Cargo Holds

Both cargo holds are designed to confine a fire without endangering the safety of the aircraft. A cargo fire detection and extinguishing system may be fitted as an option (see Fire Protection). NB If a hold ceiling light lens cover is broken or missing this is a fire hazard and the bulb should be removed if the cover can not be fixed.

Cargo-Compartment Class
Model	Lower cargo compartment	Main cargo compartment
737-100/-200/-300/-400/-500 (line no. 1-3078	Class D	Not required
737-300/-400/-500 (line no. 3079 and higher)	Class C	Not required
737-600/-700/-800 (line no. 1-90)	Class D	Not required
737-600/-700/-800 (line no. 91 and higher)	Class C	Not required
737-C/QC	Class D	Class E

Class C: Compartments that contain both the smoke-detection and fire-suppression of 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 reignition or spreading of the combustion.
Class D: Compartments that depend on oxygen deprivation to prevent and suppress combustion.
Class E: Compartments that have a smoke-detection system alert the flight crew within 60 sec from the time smoke first appears in the compartment.

The holds are sealed and pressurised but have no fresh air circulation. They have no temperature control but are heated by exhausting cabin air around their walls. The forward hold also has additional heating from E & E bay air. Live cargo can be carried on either cargo compartment but the forward hold is preferred.
The optimum distribution for loading the cargo holds to keep the aircraft in balance is as follows: 

Series	Fwd	Aft
-300/700	1/3	2/3
-400/800	0	All
-500/600	All	0