Landing Gear — Page 100, Lesson 142

Let’s pick up with the multi-wheeled undercarriage and why it’s used, then move into a real operational hazard: contaminated runways. I want to start with the advantages of multi-wheeled units, because the text lists four of them, and each one matters for how we design and operate the gear. First, weight. The greater the number of wheels, the lighter the unit can become, because the wheels themselves are smaller. Now, the text admits this point is hard to prove, since with the size of today’s modern aircraft, a single-wheel unit would be impracticable anyway. So it’s a theoretical advantage more than a practical one. Second, ease of servicing. Yes, the whole unit is more complex, but changing a wheel or a brake unit is easier than on a single-wheel undercarriage, and the individual components are much nearer the ground. That’s a big deal for line maintenance — you’re not working up on a stand, you’re reaching components at a convenient height. Third, a greater safety factor. If you have a burst tyre, there will be one or more serviceable wheels remaining to carry the load. So a single tyre failure doesn’t leave you with nothing under that leg. Fourth, ease of on-board stowage. Multi-wheel units are easier to stow. But here’s the catch — most undercarriages are designed to fit in the space available, and the thickness of the wing plays a big part. Thin wings mean specially designed folding and swivelling bogies have to be used, which escalates the costs and makes general routine servicing more complex. That’s why some aircraft have their undercarriage as part of the fuselage — it eases the design problem and allows the gear to be raised and lowered vertically. Now, the main disadvantage of multi-wheel bogie units. They have a large footprint area, and that causes the unit to crab whilst turning. Because of that, the turning radius has to be increased, which creates manoeuvring problems on the ground. And there’s a knock-on effect: tyre wear caused by scrubbing. The forces applied to the tread are considerable, and the smaller the radius of the turn, the greater those forces become. The tread gets torn and can split, exposing the casing fabric. To minimize this, the recommendation is to manoeuvre the aircraft on the ground using the largest turning circle possible, avoid tight turns if at all possible, and move the aircraft in a straight line for a short distance before stopping. Now let’s move to a very practical operational problem: landing gear operation on contaminated runways. The scenario is slush — a mixture of water, wet snow and ice. Problems have occurred on aircraft that have taken off from runways contaminated with slush. Here’s the mechanism: slush is deposited on the gear during the take-off run, then it freezes in the landing gear bay during the climb and cruise. On more than one occasion, the crew have been unsuccessful in lowering the gear upon arrival at their destination, because that frozen slush has jammed the mechanism. So what’s the procedure? If it is absolutely essential that you take off in such poor conditions, you are advised to cycle the gear just after take-off — selecting the gear UP, DOWN, and then UP again. The reasoning is that the shocks inflicted on the gear during this cycle should be sufficient to remove any deposits from it. So you’re deliberately working the gear through a full cycle to break off the slush before it has a chance to freeze solid in the bay. That’s the whole picture here: why we use multi-wheeled bogies, the trade-off in ground handling, and the contaminated-runway procedure that keeps the gear free to extend when you need it.