Aeroplane Equipment and Instruments — Page 90, Lesson 143

I want to walk you through the regulatory requirements for supplemental oxygen in unpressurised aeroplanes. This is a critical safety topic because as you climb, the air gets thinner, and without supplemental oxygen, your body — and your passengers' bodies — can suffer from hypoxia. Let's start with the notes that set the foundation. The first note tells us that the oxygen supply provided must take account of the cabin pressure altitude descent profile for the routes concerned. In plain language, when you plan a flight, you have to consider what the cabin altitude would do during a descent — not just a straight line, but the actual profile — and size your oxygen accordingly. Now, notes 2 through 4 give us three different minimum supply calculations, and they apply to different situations. Note 2 says the required minimum supply is that quantity of oxygen necessary for a constant rate of descent from the aeroplane’s maximum certificated operating altitude to 10,000 feet in 10 minutes, followed by 20 minutes at 10,000 feet. So you need enough oxygen to descend steadily from your highest possible altitude down to 10,000 feet over ten minutes, then cruise at 10,000 feet for another twenty minutes. Note 3 is similar but extends the time at 10,000 feet: it's a constant rate of descent from maximum certificated operating altitude to 10,000 feet in 10 minutes, followed by 110 minutes at 10,000 feet. That's a much longer holding period — nearly two hours at 10,000 feet. Note 4 is different: it's a constant rate of descent from maximum certificated operating altitude to 15,000 feet in 10 minutes. No holding time at 15,000 feet is specified — just the descent. Note 5 is important for interpreting the passenger numbers. It says that for the purpose of this table, 'passengers' means passengers actually carried and includes infants. So an infant in arms counts as a passenger for oxygen requirements — they are not excluded. Now let's move to the table itself, which lays out the minimum supplemental oxygen requirements for unpressurised aeroplanes. The table has two columns: "Supply For" and "Duration and Pressure Altitude." Row 1: All occupants of flight deck seats on duty — that means the pilots — must receive supplemental oxygen for the entire flight time at pressure altitudes above 10,000 feet. So if you're flying above 10,000 feet pressure altitude, you need oxygen on from the moment you pass through that level until you descend below it. Row 2: All required cabin crew members must receive oxygen for the entire flight time at pressure altitudes above 13,000 feet. Additionally, for any period exceeding 30 minutes at pressure altitudes above 10,000 feet but not exceeding 13,000 feet, they also need oxygen. So between 10,000 and 13,000 feet, you have a 30-minute grace period; after that, oxygen is required for cabin crew. Row 3: 100% of passengers — and remember, that includes infants — must receive oxygen for the entire flight time at pressure altitudes above 13,000 feet. So above 13,000 feet, every single passenger on board needs supplemental oxygen. Row 4: 10% of passengers must receive oxygen for the entire flight time after 30 minutes at pressure altitudes greater than 10,000 feet but not exceeding 13,000 feet. So between 10,000 and 13,000 feet, after the first 30 minutes, you need to have oxygen available for at least 10% of your passengers. This is typically to cover those who might be more susceptible to hypoxia. There's a repeated note at the bottom of the table: for the purpose of this table, 'passengers' means passengers actually carried and includes infants under the age of 2. So the same definition applies — infants are counted as passengers for oxygen requirements. Let me tie this together with a practical picture. If you're flying an unpressurised aeroplane at 12,000 feet pressure altitude, here's what you need: pilots on oxygen the whole time, cabin crew after 30 minutes, and 10% of passengers after 30 minutes. If you climb to 14,000 feet, now pilots are still on oxygen, cabin crew are on oxygen immediately, and 100% of passengers are on oxygen immediately. The descent profiles from notes 2 through 4 tell you how much total oxygen to carry to cover an emergency descent and holding. That figure on page 178 shows the oxygen requirement table visually — it's the same data we just covered, but in a format that helps you see the thresholds at a glance.