Aircraft environmental control — most used air-cycle system. Which air cooling (reverse Brayton) system is most commonly used on transport-type aircraft for cabin air-conditioning and cooling?

Introduction / Context:
Modern transport aircraft rely on air-cycle (reverse Brayton) refrigeration packs to condition cabin air using bleed air or compressor stages. Among several air-cycle layouts, one configuration dominates because it efficiently reaches the pressure and temperature levels required at altitude while remaining compact and lightweight.

Given Data / Assumptions:

• Transport (civil) aircraft with engine bleed air available.
• Need for compact, reliable, and efficient cooling at high altitude.
• Typical ECS (Environmental Control System) architectures.

Concept / Approach:
The boot-strap air-cycle system uses two compressors (or compressor stages) with an intercooler and a turbine expander. The “boot-strap” name reflects how one compressor stage feeds another to achieve a higher pressure ratio before expansion. This improves the temperature drop across the turbine and boosts cooling capacity for a given mass flow, which suits long-range transports.

Step-by-Step Solution:
Compare systems: Simple air-cycle offers minimal components but limited pressure ratio and cooling at altitude.Evaporative variants add moisture and are uncommon in aircraft ECS due to icing and water handling challenges.Boot-strap integrates staged compression and effective heat rejection to maximize turbine work output and cooling.Therefore, the most used arrangement is the boot-strap system.

Verification / Alternative check:
Standard aircraft systems diagrams in aerospace texts show boot-strap or boot-strap with reheat/recool arrangements as baseline for transport jets, verifying its prevalence.

Why Other Options Are Wrong:
(a) The simple system lacks performance at high altitude. (b) Evaporative systems are not practical for aircraft ECS. (d) “All of these” is too broad; one layout is predominant.

Common Pitfalls:
Confusing auxiliary ground-cooling methods with in-flight ECS. Also, assuming evaporative cooling is favored because it is common in buildings; aircraft constraints differ dramatically.

Final Answer:
boot-strap air cooling system