We’re starting a brand-new chapter now — Chapter 4, Aircraft Wheels — Page 115, Lesson 155

We’re starting a brand-new chapter now — Chapter 4, Aircraft Wheels. So let’s set the scene first, because this is the foundation for everything else in the landing gear system. The wheels and tyres of an aircraft do three jobs. They support the aircraft when it’s on the ground. They give it mobility for take-off, landing, and taxiing. And the pneumatic tyres — that’s the air-filled rubber part — cushion the aircraft from shocks caused by irregularities in the ground surface, and occasionally, by a lack of landing technique. That last phrase is the book’s polite way of saying a hard landing. Now, the main wheels, and in some cases the nose wheels, house the brake units. Those brakes control the movement of the aircraft on the ground and give you a means of deceleration on landing. So the wheel isn’t just a rolling thing — it’s the mounting point for the braking system. Aircraft wheels are designed specifically to make tyre replacement easy. And they’re classified into two types: the loose and detachable flange wheel, and the divided wheel. Let’s take them one at a time. First, the loose and detachable flange wheel. Look at Figure 4.1 in your materials — that’s the loose flange wheel. In this type, one flange is integral with the wheel body. That means it’s machined as one solid piece with the wheel — it can’t come off. The other flange is loose, and it’s machined to fit over the wheel rim. So you have one fixed flange and one removable one. Now here’s the key distinction the book draws. The difference between the loose flange type and the detachable flange type is the method by which the removable flange is secured. The loose flange is retained by a locking device on the wheel rim. The detachable flange, on the other hand, is secured to the wheel body by nuts and bolts. And a detachable flange can be a single piece, or it can be two or three pieces bolted together. So remember: loose flange = locking device on the rim. Detachable flange = nuts and bolts to the wheel body. Now the second classification — the divided wheel, also called the split hub. This one is built from two half wheels that are matched up and connected by bolts passing through both halves. Those bolts are fitted with stiff nuts. Or, if one half of the wheel is tapped — meaning it has threads cut into it — then each bolt is locked with a locking plate instead. In the wheel shown in Figure 4.2, the two halves are clamped together by bolts, nyloc nuts, and washers. A nyloc nut is a nut with a nylon insert that grips the bolt threads to stop it vibrating loose — that’s your standard aviation locking nut. Now, this divided wheel is designed to be used with a tubeless tyre. That means there’s no inner tube — the tyre seals directly against the wheel. So a seal is incorporated at the joint between the two halves. That seal does two things: it prevents abrasion between the two halves, and it provides an airtight joint. Without that seal, you’d lose air pressure at the split. But here’s the interesting bit. When this same divided wheel is used with a conventional tyre — that is, a tyre with an inner tube — the wheel inflation valve is removed. Why? Because you need to fit the tube’s inflation valve through the rim instead. So the wheel has its own valve for tubeless operation, and you take it out to let the tube’s valve poke through when you’re running a tube-type tyre. That’s the core of the chapter so far: the two wheel classifications, how the removable flange is secured in each, and how the divided wheel handles both tubeless and tube-type tyres.