Dem Bones
By Bernie Willis
“The thigh bone connected to the back bone, the back bone connected to the neck bone, the neck bone connected to the head bone, oh hear the word of the Lord!”
Do these lyrics ring in your mind? Sometimes called “Dry Bones” were written in the 1920’s by African-American song writer James Weldon Johnson. The catchy rhythm and melody has a spiritual meaning as well as fun way to learn about our bodies… and our airplanes. This article is about a particular circuit that exists in almost all of our airplanes. Call it the starting circuit. If you press a button or turn a key to start the engine you have a starting circuit.
A circuit is in fact a circle. It has no end. Electronic action moves from part to part to activate appliances that turn the engine in an attempt to get it running on fuel. Hydrocarbon fuel is more concentrated than electricity so is easier to carry. For many years engines running on gas or diesel have been preferred over electricity. But electricity is needed to begin the process. The only way to avoid a starting circuit is to turn the engine by hand or prop it. If when you press the start button or key nothing happens, what do you do?
This starting circle, circuit, is about the same whether you fly a Cub, C- 185 or Bonanza. Each electrical connection must be able to process the necessary flow of electrons to the next section for the system to work.
Follow the steps with me and see how many places there are for a malfunction to occur. Beginning with the battery, a large wire connects the positive terminal to the master/battery solenoid. The master or battery switch on the panel when turned on connects the positive battery terminal through the small stud on the master relay to the negative side of the battery which powers an electromagnet switch (master solenoid) to power another electromagnet switch (starter solenoid). The start button/key takes power from the battery to connect the large wire on the solenoid to close the switch and power the large wire on the other side of the solenoid which powers the starter motor. The starter motor spins only if the negative side of its electromagnetic windings are connected to the engine through one of the carbon brushes running on its armature and connected to the
engine case which is connected to the airframe around the rubber engine mount which is connected to the battery negative post. This completes the circle/circuit.
Each of these connections has 7 parts, the wire, the 2 ends of the wire, the terminals or crimped end to the wire and the nut or bolt connecting the terminal to the device. If you came up with 57 places for a disconnection we agree. Hope that’s about right.
What can be done? Lets’s say the tide is flowing or rain is rising the river shortening the takeoff area and the starter won’t turn the engine. Is the problem a dead battery? Not necessarily. Check each of these connections. Are any of them loose? Tighten them. If dirty looking, take apart, scrape with a knife then retighten. Looseness or corrosion causes resistance which will feel warm compared to surrounding parts. This is a warning sign to clean the contact surfaces. A broken wire should be obvious. It’s likely to be a small 20 gauge wire easily replaced temporarily with a spare from your tool kit. If you left your battery/master switch on the master solenoid will drain the battery in time. Now the choices are few. A substitute battery, a jump from another battery or a hand start. If as a last resort you must start by hand and have an alternator instead of a generator the charging system won’t work so all your new avionics and engine monitors will be out of service too. If you have at least one old fashion mag your engine will run and you can limp home
Den bones must be connected to make us run. Those wires must be connected to get us get started. The charging circuit another later.