Let’s pick this up right where the landing gear story gets interesting — Page 115, Lesson 154

Let’s pick this up right where the landing gear story gets interesting. We were talking about the danger of a gear that won’t come down properly, and now I want to walk you through what happens when the normal extension system fails, and then into a clever piece of logic that every airliner uses. First, the emergency extension. On some aircraft, if the gear has to be lowered by an emergency system, there’s a real problem: the gear doors might not be fully open, or the gear might not be locked in the right position. Because of that, there’s a chance the gear will contact the ground on touchdown unless the landing is exceptionally gentle. So to minimize the damage and the replacement cost, some aircraft have doors fitted with a frangible portion at their lowest extent. “Frangible” means it’s designed to break away cleanly. So if the gear does strike the ground, that sacrificial piece takes the hit, and you only have to replace that small part instead of the whole door. Now, how do we actually get the gear down in an emergency? On other aircraft, the landing gear is extended by an emergency pressure system. This system often uses alternative pipelines to the jacks — meaning it doesn’t rely on the normal hydraulic lines; it has its own separate plumbing to the same jacks that move the gear. The pressure for this emergency system can be supplied by one of four sources: a hydraulic accumulator, which is a stored-pressure bottle; a hand pump, which the crew physically operates; a pneumatic storage cylinder, which uses compressed gas; or an electrically powered pump. So you have multiple independent ways to get pressure to the gear. And regardless of how the gear is extended, you need to know it’s actually locked down. That’s where the Mechanical Indicator comes in. It’s provided to indicate gear locked down — a physical, mechanical signal, not just an electrical light, so you have a positive confirmation that the gear is secure. Now let’s move to the Air/Ground Logic System. This is a fundamental concept. Inevitably, there are systems of all types which need to be selected on or off in response to the criterion of whether the aircraft is airborne or not. Think about it: you don’t want the ground spoilers arming while you’re still in the air, and you don’t want the weight-on-wheels systems thinking you’re on the ground during flight. So the aircraft needs to know its state. The simplest way to get this effect is by placing microswitches on the main landing gear oleos. The oleo is the shock-absorbing strut of the landing gear. When the aircraft is on the ground, the weight of the aircraft compresses the oleo, which changes the position of the microswitch. Alternatively, on take-off, when the weight of the wheel and bogie assembly extends the oleo, the switch position changes again. So the switch tells you whether the strut is compressed — on the ground — or extended — airborne. On more modern aircraft, microswitches have been superseded by proximity sensing devices. These work essentially in the same manner as the microswitches, but instead of physical contact, they deduce the extension or retraction of the oleo by capacitive or inductive sensing equipment fitted to the oleo. Capacitive sensing measures changes in electrical capacitance; inductive sensing measures changes in magnetic fields. Either way, the sensor detects the position of the strut without touching it. Whichever system is used, a controlling signal will be sent to a relay or bank of relays. A relay is an electrically operated switch. These relays are capable of switching the affected circuits on or off as required. So the air/ground signal from the oleo sensor drives the relays, and those relays then control all the systems that need to know whether you’re flying or taxiing. One more important detail: some aircraft use sensors on just one main landing gear oleo, but it is common to find the sensors duplicated on both main oleos to provide a degree of redundancy in the system. Redundancy means you have a backup — if one sensor fails, the other still gives you the correct air/ground state. That’s a safety feature you’ll see throughout aircraft design. So to tie it together: the gear doors have a frangible section for emergency touchdown protection, the emergency extension uses alternative pipelines and one of four pressure sources, the Mechanical Indicator confirms the gear is locked, and the Air/Ground Logic System uses oleo-mounted sensors — microswitches or proximity devices — feeding relays to switch systems based on whether the aircraft is airborne or on the ground, with duplication for redundancy.