Aeroplane Equipment and Instruments — Page 90, Lesson 142

Let’s start with the definition of supplemental oxygen. Supplemental oxygen means an oxygen supply that adds to the natural oxygen already in the atmosphere — it’s not pure oxygen from a separate source for every breath, it’s extra oxygen you breathe when the air around you doesn’t have enough. The regulations for carrying it are split into two tables, and we’re looking at the first one: the minimum requirements for pressurised aeroplanes. Pressurised aeroplanes are the key here — these rules apply when the aircraft has a pressurisation system that can keep the cabin at a lower altitude than the aircraft is flying at. But even in a pressurised aeroplane, the cabin pressure altitude can rise if the system fails or if you’re operating at very high altitudes. So the rules are based on cabin pressure altitude, not the actual flight altitude. Let’s go through the five categories one by one. Category 1 — all occupants of flight deck seats on duty. That means the pilots, and any other crew member who must be at a flight deck station during the flight. For them, you must supply supplemental oxygen for the entire flight time when the cabin pressure altitude exceeds 13 000 feet. Also, when the cabin pressure altitude is above 10 000 feet but not above 13 000 feet, you must supply it for the entire flight time after the first 30 minutes at those altitudes. So between 10 001 and 13 000 feet, you get a 30‑minute grace period; above 13 000 feet, no grace period — oxygen immediately for the whole time. But there’s a minimum duration you must carry, regardless of the actual flight time. For aeroplanes certificated to fly at altitudes not exceeding 25 000 feet, the minimum is 30 minutes. For aeroplanes certificated to fly at altitudes more than 25 000 feet, the minimum is 2 hours. And there’s an important equipment note: the oxygen masks fitted to these aircraft must be quick-don style — that means a mask the pilot can put on rapidly with one hand, usually stored right next to the seat, so they don’t waste time fumbling in an emergency. Category 2 — all required cabin crew members. For them, you must supply oxygen for the entire flight time when cabin pressure altitude exceeds 13 000 feet, but not less than 30 minutes. And again, when cabin pressure altitude is greater than 10 000 feet but not exceeding 13 000 feet, you supply it for the entire flight time after the first 30 minutes at those altitudes. So the cabin crew get the same 30‑minute minimum as the flight deck for the high‑altitude case, but note: the 2‑hour minimum for aircraft above 25 000 feet applies only to the flight deck crew, not to cabin crew in this table. Category 3 — 100% of passengers. For all passengers, you must supply oxygen for the entire flight time when the cabin pressure altitude exceeds 15 000 feet, but in no case less than 10 minutes. So above 15 000 feet cabin altitude, every passenger gets oxygen for the whole time, with a floor of 10 minutes even if the exposure is very short. Category 4 — 30% of passengers. When the cabin pressure altitude exceeds 14 000 feet but does not exceed 15 000 feet, you need enough oxygen for 30% of the passengers for the entire flight time at those altitudes. So between 14 001 and 15 000 feet, you don’t need to cover everyone — just 30% of them. Category 5 — 10% of passengers. When the cabin pressure altitude exceeds 10 000 feet but does not exceed 14 000 feet, you need oxygen for 10% of the passengers for the entire flight time after the first 30 minutes at those altitudes. So between 10 001 and 14 000 feet, there’s a 30‑minute grace period, and then you need to cover 10% of the passengers. Notice the pattern: the higher the cabin altitude, the greater the percentage of passengers you must protect, and the shorter or non‑existent the grace period. Also, the flight deck crew always get the most stringent requirements — they must have quick‑don masks and longer minimum supplies for high‑altitude aircraft. The tables in the book also have notes, which I’ll mention as we go. Note 1 tells us these are for pressurised aeroplanes. Note 2 and 3 refer to the minimum durations I already covered. Note 4 sets the 10‑minute floor for the 100% passenger case. Note 5 is a general reference to the passenger percentages. That figure shows the first‑aid kit requirement, but the oxygen table is the core of this excerpt. So to summarise: you carry supplemental oxygen based on cabin pressure altitude, with different rules for flight deck crew, cabin crew, and passengers, and with minimum durations that depend on the aircraft’s certification altitude. The masks for the flight deck must be quick‑don. That’s the foundation for understanding oxygen carriage in pressurised aeroplanes.