Why aircraft use air-cycle refrigeration Aircraft commonly employ air (reversed Brayton) refrigeration cycles primarily because:

Introduction / Context:
Air-cycle (reversed Brayton) systems are favored in aviation. Understanding the engineering trade-offs explains why COP is not the sole selection criterion.

Given Data / Assumptions:

• Aircraft already carry air compressors/turbomachinery (bleed air/ram air).
• Weight, reliability, and maintenance are critical constraints.

Concept / Approach:
Although air-cycle COP can be lower than that of vapour-compression, the system offers major advantages: light weight, no refrigerant handling, tolerance to leaks, and effective use of available ram air and high-altitude low ambient temperatures.

Step-by-Step Solution:
Assess competing drivers: mass and simplicity dominate in aircraft.Air cycle uses air as the working fluid; no evaporator with phase change refrigerant is required, reducing complexity and weight.Altitude provides cold sink air to enhance cooling performance.

Verification / Alternative check:
Aircraft environmental control systems (ECS) widely use bootstrap/air-cycle packs that prioritize weight and reliability over peak COP.

Why Other Options Are Wrong:

• Highest heat-transfer coefficients: not the primary justification.
• “Only” low ambient temperature: it helps but is not the sole reason.
• Higher COP than vapour cycles: generally untrue.
• No compressor: air cycle still uses a compressor or utilizes bleed/ram compression.

Common Pitfalls:
Equating selection solely with COP while ignoring weight and system integration.

Final Answer:
They provide low weight per tonne of refrigeration and simple, robust hardware.