compliance requirement.
757-200 Class C Amendment 32
(plus FAR 121 Amendment 202)
No  
757-300 Class C Amendment 72
(plus Amendment 25-60)
Yes 25.855 certification compliance requirement.
767-200/-300 Class C Amendment 32
(plus FAR 121 Amendment 202)
No  
767-300F (UPS) Class E Amendment 25-32
(plus FAA Issue Paper S-2)
Yes Required by FAA Issue Paper S-2.
767-300F (GMF) Class C Amendment 25-32
(plus Amendment 25-60)
No  
777-200/-300 Class C Amendment 25-60 /Amendment 25-72 No Fiberglass light housing provides flame penetration protection.
DC-8
DC-9
Class D CAR Amendment 4B-12, 4B-14
CAR 4B (final issue)
No
No
 
DC-10 Class D/C Amendment 25-22 No  
MD-11 Class C Amendment 25-61 No Low-temperature fluorescent lamps used. Light can be on during flight.
MD-80 Class D Amendment 25-40 No  
MD-90 Class D Amendment 25-70 No  
a-As manufactured and delivered.
b-Compartment classification changed from Class D to Class C in accordance with U.S. Federal
Aviation Administration Airworthiness Directive/Notice of Proposed Rule Making mandate.

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Figure 1.

Figure 2.

Figure 3.

Michael Kovalenko
Lead Engineer
Cargo Furnishings
Boeing Commercial Airplanes Group

Christopher Simon
Senior Designer
Cargo Furnishings
Boeing Commercial Airplanes Group

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