Page 20 - Volume 10 Number 9

Ask the Expert
Is My Gear Down?
by Tom Clements
Well of course it is! “Three Green, No Red;” I’m good to land.
And so you are. You have verified that the individual green gear down lights are illuminated and the two lights in the landing gear handle with the red lenses are both extinguished. You have proper confirmation that your gear is safe for landing. Go for it!
But what about when the system throws us a curve ball and the normal “Three Green; No Red” is not what we see? Now what? The intent of this article is to review the operation of the landing gear indication lights and the landing gear warning horn to remind us there are at least one or two additional ways to decide if the gear is safely extended or not.
With the exception of the original King Air 65-90 model of 1964 and 1965, the landing gear indication and warning horn system is basically the same for all King Air models. Yes, there are minor changes between the various models and years, but overall the system is nearly identical. Each of the three gear legs has both a “downlock” and “uplock” switch installed. The name for these switches sometimes leads to a bit of confusion because the switches themselves have nothing whatsoever to do with actually locking the leg safely in the down or up position. But the switches should be installed and adjusted in such a way that they do not activate – the plunger on the switch is not depressed – until the gear leg has indeed reached its fully extended or fully retracted, locked position.
The downlock switch, when activated, turns on its respective green light – very simple. The uplock switch, on the other hand, does not turn on a light. Instead, when all three uplock switches are properly activated, the red lights in the gear handle are extinguished. The red handle lights serve two purposes: (1) To advise us that the landing gear is “in transit,” not locked up or down, and (2) To advise us that the gear is “unsafe.” Let’s discuss each of these functions in a bit more detail.
The “in transit” nature of the red lights is telling us that at least one downlock or uplock switch is not being activated. I am sure some of my readers can relate to what I am about to write. You are in flight and encounter some moderate or severe turbulence. As the airplane gets bounced around you observe that the red handle lights blink on and off. As you watch more intently, it even becomes apparent that the lights glow when a positive
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G force is experienced and go out when a neutral or negative G force exists.
What’s happening here is that the positive G force is causing at least one gear leg to sag down enough to deactivate its uplock switch. Is one leg sagging, or two, or all three? There is no way to know. The activation of the red lights requires only one leg to move off of its switch, so there will be no change in indication if another one or two are also moving. There is, however, a way to know if the right main gear is sagging in those models that have the Type I, older prop sync system and hence also have a yellow “Prop Sync On” annunciator. That light is triggered, with the sync switch on, when the right main gear’s uplock switch is not activated. So if you observe that the annunciator blinks in tune with the red lights, you can tell your mechanic that at least the right side needs adjustment, needs improved rigging. As for the left and the nose? No way to know if they, too, are involved or not.
By the way, this sagging gear phenomenon is highly unlikely to happen on the later models with the electro- hydraulic landing gear system. It is almost exclusively applicable to the older electro-mechanical operating systems. Why? In the mechanical system, only the friction of the jackscrew is providing the force that holds the gear up. In the hydraulic system, positive pressure on the retraction side of the hydraulic actuator piston is the holding force and it is mighty potent!
SEPTEMBER 2016