Aircraft fly between 30,000 and 43,000 feet in the earth’s stratosphere region. At such high altitudes, the atmosphere provides less than four(4) psi of air pressure, insufficient for normal humans to breathe. Hence, aircraft use Cabin Pressurization Units (CPU) to pump sufficient air into the airplane, so the internal pressure is high enough to breathe.
Aircraft CPU units constantly pump fresh, outside (atmospheric) air into the fuselage. A motorized door or an outflow valve is located near the aircraft’s tail is used to control and maintain the interior pressure within the cabin and exit the air. Heavier aircraft often have two outflow valves. The aircraft’s pressurization system automatically controls the valves. When higher pressure is required within the cabin, the outflow door closes. And to reduce the cabin pressure, the door slowly opens, allowing more air to escape. This automatic pressurization mechanism ensures a constant flow of clean, fresh air moving through the aircraft cabin.
During normal operations, the air inside the cabin is completely changed every two or three minutes making it far cleaner than the air in the sea-level closed room conditions. CPUs are designed to maintain the interior air cabin pressure in the range of 12 ~ 11 psi at cruise altitude. On a typical flight, while the aircraft climbs to 36,000 feet, the plane’s interior “climbs” to between 6000-8000 feet.
While CPUs are designed to maintain constant cabin pressure, changes in cabin pressures are abrupt during rapid ascent and descent. Two types of mechanical devices are installed on the fuselage to protect the pressurized section of the aircraft against excessive pressure differential. Differential pressure is the difference between the air pressure inside and outside the aircraft. Exceeding the differential pressure limit makes a balloon pop when it’s overinflated. To protect the fuselage from excessive differential pressure, two types of relief valves are used:
Advancements in technology paved the way for several CPU technologies such as:
As mentioned in the above section, Isobaric CPUs pressurize and maintain steady air pressure within the cabin. To develop the pressure, aircraft designs have been using different technologies and machines, as follows:
Modern airline CPUs are integrated with the Environmental Control System (ECS) of an aircraft, which in turn are responsible for regulating and conditioning the airflow into the cockpit, cabin, and avionics bay. The ECS of an aircraft consists of three different complex subsystems to provide conditioned air with the correct temperature, pressure, and humidity. They are:
Preventative maintenance of CPU or ECS should involve the maintenance of BAS, PACK, and the ADS components and systems.
Preventive maintenance pertains to the activities and the personnel authorized to perform them (as listed in FAR 43.3, Appendix A), provided it does not involve complex assembly operations.
According to the FAA, there are several types of maintenance -Preventative Maintenance, Maintenance, Alterations, and Repairs. Following are the Regulations for Preventative Maintenance of CPU and ECS:
Preventive maintenance also involves fault analysis, regular training, and maintaining data records of all the activities performed.
Establishing and maintaining an aircraft PM Program for ECS appears complex and time-consuming, with several long-term benefits in terms of uptime and aircraft availability.
Regulatory authorities also produce a list of required maintenance activities. For small aircraft under FAR Part 91, the list of necessary maintenance is confined to:
The time between overhauls (TBO), service bulletins, and service letters are also included for larger aircraft. As per the guidelines for new aircraft under FAR Part 21.183 (a), (b), and (c), two conditions are to be met to deem the aircraft airworthy:
With a standard PM Plan for CPUs and ECS, airlines can provide safe cabin space for passengers.
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