Electrical System Safety
There are ultralights being flown that do not have an electrical
system with a battery which allows you to have a starter, anticollision
lights, radio and all the other good things that system provides.
I have noticed a significant difference between the electrical
system used in automobiles as compared to boats and airplanes.
Automobiles have no master switch. No. The ignition switch is
not a master switch. All airplanes and some boats with inboard
engines generally have a master switch.
What is the master switch, you ask? The master switch is the first switch on the checklist for starting. It "turns on" the battery and all the power for the electrical systems except ignition, which is powered by the magneto. The master switch actuates a solenoid, which has large contacts capable of handling the current needed by all the electrical systems and the very high current required by the starter. In short, it connects and disconnects the battery to the airplane.
Automobiles don't have such a thing and seldom need to. Aircraft need them because there are times when your safety will depend upon it. I will cite some examples I have been a part of. A Cessna 150 was returning from a cross-country flight when something happened to the power system. It was evening and the pilot (grandson of the plane's owner) turned on the landing lights, which went on momentarily and then went out. A short-circuit condition was suspected. He decided not to turn on the radio but flew the pattern and made sure it was safe for him to land. Fortunately it was an uncontrolled airfield without much traffic at that hour. After landing he found that the landing lights were blackened from being burned out and the fuse was blown.
The next day I was called by the A&P mechanic to find out what happened. I suspected that the voltage regulator had failed, so we started the engine and checked the voltage of the generator at the battery terminals, which showed 16.5 volts. The mechanic opened the battery box and the battery was found to have been boiling so there was a clean-up job to do. Usually the voltage regulator fails by low voltage output, but this was a high-voltage condition.
"Why didn't the fuse protect the landing lights?" the mechanic asked. When the landing lights were switched on a large current flow, resulting from the excessive voltage, surged through the filaments of the bulbs so quickly the fuse blew as the bulbs blew. An interesting aside: The tungsten filament of a bulb, when cold, has a low resistance such that the initial instantaneous current is from 8 to 10 times the steady state current. Bearing that in mind you can understand why everything burned out in a flash.
If the plane had a generator switch and a voltmeter, the pilot could have identified the problem and turned off the generator. Then he could have used the radio and landing lights from battery power. Some larger planes have a generator switch and voltmeter but the little ones don't. Luckily the only other thing that got burned out was the directional gyro.
Some months earlier, on this same plane, smoke was detected in the cabin. Fearing the possibility of fire the master switch was turned off. Later a short circuit was found. Usually this condition causes the circuit breaker or fuse to blow, but sometimes that doesn't happen. At those times it's nice to have another means of cutting off the electrical power.
Here is one that happened to me. In starting my ultralight one day the contacts in the starter solenoid welded closed. With the engine turned off the starter kept on spinning the engine over. I had wired my machine like an automobile so the only thing I could do was to break the battery connection. Knowing that the certified aircraft's master solenoid was rather expensive, I looked in a catalog for boat supplies and there I found a look-alike without a metal case but having all the power-handling capability I needed and more. To give you some idea of what it is capable of, its label says it is rated to handle 65 amps continuous and 750 amps for 10 seconds.
What is the downside, you ask? Its weight is a little less than 1 pound. Sure it requires some current to keep it closed but that is only .66 amps, about the same as a small light bulb. Not bad for what it can do. Also, you have to remember to put it on your checklist to be turned off during shutdown because if you forget, the next time you are going to fly, the battery will be discharged.
If your radio or other electronic equipment is turned on when you are turning off the master switch, the diode is necessary to suppress the high voltage kick that occurs when the actuating coil is turned off. The idea is to protect semiconductors from the resulting voltage transient that can puncture them. If you don't have semiconductor devices in the system, the diode can be left out.
If your local marine equipment supplier doesn't have this item, contact: Cole Hersee Co., 20 Old Colony Ave., South Boston, MA 02127. Phone: (617) 268-2100 (ask for part #24117-01).
Arnold C. Anderson has been flying ultralights since 1982, logging more than 300 hours in his Kasperwing. After 37 years in the engine and aerospace industry as a mechanical engineer, designing electro-mechanical equipment and solving reliability problems in equipment for unmanned deep space missions, Arnold is now retired. He lives in Bellevue, Washington, where he pursues his hobbies, including aerial photography and flying RC airplanes and gliders.
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