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 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!
It is understandable why so many King Air pilots incorrectly believe that the red handle lights are “disagreement” lights, telling them that the gear handle position and the actual gear position do not agree. Of course, every time we move the landing gear handle from one position to the other, we expect to see the red lights illuminate until all gear legs have properly reached their new positions, and then the lights extinguish. So when disagreement exists, we have the lights. True, but that is not what is triggering them. You see, what if the gear never moved at all? What if we have a tripped CB or a bad motor so that when we move the handle nothing happens? We certainly have disagreement now, right? But we won’t have red lights. At least one of the gear legs has to move enough to deactivate its uplock or downlock switch before the red lights will appear. That is why, technically, they are not disagreement lights, but truly are in transit lights.
In addition to the in transit trigger mechanism of the red handle lights, we have said that the other trigger is “unsafe.” What exactly does that mean?
Unsafe, in this context, means that we may be getting ready to land and yet all three gear legs are not fully in the down and locked position. Unless you are willing to touch down extremely fast, it is rather difficult to land an airplane while a lot of power is still being applied. So the King Air series – just like almost every other retractable gear airplane – has a landing gear warning horn system that is triggered by the combination of low throttle (power lever) position and “not down” gear position. Inside the power quadrant in the cockpit, both the left and the right power levers activate switches when those levers are not well forward. Depending upon your exact model and serial number, these may be set for an N1 of about 79 percent to as high as 86 percent. (TPE331 powered B100s? Oh, about half-way from Flight Idle to Full Forward.) Keep in mind this is power lever position only, not actual engine power output.
In the King Air, the designers decided to include the red handle lights into this warning horn system. When the horn sounds, the lights illuminate … a double indication that landing is not advised yet! As you know, the obnoxious horn may be silenced with a button – since it is not at all uncommon in the turbine-powered airplanes to intentionally pull power back so far as to trigger the warning during some descents – but the red lights remain lit until either the power levers are moved forward past the switches or all three gear legs activate their downlock switches.
In the mid-1970s, the FAA changed the rules and required that another mechanism be added to trigger the “gear unsafe” warning. Apparently there had been a few close calls or actual gear-up landings that followed a too low, too slow, airplane fully dirty, “drag it in” type of landing approach. Overcoming so much drag, the power being applied was above that which triggered the warning. The change, therefore, was to add a flap position trigger into the unsafe warning circuit. In the King Air series, Beech added a switch beside the flap limit switches attached near the inboard track of the right inboard flap. This switch was adjusted to be activated whenever the flaps exceeded the approach position. So most King Airs, now, will have the horn blow and red lights illuminate whenever the flaps exceed approach if the gear is not correctly down, regardless of power lever position. There was no requirement to retrofit this flap trigger to earlier airplanes so many vintage King Airs still will not have it.
If you have operated a King Air for very many hours, I am sure you have experienced at least one case of “Two Green; No Red.” Huh? That ain’t right! By far, the most likely reason for this is a burnt out green light, right? Merely test the bulb by pressing it (or the module), verify it is bad, replace it with a spare bulb, and now we are relieved to see the “Three Green” that we were expecting. (If you are not 100 percent sure how to test and change the bulb, ask your mechanic or another pilot to show you the procedure!)
Unfortunately, there will come a time when the “Two Green; No Red” situation reveals that the bulb is good; it tests just fine! Now wait. How can that be?! If all three downlock switches must be activated to turn out the in transit red lights, how come that downlock switch is not also activating its greenie?
The answer comes from the fact that even though there is only one downlock switch per gear leg, that switch contains more than one set of contacts and has a lot more than just two wires attached to it. History has shown that it is not uncommon for the portion that deactivates the red lights to still function properly after the portion that activates the green light has failed.
In this situation, I like to say that we have a tie vote: One Aye and one Nay. The Aye vote – that the gear is safely down – comes from the absence of the in transit red lights. The Nay vote – that all three gear legs are not safely down – comes from the lack of that one Green. How do we break the tie?
Yes, you are correct: Use the gear warning horn. Pull one or both power levers back to idle, or go ahead and select full flaps. No horn? Whew, we are safe to land. We have two safe votes now and only one unsafe vote. I am not saying that the old tower fly-by is incorrect now, and feel free to do so if you prefer. But the situation I am describing is common enough in the King Air fleet that most experienced King Air pilots think the visual check is unnecessary.
Two more comments concerning this “Two Green, No Red” scenario: First, it is also quite common that sometime prior to touchdown the @%&! last green light will finally appear! The downlock switch contact had not totally failed but was just “sticky.” Second, for the older electro-mechanical systems, realize that all three gear jackscrews are driven from a common source, a common transmission that has torque tubes attached to the main jackscrews and a chain drive attached to the nose jackscrew. Unless there was a rather catastrophic failure of that transmission assembly, if one jackscrew has been driven properly to “full” extension then the chances are good that the other two are also fully extended. You didn’t hear a big, loud, “Sprong, Clackity-clack!” did you? If not, all three legs are likely operating together just fine.
Before I end this article, allow me also to review landing gear limit switches. It is surprising for a lot of King Air pilots and mechanics to find that the uplock and downlock switches play absolutely no role in telling the motor when to stop running! (With one exception that I will cover.) No, for the electro-mechanical system, the switches that tell the motor it’s time to quit are associated with that transmission/gearbox in the belly. If they activate too soon, the gear remains in transit without good contact on the uplock or downlock switches. Activate too late, and we put undesirable and possibly damaging stress on the jackscrews, torque tubes, and chain drive when they are asked to travel further than they should.
Lastly, what shuts off the motor that drives the fluid pump in the electro-hydraulic systems? Where is its “Limit Switch?” On the retraction side, the shutoff is triggered by a pressure-activated switch. As all three gear legs hit the stops on the upside and stop moving, the pressure from the pump – that was causing the three actuators’ pistons to retract and expel fluid back to the reservoir via the normal extension lines – rapidly builds up as the piston motion ceases. In the C90A (and after) system, the shutoff pressure value is near 1,800 psi. In the B200 (and after) and 300-series, the shutoff pressure is just slightly under 3,000 psi. It is this pressure that holds the gear up and leads to the conclusion that a sagging leg is highly unlikely so long as the pump can still develop pressure and the motor that operates the pump has not died.
It’s a different story entirely on the extension side. Now what tells the pump motor to stop running are our “old friends” the downlock switches. For the left and right mains, the same switch that triggers the respective green light – and eliminates the red in transit and unsafe lights in conjunction with the other downlock switches – does double-duty in telling the motor to stop. For the nose? That is slightly different. Now the designers added a new switch that looks at the position of the actual ball-in-groove mechanical downlock in the nose gear’s hydraulic actuator. When that switch senses the “all safe” condition, it feeds that information also to the motor’s shutoff circuitry.
In addition to what has already been covered – namely, using the gear horn to cast the deciding vote whenever a green/red disagreement occurs – there is one last way to feel sure that your gear is down when in a King Air with the electro-hydraulic system. In the C90A (and after) series: Is the pump motor still running? If it is not – and you can usually tell whether it is running or not by its noise – then that is another vote that all three gears legs are safely extended. The 200- and 300-series models have one last thing I have not yet mentioned: A 14-second time delay relay that shuts off the pump motor if it operates too long, before it burns itself out. This circuit, when it activates, also provides a ground to the Landing Gear Relay Circuit Breaker – the CB right next to the gear handle – causing it to trip or pop. So if that CB has not tripped, then you have a vote that all three gear legs are safely extended.
Is my gear down? Now you have a few more ways to feel certain that indeed it is.