GLASS GOOSE GAZETTE * ISSUE #17, June 15, 2000

Page 11

I can also tell you that the adjustable pressure switches are very hard to adjust and impossible to adjust so they work consistently and dependably. I am also concerned about the ability of most of the switches and circuits I have seen to handle the loads they are subjected to in this application. The pressure switches I have seen are just too small to have contacts inside them capable of handling the arcing and the amperage drawn by the pump. If they are actually controlling a solenoid switch that is capable of handling those loads, then there have got to be 2 large solenoids the size of starter solenoids somewhere else in the circuit. Based on my own experience with both approaches, I can see no justification for the expense, complexity, and potential for failure of such a complex system with so many different elements that can fail when there is a simple, inexpensive, absolutely dependable, and proven alternative.

The old hydraulic pump drew upwards of 50 amps when it was pumping 1700 pounds of pressure. The new pump runs more like 30 - 35 amps at the same pressure. Either way, a fair amount of current. Any solenoid or pressure switch capable of carrying that much current is going to be a fair sized piece of equipment. If it’s not, the points will be too small and it won’t be long before they are fried and begin to stick or not make contact or whatever. I’m not going to go into the weight aspect of the whole thing, but it is self evident that every unnecessary element in a circuit is another opportunity for failure. If it’s not there, it can’t fail? Having said that, what basic element of a circuit must be there to turn a pump on and off? A simple switch. Nothing else. You can make it more complicated, but in reality, that’s all that’s absolutely necessary. An on/off switch.

Now, there is one unavoidable complication to consider when controlling the pump in our gear system. It must be reversible. Since the pump has a DC motor, all you have to do is reverse the current to the motor and it runs the opposite way. What is the simplest, easiest, and most fail safe way to reverse DC current? A simple single pole double throw switch!

So, here we have narrowed it down to 2 very basic small items that are the grand total of what is absolutely and unavoidably necessary to

control the gear system on our aircraft. A simple on/off switch and a simple single pole double throw switch. The on/off switch supplies the current to the double throw switch anytime the pilot turns the switch on. The double pole switch then routes the current to the pump in whichever direction the switch is set to cause the pump to run in that direction. The pilot merely selects the direction he wants the gear to travel with the selector switch and then when he is ready he turns the power switch to "on" and the gear responds accordingly. As soon as the pilot sees the gear position lights indicate the sequence is completed, he simply turns the switch off. Many of you have flown with me and watched me set the selector switch to retract while going through my check list prior to takeoff. With the switch in that position, as soon as the gear comes off the ground, all I have to do is hit the power switch and the gear retracts. As soon as the lights come on, I flip the switch off. Prior to landing I reverse the process. I flip the selector switch to the down position when I drop the flaps and then when I’m ready, hit the power switch for about 5 -6 seconds until the down position lights come on. I use a switching breaker for my off/on switch. This saves space and the one device serves as both the switch and the breaker to protect the circuit. The part # for the selector switch I have been using for the last 13 years is ________(available later)

You can tell by the part # that it is a true aircraft quality switch and it handles the loads no sweat. It has a locking toggle. You have to pull the handle out just a little in order to switch positions. Sometimes you can find them at air shows cheap, but they are available from supply houses.

For all the complexity and expense and potential problems of more complex systems designed to be more automatic, what is the actual difference in the pilot work load of such a system and the basic, simple circuit I just described? ANY control circuit is going to have to have a switch of some kind the pilot is going to have to exercise, and that is no different than the simple system. The only actual difference is that the pilot must select whether he wants the gear to go up or down with the selector switch! But-----, oh my gosh I forgot. The pilot is actually going to have to turn the switch OFF as well. Well, now that blows it doesn’t it. That’s just

GLASS GOOSE GAZETTE * ISSUE #17, June 15, 2000

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