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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. KA
King Air expert Tom Clements has been flying and instructing in King Airs for over 44 years, and is the author of “The King Air Book.” He is a Gold Seal CFI and has over 23,000 total hours with more than 15,000 in King Airs. For information on ordering his book, go to www.flightreview.net. Tom is actively mentoring the instructors at King Air Academy in Phoenix.
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at kblonigen@cox.net.
SEPTEMBER 2016
KING AIR MAGAZINE • 23