Pressurisation & decompression

070 Operational ProceduresChapter 2

Cabin altitude warning

Key fact

Excessive cabin altitude horn sounds above 10,000 ft cabin altitude. Flight deck requires quick-donning masks when operating above FL250. Above FL410, one pilot must be on oxygen at all times. Masks sufficient to cover 110% of seating capacity must be carried above FL250.

Decompression types

Definition — Rapid decompression

First sign is often a BANG, followed by mist and cabin altitude increase. First action: don oxygen mask, then initiate emergency descent to 8,000 ft (crew efficiency not impaired) or MSA (whichever is higher).

Definition — Slow decompression

Caused by leaks — gradual cabin altitude rise. Same initial actions: oxygen mask, then controlled emergency descent.

Physiological limits

Altitude	Effect
6,000 ft	Lack of O₂ begins to degrade performance
8,000 ft	Maximum altitude for 100% crew efficiency without supplemental O₂
FL320	Pure O₂ required (diluter-demand system)
FL250	Quick-donning masks compulsory
FL410	One pilot continuously on oxygen

Key fact

TUC at 40,000 ft: approximately 12 seconds useful consciousness without oxygen. PBE (Protective Breathing Equipment) provides oxygen for over 15 minutes.

Emergency descent procedure

Key fact

Emergency descent to 8,000 ft or MSA — performance is not affected by the descent itself. After donning masks, crew should establish communication, descend promptly, and advise ATC. Cabin crew attempt to determine source of decompression; if source cannot be determined, divert.

Pressurised vs unpressurised oxygen rules

Key fact

After decompression, flight crew can operate without supplemental O₂ with no time limit at 10,000 ft. All crew above 13,000 ft need supplemental O₂. Passengers: 100% for entire flight above 15,000 ft pressure altitude (minimum 10 minutes).

First aid oxygen

Key fact

Required when operating above 25,000 ft or when cabin altitude is between 8,000 ft and 15,000 ft for remainder of flight. Sufficient for 2% of passengers carried (minimum 1 passenger). Average flow rate 3 L/min STPD (starts at 4 L/min, decreases to 2 L/min minimum). Minimum 2 dispensing units.

RVSM altimetry (related)

Check	Tolerance
Pre-flight (aerodrome elevation)	± 75 ft
In-flight primary altimeter cross-check	± 200 ft (max 1 hr intervals)
Assigned altitude error tolerance	± 300 ft
Level-off rate	under 1,500 fpm
Level overshoot/undershoot	± 150 ft

Exam trap

First action in any decompression is don the oxygen mask — before descent, communication, or troubleshooting. Do not confuse cabin altitude horn (10,000 ft) with passenger drop-out masks (15,000 ft cabin altitude). Emergency medical kit required when MOPSC over 30 AND route over 60 min from qualified medical assistance.