Aircraft General Knowledge (CPL Level): Pressurisation and Hydraulic Systems Explained

Pilots at the Private level are usually at home in a very simple aircraft with few systems. However, when it comes to Commercial Pilot Licence (CPL) level training, the planes and systems become more advanced, complex and technical. This is a reflection of the aircraft types that one will end up operating as a professional, transport-category (passenger) aeroplane. Two of the most important aircraft systems in a pilot’s studies and training include the pressurisation system and the hydraulic systems. You must have heard it a hundred times before – at the CPL level, your theoretical knowledge and technical understanding are put under the spotlight. How? During your aircraft safety inspections or in your simulator training exercises where your instructor may put you through a number of failures to test your aircraft systems knowledge.

Why is it Important to know about Pressurisation?

Did you know that at 10,000 ft above sea level, the air has only about 2/3 the amount of oxygen available as on the ground? Passengers and crew are at risk of developing hypoxia at high altitudes, so it’s crucial for aircraft to have a pressurisation system. With pressurisation, we are able to fly safely at a cruising altitude of over 30,000 ft (yes, even higher in some cases) and maintain a cabin environment that’s similar to that at 6,000 – 8,000 ft above sea level. So, the aircraft systems for pressurisation are important to know for CPL candidates because they are related to safety and passenger comfort.

How does Pressurisation work?

The concept is simple: compressed air is provided, usually from the compressor stages of the engine (called engine bleed air) and then this air is conditioned (filtered, heated, cooled, humidified) and pumped into the cabin. The pressurisation system includes an outflow valve that vents air from the cabin to regulate the pressure inside. The rate at which air enters the cabin must be equal to the rate at which it leaves; otherwise, the internal pressure will rise or fall and may cause structural damage or discomfort to passengers and crew.

The main components of a pressurisation system are:

● 1. Cabin Pressure Controller (CPC)

● 2. Outflow valves

● 3. Safety relief valves and emergency pressure relief valves

● 4. Air conditioning packs

Pressurisation Malfunctions

There are a few key malfunctions to look out for, including:

● 1. Slow loss of pressure – can go undetected for a while

● 2. Rapid decompression – must descend to a lower, safe altitude and fit oxygen masks to passengers

● 3. Pressurisation controller failure or outflow valve stuck open – pressure will fall

● 4. Safety valve or pressure relief valve failure – pressure may increase rapidly

● 5. AC pack failure – discomfort to passengers and crew

The systems for pressurisation are an important area of aircraft general knowledge at the CPL level. The student is trained to know how to identify malfunctions early and to react quickly.

The Role of Hydraulics in an Aircraft

In many small general aviation (GA) airplanes, cables and pulleys do a fine job moving the flight controls. However, on more complex and larger aircraft, hydraulics provide smooth, powerful operation of critical components such as:

● 1. Flight controls (ailerons, elevators, rudder)

● 2. Landing gear extension/retraction

● 3. Flaps and slats

● 4. Brakes and thrust reversers

Without hydraulics, pilots would lack the physical strength required to move control surfaces against the loads on high-speed flight.

How Hydraulic Systems work

The incompressibility of fluids is the key principle. A hydraulic pump pressurises the hydraulic fluid which is then directed by valves to actuators which convert the hydraulic pressure into mechanical force and motion.

Components of a hydraulic system include:

● 1. Hydraulic pumps – may be engine-driven, electric or even ram air turbine (RAT) driven in an emergency

● 2. Reservoirs

● 3. Actuators

● 4. Accumulators – store pressure for emergency use

Reliability and Redundancy

Because hydraulic systems are so critical, there are usually multiple independent hydraulic systems on transport category (airliner) aircraft. This is so that if one system fails, the aircraft can still maintain its essential hydraulic functions. In many airplanes, components are powered by more than one hydraulic system to provide redundancy.

Hydraulic Malfunctions

Key malfunctions to understand include:

● 1. Hydraulic fluid leaks – will reduce system pressure and performance

● 2. Pump failures – may be backed up by redundant pumps or use demand pumps, which only operate when needed

● 3. Contamination – may lead to wear or valve/actuator malfunction

At the CPL level, there is no shortage of topics to cover and understand to pass the exams. However, systems like pressurisation and hydraulics are vital to know well because of their importance to safety and aircraft function. Many aircraft have simulated failures in the sim to ensure the student not only has the aircraft systems knowledge but also the airmanship and presence of mind to deal with issues while remaining in control and prioritising tasks.

This depth of knowledge is exactly why structured programs like the Diploma of Aviation in Australia are so valuable—they bridge the gap between theory and practical application. Similarly, international schools, such as a Hong Kong Flight Training Centre, emphasise advanced system knowledge for students aspiring to fly complex aircraft in airline environments.