By Vic Syracuse, EAA Lifetime 180848
This piece originally ran in Vic’s Checkpoints column in the August 2021 issue of EAA Sport Aviation magazine.
Some of the most common calls I receive from pilots have to do with starting problems. It’s usually one of four scenarios. First, “I turn the key and nothing happens.” Second, “I haven’t flown for a few weeks, and it just makes a clicking noise when I turn the key to START.” Third, “The prop turns really slowly and then stops.” And, lastly, “I just landed and got fuel at the pumps, and now it won’t restart.”
Perhaps some of you can relate to one of the four I just mentioned, or you have your own story to tell. Usually, these problems happen not at the home field but when you’re on a vacation trip away from home, and they can put a real damper on the fun factor, especially when the family is standing around waiting. So, I thought it might be beneficial for all of us if I spent some time on this topic and covered all four scenarios.
We all learned that combustion engines require three things to make noise: air, fuel, and spark. They need to be available at the right time and in the right quantities to get the fire lit. But, ahead of that, there needs to be some rotational force applied so that things can begin happening in the right sequence. That rotational force can be anything from an “Armstrong” starter for those aircraft without electrical systems to an explosive shell, which was used on early jet fighters that were on alert status. Most general aviation pilots have become used to pushing a button or turning a key to get things started. Turbine pilots are used to having an aircart or APU that supplies the air to start things spinning.
Let’s start with the first scenario. For the typical pilot (that’s us), that sound of silence when the key is turned is “deafening.” Let’s begin with some basics. First, the heart and soul of a good starting system begins with the battery, but it doesn’t end there (more about that in a bit). On the typical airplane that most of us fly, with a four- or six-cylinder reciprocating engine, the battery is expected to deliver 80-200 amps when energized. Yep, that’s a lot of power, and more than some welding units can supply. So, having a fully charged battery is important. Most batteries have a specified OCV (open circuit voltage), which is the voltage of the battery at rest with a voltmeter. This should be more than 12.5 volts DC. Most of our sealed batteries today will read 12.8 volts or higher, depending upon the chemistry. One way to check the health of your battery is to take a look at the voltmeter when you energize the master switch, as a good battery should be more than 12 volts. When you activate the starting circuit, the voltage should remain greater than 9.8 volts, and anything less than that may indicate a weak battery or potentially some other issues that I will explain later.
Some normal causes for a weak battery could be the age. When treated well, most batteries can have a life expectancy of five to eight years or more. I’m a believer in replacing every four to five years for reliability, especially if you fly on instruments or land regularly at places without facilities. Treating the battery well begins with paying attention to the charging circuit in your aircraft. I know we’ve all learned to check the oil pressure as soon as we start up, but then the next check should be to check the charging system with the alternator ON, both for volts and amps. A good voltage should be between 13.6 and 14.4 volts. Any less and you will not adequately charge the battery. Anything more and over time you will potentially “cook” the battery. Amps at startup should momentarily be high and then gradually taper off to a steady state. By the way, depending upon how your ammeter is wired, it may show only a battery charging flow or the whole electrical load on the aircraft. If you see the ammeter reading a steady low reading but the voltage is normal, then it is wired to show the charging current to the battery. Normal indication in flight should be a steady 2-5 amps unless the load exceeds the alternator’s capability. Then you may see a negative number as the battery will try to make up the difference. If you see a negative number at runup or cruise rpm, there’s a pretty good chance that the alternator has failed. I’m always amazed at how many pilots can’t tell me what the charging voltage is on their airplane. Amps and volts should be a regular part of your scan.
Another common cause for a weak battery is leaving a battery charger connected to your aircraft. Unless you are using the absolute right charger for your battery, you will do more damage by leaving it constantly connected. Most modern sealed batteries can be left for months without needing a charge and will start your engine just fine. Don’t be so quick to attach that charger every time you land.
