They're grouped together because they're related, but they're separate entries — Page 348, Lesson 441

This is the index of the book — the alphabetical listing of topics and the pages where they appear. So what I want to walk you through is how to read this index, because it tells you exactly where every subject lives in the manual. Let's look at the entries under "S." We have "Safe Life" on page 5, and right next to it "Safety Factor" also on page 5. These are two distinct concepts — Safe Life is a structural design philosophy, and Safety Factor is the margin built into that design. They're grouped together because they're related, but they're separate entries. Then we have "Safety valve" on page 220 — that's a pressure-relief device in a hydraulic or pneumatic system. "Sandwich Construction" on page 26 — that's a composite structural method. "Sangamo Weston Ice Detector" on page 243 — that's a specific ice detection instrument. "Schrader valve" on page 121 — that's the tire inflation valve you'd find on landing gear. Now notice "Seals" on page 51 — that's a general entry covering all the sealing elements in the airframe. Then we get into the valve family: "Selector Valves" on page 68, "Sequence Valves" on page 69, "Shut-off Valves" on pages 317 and 73, and "Shuttle Valves" on page 69. Each of these is a different type of hydraulic control valve, and the index lists them separately so you can find each one's specific function. We also have "Self-centring" on page 96 and "Self-centring Operation" on page 97 — these relate to control systems that return to a neutral position. "Semi-monocoque" on page 12 and "Semi-monocoque Construction" on page 14 — that's the fundamental airframe structure where the skin carries the load. For flight controls, we have "Servo Tab" on page 171 and "Spring Tab" on page 171 — both are trim tab mechanisms. "Speed Brakes" on pages 160 and 174, "Spoilers" on page 174, and "Spoiler System" on page 161 — these are the drag and lift-dumping devices. Under landing gear, "Spring Steel Legs" on page 89 — that's a type of landing gear leg. "Spring Bias" on page 179 — that's a spring-loaded force in a control system. Then we have "Shear" on page 3 — that's a structural load type. "Skin" on page 23 — the outer surface of the airframe. "Smoke Hoods" on page 287 — that's cabin safety equipment. "Speed Rating" on page 122 — that's a tire speed limitation. "Spool Valve Selector" on page 68 — that's a specific type of selector valve. And "SQUIB" on page 298 — that's an explosive initiator used in emergency systems. So the index is your map. When you're studying, you use it to jump straight to the page where each topic is explained in full. That's the purpose of this page — it's not content itself, it's the navigation tool for the entire manual.