12.6 and 12.13 Helicopter air conditioning, ice and rain protection

Scope

• 12.6.1 Air supply (A 1, B1.3 2): sources of air including engine bleed and ground cart.
• 12.6.2 Air conditioning (A 1, B1.3 3): air-conditioning systems, distribution, flow and temperature control, protection and warning devices.
• 12.13 Ice and rain protection (ATA 30) (A 1, B1.3 3): ice formation, classification and detection; anti-icing and de-icing (electrical, hot air, chemical); rain repellent and removal; probe and drain heating; wipers.

Helicopters are normally unpressurised, so ATA 21 on helicopters is about heating, ventilation and cooling, not pressure control.

Air supply sources

Source	Notes
Engine bleed air	Tapped from the turbine-engine compressor; hot and at pressure. Used for cabin heating, ACM drive and anti-icing. Taking bleed reduces available power and raises turbine temperature
Ram / ventilation air	Through scoops and NACA inlets; used for fresh-air ventilation
Fans/blowers	Electrically driven; circulate cabin air and push air through heat exchangers
Ground cart / ground conditioning unit	Supplies conditioned air on the ground without running engines
Exhaust-shroud heater	Piston helicopters: ambient air heated by passing over the exhaust muffler — risk of carbon monoxide leaks if the shroud cracks
Combustion heater	Burns aircraft fuel in a sealed chamber to heat ventilation air; needs overheat protection

Cooling: vapour cycle and air cycle

Vapour cycle (most common on helicopters because it works on the ground and needs no bleed air):

1. Compressor (electric or belt/gearbox driven) raises refrigerant vapour pressure and temperature.
2. Condenser rejects heat to outside air; vapour condenses to high-pressure liquid.
3. Receiver-dryer stores liquid and removes moisture; sight glass shows bubbles if undercharged.
4. Expansion valve meters liquid into the evaporator, dropping its pressure.
5. Evaporator in the cabin airflow — the refrigerant boils, absorbing heat from cabin air.

Refrigerant is normally R134a (older systems used R12). The two must never be mixed; servicing needs gloves and eye protection.

Air cycle machine (ACM) — used on larger turbine helicopters: bleed air is cooled in a heat exchanger, compressed, cooled again, then expanded through a turbine that drives the compressor ("bootstrap"); the expansion produces cold air. A water separator removes condensed moisture.