There’s another cause for a weak battery that can cause some consternation, and I know there are some of you reading this right now who are going to tell me that unless you leave the charger connected, the battery will be dead within two weeks. That scenario is usually caused by a parasitic drain on the battery, typically caused by something being wired to the hot side of the battery. Some of the primary culprits I’ve seen are 406 MHz ELTs (as they require power to them), USB charging outlets that are hot-wired and have the device still plugged in after the aircraft is shut down, and baggage compartment lights that are hardwired (it’s very easy to accidentally flip the baggage compartment switch during the daytime and not even notice it). A quick test for parasitic drain is to disconnect your battery’s positive terminal and put an ammeter between the battery and the cable. It should read zero. If you see anything else, which usually will be some very low reading in the milliamps, you need to start chasing down circuits to find the culprit, as even a milliamp draw will drain the battery within a few weeks.
Assuming we have a known good battery and there is still silence when the starter switch is activated, let’s continue down the path to the next possible culprit — the starter solenoid. Listen carefully when you hit the starter button. You should be able to hear the click or “thunk” that the starter solenoid should make when it is activated. If you don’t hear that, then the first thing to check would be the fuse or circuit breaker. If all is well there, it may be time to get a mechanic involved, unless of course you have built the airplane or understand some of the next steps. A typical starter solenoid requires an activation voltage that will be the same as your battery voltage, so look for the terminal labeled S1 on the solenoid, and you should measure buss voltage when the starter circuit is energized. Be sure to stay clear of the prop just in case it decides to work! If you have no voltage, then it is a circuit problem. If you have voltage, then it is most likely a solenoid failure, and a replacement should rectify the problem. Sometimes, tapping on the solenoid with a screwdriver handle or rubber mallet will free up a stuck one. On some aircraft, and especially cars, the starter solenoid is part of the starter motor, and tapping on it can rectify the problem as well, and at least get you on your way.
If the solenoid is working properly and the propeller doesn’t move, there is a good chance that the starter is defective. Please be careful with this next test! Place your voltmeter at the starter connection and verify that you have buss voltage at the starter when the circuit is activated. If you do, then there’s a high probability that the starter is defective. Try the rapping it with the hammer trick. It might work and get you going, but it still needs to be replaced.
Another possible outcome is that you measure very low voltage at the starter when activated. This is scenarios three and four. If at the starter you have a voltage lower than 9.8 volts, there’s a chance that you really do have a weak battery, so measure the voltage right at the battery as you activate the starter circuit. If it’s the same or close to the voltage reading at the starter, then it’s battery replacement time. If the battery maintains a high voltage, we have some other things to look at.
The starter solenoid could still be the culprit, as the contacts inside it can get pitted over time from arcing and start to cause high resistance. Measure the voltage on both sides of the solenoid when activating the starter circuit. They should be really close to being the same. If they are, start checking for a loose or broken ground connection. The ground wire to the engine from the aircraft frame should be the same size as the starter wire, usually 02 AWG or larger. I have seen ground wires pull right out of their connections when tugged on. In fact, Van’s had a service bulletin for some prefab cables that were improperly swaged, and we have found many of these over the years. What happens when the ground is broken is that all of that current has to find other paths, which can end up being control cables or even signal wires, which aren’t usually big enough to handle all of the amps required by the starter. Sometimes, control cables have even been welded stuck by having all of the current pass through them.
In this column, I covered the reasons for deafening silence when the key is turned. I’m out of space, so next month I will cover reasons why the prop could be turning, but the engine still won’t start.
In the meantime, keep the fun factor alive! For those who read last month’s column and learned about my helicopter escapades, know that I am now having fun with it. I think I am about a month away from a checkride and can’t wait.
Vic Syracuse, EAA Lifetime 180848 and chair of EAA’s Homebuilt Aircraft Council, is a commercial pilot, A&P/IA, DAR, and EAA flight advisor and technical counselor. He has built 11 aircraft and has logged more 9,500 hours in 72 different types. Vic also founded Base Leg Aviation and volunteers as a Young Eagles pilot and an Angel Flight pilot.