When I designed Melmoth's replacement in the early '80s, I decided to simplicate in a big way. I got rid of the three hydraulic cylinders, one at each gear leg, and replaced them with a single actuator located in the wing centersection, inches from the hydraulic pump and the up/down control valve. The three gear struts were mechanically linked, and the position of components of the system passing through the cabin indicated whether the gear was up or down; no lights, with their attendant microswitches and wires, were needed. There was hardly any plumbing at all. In place of several electrically powered valves, I provided a cable circuit, traveling through bicycle brake cable housings from the gear handle on the panel to the valve under the seat.
So last week I was hooking up that cable circuit when I had one of those sensations that my friend Russ Hardwick calls a "klong." He defines this word as a "rush of s--t to the heart." What I realized was that my seemingly simple system had an unexpected failure mode.
The cable connecting the handle on the panel to the valve beneath the seat wraps halfway around a grooved disc, like a pulley, at each end; these discs are fastened to the panel handle and the valve's stem. To keep the cable from slipping on the disc, a barrel-shaped thing about the size of a .22 short, secured to the cable with a set screw, rests in a notch in the perimeter of the disc.
Now, here's the rub: if the set screw were to back out and the cable were to slip on the disc, the valve could conceivably be left in the "gear up" position while the control handle would return to center, as it is supposed to do when not in use. If this happened on the ground, then as soon as I operated the flaps or speed brake, both of which are hydraulic, all three gears would retract.
Any retractable landing gear poses a risk of inadvertent retraction. My first airplane had no protection; I simply kept my hand away from the gear switch (which was electric) while on the ground. Of course, I never dreamed that I would be so silly as to unlock the gear by pulling on the nosewheel doors, but none of the usual kinds of protection would have helped in that case anyway.
Many airplanes with retractable gear have some sort of system that disables the gear actuator while on the ground. For example, there might be a pressure switch in the pitot line that allows the gear to retract only when the airplane is at flying speed, or a "squat switch" on an oleopneumatic gear strut might sense when the strut is compressed and turn off power to the gear actuating system.
A problem with all such systems is that the protection itself can fail. It can either not prevent retraction on the ground or not permit it in the air. In the latter case, multiengine airplanes whose ability to climb on a single engine depends on cleaning up gear and flaps need a way to bypass the squat switch. And so the complexity, and the number of potential points of failure, mounts.
Once I recognized the vulnerability of Melmoth 2's gear system, I felt that I had to find a way to protect it. My first thought was naturally the conventional squat switch, which has the virtue of preventing both commanded and uncommanded retraction on the ground. I posed the question to a pilot friend who saw it mainly in terms of information-I needed to know the true status of the hydraulic valves at all times. He first proposed that I put three lights on the panel to indicate that the three valves were or were not in a safe-that is, closed-position.