Landing Gear — Page 106, Lesson 149

Let’s start with gear position indication. When you select the landing gear down, you might be able to see the gear from the cockpit, but you can’t be certain each undercarriage is securely locked just by looking. So we use an electrical indicating system to give the crew a positive indication of both the lock operation and the gear position. The system usually uses microswitches on the uplocks and downlocks. These switches make or break when the locks operate, and they connect to a landing gear position indicator on the instrument panel. So the microswitch is the sensor, and the indicator is the display. There may also be a mechanical indicator, which shows the gear is down and locked even if the electrical system fails. That’s your backup. Now, the typical electrical indication works like this: a green light means the undercarriage is locked down. A red light means it’s in transit—that is, moving between up and down. And no lights at all means it’s locked up. The bulbs are usually duplicated, so if one bulb fails, you don’t get a false indication. On other aircraft, you might see magnetic indicators or lights instead. On some light aircraft, a single green light means all undercarriages are locked down, and an amber light means all are locked up. Many large aircraft also have main gear door lock indicators, to confirm the doors are locked in their correct position. Now, gear safety features. Because correct gear operation is critical, the retraction system includes several safety features. One is nose wheel centring. To avoid damaging the airframe, the nose wheel must be aligned fore and aft—that is, front to rear—during retraction. One method is hydraulic nose wheel centring on aircraft with powered steering, which we discussed earlier. Another safety feature is the gear selector lock. To prevent inadvertent retraction when the aircraft is resting on its wheels, a device stops the selector lever from moving. This device is a spring-loaded plunger that holds the selector in the down position. It’s released by a solenoid. Electrical power to that solenoid is controlled by a switch on the shock absorber strut, which is part of the air/ground logic circuits. So when the aircraft is on the ground, the switch keeps the plunger engaged, and the gear can’t be retracted by mistake.