Aircraft Cabin Pressurization Unit Calibration

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.


CPU Mechanism – Operation Principle:

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.


Aircraft CPU – Auxiliaries:

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:


  1. Positive Pressure Relief Valve: All aircraft cabins are structurally designed for a maximum pressure differential limit. Exceeding this limit (CPU pumps too much air pressure into the fuselage) can cause damage by blowing out doors and windows. To ensure aircraft from over pressurizing, positive pressure relief valves are installed. These devices (also called butterfly valves) are spring-loaded to vent excess air pressure when cabin pressure exceeds the designed (Max) limit.
  2. Negative Pressure Relief Valve: Negative pressure differential condition is the reverse situation of Positive differential, meaning the pressure outside the cabin is greater than the cabin pressure. This may occur during the rapid descent. Negative pressure is dangerous as it pushes the cabin inward, pushing the doors and windows. To avoid this situation, similar to positive pressure relief, spring-loaded devices protect the fuselage from damage. Air pressure of less than 1.0 psi against the outside of the doors causes them to open inward against the spring load, venting air into the fuselage to equalize the pressure.


Types of Aircraft Cabin Pressurization Systems:

Advancements in technology paved the way for several CPU technologies such as:


  1. Isobaric: The most commonly found CPU system maintains the cabin pressure at a constant value regardless of the outside air pressure.
  2. Isobaric Differential: Found in certain Military fighter aircraft, these CPUs begin pressurization on ascent until the cabin reaches a pre-set altitude. If the altitude is reached, a constant pressure differential is maintained in correlation with the ambient pressure outside the airplane.
  3. Sealed Cabin: Used in spacecraft applications, where the vessel carries its own supply of gases.


Mechanism of Modern (Isobaric) Aircraft CPU Systems:

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:


  1. Electric Compressors: These comprise the conventional Piston – Cylinder powered system to pump air into the cabin. Though the system worked well, these heavy compressors added a lot of weight to the aircraft, affecting the operation efficiency. However, advancements in metallurgy and material science have reduced compressor weights, so modern aircraft such as B787 use electric compressors to pressurize the air.
  2. Turbocompressors: Early jetliners, such as the Douglas DC-8 and Boeing 707, started using bleed air from the engines to spin the turbo compressors, which in turn pumped fresh outside air into the cabin.
  3. Engine Bleed Air: Most modern airliners use bleed air from the compressor section of the engines to pressurize the cabin. This scorching air is cooled to a comfortable temperature using intercoolers before directing into the cabin.


Preventative Maintenance Program for Aircraft Cabin pressurization:

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:


  1. Airworthiness Directives (ADs): If the CPU or ECS system or component is deemed unsafe, the FAA may issue an AD, requiring it to be fixed before the flight to be an approved unit. These required safety alterations can be conducted during preventive maintenance.
  2. Proper documentation: Irrespective of the personnel performing the preventive maintenance activity, the work should be recorded in the appropriate aircraft logbook.
  3. Proper displays: Post maintenance activity, the required placards should be installed, and that the aircraft registration and certificate of airworthiness are in the aircraft and accessible.
  4. Cleanliness standards: The Occupational Safety and Health Administration (OSHA) has a wide range of regulations in place to protect workers, including the need for a clean and safe workspace. These steps are to be followed within a preventive maintenance activity.


Preventive maintenance also involves fault analysis, regular training, and maintaining data records of all the activities performed.


Advantages of Preventative Maintenance Program – Why is it essential:

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.


e2b calibration offers industry-leading consultancy and certified PM services for your aircraft cabin pressurization systems. Our labs are ISO/IEC accredited and operated by a team of qualified experts providing training and consultancy services on Aircraft Cabin Pressurizer maintenance and inspection. Our verifiable services are unmatched in the industry. Contact e2b calibration for all your equipment calibration needs.



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