Pressurisation & ice protection

021 Aircraft General KnowledgeChapter 7

Pressurisation fundamentals

Key fact

Above 10 000 ft, ambient pressure insufficient for normal breathing — pressurisation required. Differential pressure typically 7–9 psi on large jets.

Cabin altitude = Flight level pressure − differential pressure. Positive pressure = cabin above ambient. Negative pressure relief valves open if cabin below ambient.

Percentage of oxygen in cabin air remains constant regardless of pressurisation — it is partial pressure that matters.

Key fact

Cabin pressure controller: Maintains cabin altitude, rate of change, and max differential by constant inflow + controlled outflow via outflow valves (partially open in cruise). Cabin pressure regulated by air leaving the cabin.

Two modes: (1) Constant differential — slowly changing ΔP in climb/descend. (2) Isobaric — same cabin pressure during climb/descend in cruise band.

Trigger	Action
10 000 ft cabin	Excessive cabin altitude warning — don masks
14 000 ft cabin	Pax oxygen auto-deployed
15 000 ft cabin	Outflow valve auto-closes

Exam trap

Cabin altitude decreasing = pressure inside increasing = cabin rate of climb decreasing. If outflow valves locked in level flight, cabin pressure rises (cabin alt indicator shows descent). Ditching control: Closes outflow valves for water landing.

Pre-pressurisation on ground: ~0.1 psi below ambient when throttles advanced — smoother climb transition, prevents exhaust ingestion.

Bleed air and air conditioning

Key fact

Bleed air: Usually LP stage of HP compressor (5th stage); at low RPM from HP stage (9th) via check valve. Used for pressurisation, air conditioning, reservoir pressurisation, anti-ice, engine start.

Bootstrap / air cycle machine (ACHE): (1) Primary heat exchanger → (2) Compressor → (3) Secondary heat exchanger → (4) Expansion turbine → water separator. Turbine drives compressor; expansion reduces T and P.

Key fact

Combustion heater (MEP): Separate from exhaust — prevents CO poisoning. Pack cooling fans: Ram air through heat exchanger during slow flight/ground. Short-haul turbojet: Cabin humidity often not controlled.

Piston pressurisation: turbocharger, Roots blower, or ram/manifold air. Roots blower on AGB avoids robbing engine compressor/turbo efficiency.

Ice protection

Key fact

Icing conditions: Visible moisture with OAT below 10°C (ground/T/O) or TAT below 10°C (in flight). Critical range +10°C to −10°C for large supercooled droplets.

Anti-ice: Prevents formation — bleed air (large jets, HP compressor), electrical (pitot, windshield, props), fluid (weeping wing). De-ice: Removes ice — pneumatic boots (turboprops, ~1.5 cm ice visible before cycle).

Method	Application
Bleed air / thermal	Wings, engine inlets — reduces max thrust
Electrical	Pitot, static, windshield (3-phase AC, 18–35°C cycle)
Pneumatic boots	Turboprop wings — de-ice only
Slinger rings	Propeller de-ice

Key fact

Ice detectors: Vibrating rod (frequency change), pressure/Smith (holes freeze, ΔP drop), hot rod (visible probe + floodlight), rotor (torque-based). Advisory: Crew activates per manual; detector backup. Automatic: Detector activates systems at optimal moment.

Windshield heat: Continuous use reduces thermal gradients and extends life. Prop anti-ice: lower blade section heated; spinner oscillates to shed ice.

Exam trap

Turbo-prop typical: Anti-ice at engine inlets, pitot/static; de-ice on wings; prop electrically protected. Bleed anti-ice: over-temperature warning; reduces thrust but not aero performance of wing shape.

Oxygen systems

Key fact

Flight deck: Diluter-demand gaseous regulator. Normal/dilute: O₂/air ratio increases with altitude; not used with smoke (mixes cabin air). 100% above 33 000 ft or for smoke. Emergency: Continuous 100% under positive pressure.

Cabin chemical generators: 15 min supply; pull mask down to start; cannot switch off; heat shield fitted; poor for severe smoke (toxic mix). Gaseous cabin: Modulated flow, greater capacity, more fire/explosion risk.

Key fact

Supplementary oxygen: Protects against accidental depressurisation — masks before cabin exceeds 15 000 ft on rapid decompression. No grease near oxygen (spontaneous combustion). Portable bottles: flow indicated by pressure.

Pressurised aircraft above 25 000 ft cert: emergency oxygen required. Unpressurised above 10 000 ft: oxygen required.

Fire and smoke detection

Key fact

Engine fire bottles (×2 per engine): Halon (BCF) — interrupts combustion, not cooling; electrically fired squib ruptures seal; agent to LP end of compressor exterior. Fire handle: Arms extinguishers, closes LP fuel valve, closes bleed/isolation valves, deactivates generators.

Detection types: Continuous loop (resistance/capacitance — R↓ C↑ triggers alarm); bimetal strip (parallel, delay for vibration); gaseous (tube pressure switch).

Exam trap

Smoke detectors (cargo): Optical (light scatter to photocell) or ionisation. Toilet fire: Automatic heat-activated extinguisher.

Engine overheat vs fire: EGT drops when throttle retarded = overheat; EGT stays high = likely fire.

Rain and cold-soak

Key fact

Rain repellent: Hydrophobic chemicals — use only in heavy rain; bleed air not used. Cold-soaked wing: After high-altitude flight, cold fuel causes hoar frost or clear ice on wing surface — refuel after last flight of day to warm tanks overnight.