6.5 Environmental Systems: Pressurization, Heating, Cooling, and Oxygen

Cabin Pressurization

Pressurization lets an aircraft fly high (less drag, above weather) while keeping the cabin altitude low enough for comfort and safety. Conditioned air — bleed air on turbines, or air from a turbocharger/engine-driven compressor on pistons — is pumped continuously into the sealed cabin, and a controlled amount is released through an outflow valve. A transport cabin is typically held near or below an 8,000 ft cabin altitude at cruise.

The key parameter is differential pressure (delta-P) = cabin pressure minus ambient pressure. Each fuselage is certified to a maximum delta-P (often roughly 8-9 psi on transports); exceeding it overstresses the structure.

Troubleshoot pressurization through source air → controller → outflow valve → safety/relief valves → ducts and door/window seals → leaks, before replacing components. Door and window seals and structural sealing are common slow-pressurization causes.

The crew sees three indications: cabin altitude (feet), cabin rate of climb (feet per minute, kept gentle — often around 300-500 fpm climb, 300 fpm descent for comfort), and differential pressure (psi). A cabin that will not pressurize points to source air or a stuck-open outflow valve or a major leak; a cabin that over-pressurizes points to a stuck-closed outflow valve, with the safety/relief valve as the last-line structural protection.

Plug-type door seals that inflate are checked for the correct inflation source and for tears, because a leaking door seal bleeds pressurization continuously. Always verify the source of cabin air first — on a turbine, a failed bleed/pack will starve the cabin no matter how good the outflow valve is.

Heating and Cooling

Heating sources vary by aircraft:

• Exhaust heat exchanger (shroud): light singles route cabin air over a muff around the exhaust. The danger is an exhaust crack that admits carbon monoxide (CO) into the cabin — inspect heat exchangers and the muff carefully and check for CO contamination.
• Combustion heater: a small fuel-burning heater (its own combustion chamber, vented overboard) on many twins; cracks or leaks again risk CO, so it is leak- and crack-inspected.
• Bleed-air heat: turbines simply route hot bleed air into the air-conditioning mix.

Cooling uses two architectures:

The ACM ("bootstrap") needs no refrigerant; the vapor-cycle system must be serviced with the correct refrigerant and is subject to refrigerant recovery rules — never vent it to atmosphere. Inside the ACM pack, a water separator wrings condensed moisture out of the cold air before it reaches the cabin, and a temperature-control (mixing) valve blends hot bleed air with cold pack output to hold the selected cabin temperature.

In a vapor-cycle system the expansion valve meters liquid refrigerant into the evaporator, where it boils and absorbs cabin heat; the compressor then raises its pressure and the condenser rejects that heat overboard. Low cooling in a vapor-cycle system traces to low refrigerant charge, a faulty expansion valve, a weak compressor, or a blocked condenser, diagnosed with gauges per the manufacturer rather than by guesswork.

Oxygen Systems

At altitude, supplemental oxygen prevents hypoxia. Two main types:

• Gaseous (stored) oxygen: high-pressure green cylinders charged to about 1,800-2,000 psi, regulated down to crew/passenger masks. Use aviator's breathing oxygen (low moisture to prevent line freezing). Cylinders are DOT/hydrostatic-test dated.
• Chemical oxygen generators: solid sodium-chlorate 'oxygen candles' that release oxygen when ignited — common for drop-down passenger masks. They get very hot, are single-use, have an expiration date, and demand careful handling, storage, and shipping (a hazardous material).

The non-negotiable rule: keep oxygen systems scrupulously free of oil, grease, and hydrocarbons. High-pressure oxygen in contact with petroleum can ignite or explode. Use only oxygen-approved, clean fittings and lubricants; never use thread compounds or oils not specifically approved for oxygen; charge slowly to limit heat of compression; and observe NO SMOKING and bonding/grounding precautions. Leak-check with an approved, oxygen-safe solution, and purge/cap lines to keep contamination out.

Servicing safety summary: secure cylinders, relieve pressure before opening, respect chemical-generator heat, and treat any oil contamination of an oxygen line as a serious hazard requiring component replacement, not wiping.

A fourth source is the liquid oxygen (LOX) converter found mainly on military aircraft: it stores oxygen as a cryogenic liquid that expands enormously into gas, saving space and weight, but adds extreme cold-burn and over-pressure hazards. For all systems, fill cylinders slowly — rapid charging raises temperature (heat of compression), and topping a hot cylinder leaves it underfilled once it cools. Cylinders carry hydrostatic-test intervals (commonly every 3 or 5 years depending on type) and a service-life limit.

After any oxygen work, purge the lines and leak-check with an approved solution, never a petroleum-based detergent. The combination of high pressure, an oxidizer, and any hydrocarbon is the single most explosive situation a mechanic handles, which is why oxygen cleanliness is its own dedicated procedure rather than general shop practice.