Let me handle the answer key first, then we'll dive into the new material — Page 87, Lesson 123

This is the answer key page for the previous chapter's questions, and then the start of a brand-new chapter — Chapter 3, Landing Gear. Let me handle the answer key first, then we'll dive into the new material. The answer key is just a grid of correct letters for questions 1 through 50 from the last chapter. I'm not going to read those out to you — the app will present those practice questions to you one at a time, so you'll see the answers interactively. What matters is that we're now moving on to a fresh topic. So, new chapter: Landing Gear. This is a big one, and I want to walk you through how it's structured before we get into the details. The chapter opens with an Introduction, then moves into Landing Gear Design. From there we look at Landing Gear Types — Fixed or Retractable, and we start with the Fixed Landing Gear. Then we get into the real meat: the Construction of Oleo-pneumatic Struts and their Operation. After that, we cover Retractable Landing Gear, its Design and Construction, and the Factors Affecting Design and Construction, plus Other Factors. We then look at where the gear is mounted — Underwing Landing Gear Units versus Fuselage Mounted Landing Gear — and the Loads Sustained by the Landing Gear. Finally, we get to the Nose Undercarriage, covering Castoring, Self-centring, and Nose Wheel Steering. Let me start you off with the core concept, because everything else hangs off this. The landing gear is the structure that supports the aircraft on the ground — during taxi, take-off roll, landing, and when parked. Its design has to balance two opposing demands: it must be strong enough to absorb the massive loads of landing and ground handling, yet light enough and compact enough to not hurt the aircraft's performance in flight. That's why we have the two fundamental types. A fixed landing gear is exactly what it sounds like — it stays extended all the time. It's simpler, lighter in terms of mechanism, and cheaper to maintain, but it creates constant aerodynamic drag in flight. A retractable landing gear folds away into the airframe after take-off to clean up the aerodynamics, at the cost of added weight, complexity, and the need for a reliable extension system. Now, the heart of any landing gear — fixed or retractable — is the shock absorption, and that's where the oleo-pneumatic strut comes in. I want you to remember that name, because it's the professional term you'll see on exams and in manuals. "Oleo" refers to oil, and "pneumatic" refers to air or gas. So an oleo-pneumatic strut uses both a liquid and a gas to absorb and dissipate the energy of landing. We'll get into the construction and operation of that strut in detail shortly, but the key idea is this: when the aircraft touches down, the strut compresses, forcing oil through a small orifice while compressing the gas. The gas acts as a spring, storing energy, and the oil acts as a damper, converting that energy into heat. That combination is what gives you a smooth, controlled touchdown instead of a bounce. Then we'll look at the nose undercarriage specifically. The nose gear has to do more than just hold the front of the aircraft up — it has to allow the aircraft to be steered on the ground. That brings in castoring, where the nose wheel can swivel freely, and self-centring, where the wheel automatically returns to the straight-ahead position. And finally, nose wheel steering, which is the pilot's direct control over the direction of the nose gear. So that's the roadmap. We're starting with the fundamentals of landing gear design and the fixed versus retractable distinction. Let's begin there — the fixed landing gear and why it's used where it is.