Most retractable gear airplanes use a set of three green lights to indicate that all three wheels are down and locked and three red lights to show that they are up and locked (or in transit). I noticed, however, that some airplanes (probably older airplanes) use different indicator light systems, in which, for instance, a red or amber light means gear unsafe, but no light is illuminated for gear up and locked. What are the most common of these systems and can they be found only in small piston engine airplanes or they are also used in larger airplanes?
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