Landing Gear — Page 115, Lesson 151

I want to walk you through the safety features that surround the landing gear system, because in professional flying, the gear is one of the few systems where a simple oversight can be catastrophic. We've already covered the basic operation, so now let's look at the protections built around it. First, let's talk about the selector lever lock. When the aircraft is on the ground, the strut is compressed, and the switch in the circuit is open. That means no electrical supply flows to the solenoid. When the strut extends after take-off, the switch contacts close, completing the electrical supply to the solenoid. That releases the selector lever lock, allowing the landing gear to be selected up. So the system physically prevents you from raising the gear while the aircraft is still on the ground, because the strut hasn't extended yet. Now, there's a means of overriding that lock. It could be a separate gated switch that completes the circuit manually, or a mechanical means of avoiding the locking plunger. This override is provided for emergency use and for maintenance purposes. So if you need to cycle the gear on the ground for a test, or in an emergency, you have a way past that lock. Next, we have ground locks, also called landing gear locking pins. These are a further safety feature intended to prevent collapse of the gear when the aircraft is unpowered on the ground. They usually consist of pins or metal sleeves that interfere, unpowered, with the operation of the gear. The key word there is "unpowered" — they physically block movement so it's impossible for the gear to move while they're in position. They're fitted with warning flags, which should prevent the crew from getting airborne with them still in place. The procedure is to remove them before flight, stow them on board the aircraft, and inform the flight crew that they've been removed and stowed safely. Now, warning devices. To guard against landing with the gear retracted or unlocked, a warning horn is incorporated in the system, connected to a throttle-operated switch. Here's how it works: if one or more throttle levers are less than approximately one third open — as would be the case during approach to land — the horn sounds if the landing gear is in any position other than down and locked. So the logic is simple: throttles pulled back, gear not down and locked, you get a loud warning. There's often a horn isolation switch, which allows certain flight exercises and ground servicing operations to be carried out without hindrance. But here's the critical point: an airspeed switch is a definite advantage over an isolation switch, because unlike an isolation switch, it cannot be first used and then forgotten, with perhaps disastrous consequences. An airspeed switch can also be used to prevent the horn sounding during initial descent from high altitude. Now, the GPWS — that's the Ground Proximity Warning System. The GPWS will be inhibited below 500 feet only if the gear is locked down and the flaps are in the landing position. So below 500 feet, the system won't give you nuisance warnings if you're properly configured for landing. For further information, we'll cover that in the Warnings and Recording section in Book 5. Finally, let's look at the landing gear operating speeds. These are critical limitations you must memorize. VLO is the Maximum Velocity for Landing gear Operation. Do not exceed this speed while the landing gear is operating — that means while it's moving, either up or down. VLE is the Maximum Velocity with Landing gear Extended. Do not exceed this speed with the landing gear extended — that means when it's locked down and staying down. So VLO applies during the transit, VLE applies once it's extended. These are separate limits, and exceeding either one can cause structural damage to the gear doors and mechanism. So to tie it all together: the strut switch prevents premature gear retraction, ground locks protect the unpowered aircraft, the warning horn guards against landing with gear up, the airspeed switch improves on the isolation switch by being self-resetting, GPWS is inhibited only when properly configured, and VLO and VLE define your speed limits for gear operation and extension.