Aircraft Oxygen Equipment — Page 280, Lesson 358

I want to walk you through two separate pieces of equipment here, because this excerpt actually jumps between two different chapters of your syllabus. First we have the regulatory table for supplemental oxygen on non-pressurised aeroplanes, and then we move into smoke detection systems. Let me take them in order. We start with Appendix 1 to OPS 1.775, which is the European regulation that tells us exactly who gets oxygen, and when, on an aeroplane that is not pressurised. This is a table with two columns: column (a) tells you the supply for, and column (b) tells you the duration and pressure altitude. Let me walk you through each row because the altitudes and the percentages matter enormously. Row one: all occupants of flight deck seats on flight deck duty. That means the pilots actually working the aeroplane. They get supplemental oxygen for the entire flight time at pressure altitudes above 10 000 feet. So above 10 000 feet, the flight crew is on oxygen for the whole time, no exceptions. Row two: all required cabin crew members. They get oxygen for the entire flight time at pressure altitudes above 13 000 feet, and also for any period exceeding 30 minutes at pressure altitudes above 10 000 feet but not exceeding 13 000 feet. So there is a subtlety here. Between 10 000 and 13 000 feet, cabin crew only need oxygen if the time up there exceeds 30 minutes. Above 13 000 feet, they are on oxygen for the entire flight time, period. Row three: 100 percent of passengers. Note the "See Note" marker. They get oxygen for the entire flight time at pressure altitudes above 13 000 feet. So above 13 000 feet, every single passenger is supplied. Row four: 10 percent of passengers. They get oxygen for the entire flight time after 30 minutes at pressure altitudes greater than 10 000 feet but not exceeding 13 000 feet. So in that band between 10 000 and 13 000 feet, once you have been up there for more than 30 minutes, you need oxygen for 10 percent of the passengers. Now the note is critical. For the purpose of this table, "passengers" means passengers actually carried, and it includes infants under the age of 2. So when we say 100 percent or 10 percent, we are counting every person on board who is a passenger, including the babies. That is a regulatory definition you must hold onto. Now let me move to the second topic, smoke detection. This is a completely different chapter. Smoke detection systems are employed where it is not possible to keep a bay or compartment under constant physical surveillance. Think cargo bays or electrical equipment bays — places nobody is sitting in to watch them. As a general rule, a system of detectors is employed in each compartment or bay which can give remote warnings of smoke, can be tested from the flight deck, and can be re-set when a warning is received in order to verify it. So three capabilities: remote warning, test from the flight deck, and re-set to verify. Smoke and flame detectors operate according to several different principles. The excerpt lists two: optical and ionization. Let me deal with the optical ones in detail, because there are two distinct types described. First, the light detection system. This is designed to respond to a change in visible light or a change in infrared radiation. It uses a photoelectric cell positioned so that it can monitor the surrounding area. When a change of light or infrared radiation strikes the cell, it produces a change in current which activates a warning circuit. And importantly, this system is activated by an open flame. So this one is really a flame detector — it senses the light or infrared coming from an actual flame. Second, the light refraction system. This is shown in Figure 14.2. Here we have a photoelectric cell which is shielded from direct light from a projection lamp. The lamp directs light into a detection chamber, but the cell cannot see that direct light. Air from the compartment is drawn through the chamber. Now, when smoke is introduced into the chamber, light is reflected from the smoke particles and falls on the photoelectric cell. That change in current flow, caused by the change in conductivity of the cell, activates a visual and aural warning. So you get both a light and a sound warning in the cockpit. There is also a test lamp in this system. When the test is selected from the flight deck, the test lamp illuminates and activates the smoke detector. So the test lamp is the way you verify the detector is working, and you do that from the flight deck, which ties back to that general rule I gave you earlier. So the key contrast here: the light detection system responds to a change in light or infrared and is activated by an open flame. The light refraction system uses a shielded photoelectric cell and detects smoke particles by the light they reflect. One senses flame, the other senses smoke, and both ultimately produce a change in current that drives a warning. That is the substance of this excerpt. We have the oxygen supply table with its precise altitude bands and passenger percentages, and we have the two optical smoke detection principles.