Onboard maintenance computer Maintenance and ARINC-429 circuitry + 28 VDC Loop A power Loop A detectors Loop A Loop B Up to four (4) detectors Up to four (4) detectors Loop B detectors Control module Alarm A C A C A C A C Fault Alarm Fault Identification ARIN C-429 data bus Loop B power + 28 VDC Figure 17-8.  Aircraft detection system control module. Smoke, Flame, and Carbon Monoxide  Detection Systems Smoke Detectors $^{ }$Fire Zones Light Refraction Type Ionization Type 17-8 5.  Class X zone—area of heavy airflow and of unusual  construction, making uniform distribution of the  extinguishing agent very difficult. Areas containing  deeply recessed spaces and pockets between large  4.  Class D zone—area of very little or no airflow. These  include wing compartments and wheel wells where  little ventilation is provided. 3.  Class C zone—area of relatively low airflow. An  engine accessory compartment separated from the  power section is an example of this type of zone. 2. Class B zone—area of heavy airflow past  aerodynamically clean obstructions. Included in  this type are heat exchanger ducts, exhaust manifold  shrouds, and areas where the inside of the enclosing  cowling or other closure is smooth, free of pockets,  and adequately drained so leaking flammables  cannot puddle. Turbine engine compartments may  be considered in this class if engine surfaces are  aerodynamically clean and all airframe structural  formers are covered by a fireproof liner to produce  an aerodynamically clean enclosure surface. 1.  Class A zone—area of heavy airflow past regular  arrangements of similarly shaped obstructions. The  power section of a reciprocating engine is usually of  this type. Some aircraft use an ionization type smoke detector. The  system generates an alarm signal (both horn and indicator)  by detecting a change in ion density due to smoke in the  cabin. The system is connected to the 28-volt DC electrical  power supplied from the aircraft. Alarm output and sensor  sensitive checks are performed simply with the test switch  on the control panel. The light refraction type of smoke detector contains a  photoelectric cell that detects light refracted by smoke  particles. Smoke particles refract the light to the photoelectric  cell and, when it senses enough change in the amount of light,  it creates an electrical current that sets off a warning light.  This type of smoke detector is referred to as a photoelectrical  device. Powerplant compartments are classified into zones based on  the airflow through them. A smoke detection system monitors the lavatories and cargo  baggage compartments for the presence of smoke, which is  indicative of a fire condition. Smoke detection instruments  that collect air for sampling are mounted in the compartments  in strategic locations. A smoke detection system is used  where the type of fire anticipated is expected to generate a  substantial amount of smoke before temperature changes are  sufficient to actuate a heat detection system. Two common  types used are light refraction and ionization. closed by the averaging gas pressure at all temperatures down  to –65 °F (–54 °C). If the detector should develop a leak, the  loss of gas pressure would allow the integrity switch to open  and signal a lack of detector integrity. The system then does  not operate during test. structural formers are of this type. Tests indicate agent  requirements to be double those for Class A zones.