Airframe Systems and Components
Core concepts
This domain covers pressurization, air conditioning, oxygen, fuel, ice and rain protection, and fire detection/extinguishing. Candidates must understand the pressurization cycle: outflow valve controls cabin altitude, cabin pressure differential is limited by a relief valve, and the pressure controller schedules climb and descent rates. Know the difference between the negative pressure relief valve and the positive pressure relief valve, and why the negative relief prevents damage during rapid descent. Understand air-cycle machine operation: bleed air, primary and secondary heat exchangers, compressor, turbine, and water separator. For fuel systems, know venting, boost pumps, crossfeed, and unusable fuel. For ice protection, distinguish anti-ice (prevention) from de-ice (removal), and know thermal, pneumatic boot, and fluid systems. Fire detection uses thermocouples, Fenwal or Kidde loops; extinguishing uses halon or approved agents.
Traps in questions
A common trap is confusing cabin altitude with aircraft altitude, or assuming the outflow valve opens to increase cabin pressure. It does the opposite: opening the outflow valve decreases cabin pressure. Another trap is treating anti-ice and de-ice as synonyms. Questions may ask which valve prevents overpressure versus negative pressure; read carefully. Also watch for questions on fuel imbalance: crossfeed is used to correct imbalance, but the affected engine must be managed to avoid fuel transfer to the wrong tank.
Worked example / rule of thumb
Rule of thumb: cabin pressure differential equals cabin pressure minus ambient pressure. If cabin is held at 8,000 ft equivalent (about 10.9 psia) while aircraft is at 35,000 ft (about 3.5 psia), differential is about 7.4 psi. If the structural limit is 8.6 psi, the relief valve must open before that limit. Always check which side of the equation the question is asking for.