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Proper maintenance is the key to reliable service

By Steven W. Ells
This is a Sky-Tec starter.
The starter drive end of a Sky-Tec starter shows the starter pinion teeth in a retracted position.
The electromechanical actuator on the Sky-Tec starter replaces Bendix-type (centrifugal) starter drives.

Woe to starters for they're caught in a whipsaw — pilots expect action when they turn the key. Rain, shine, sleet, hail, or snow, the propeller must rotate. Yet starting systems need to be as light as possible, especially since they slip into the dead-weight category after a few seconds of hard work at the beginning of each flight. A starter that doesn't kick an engine into action is likely to be the target of abuse, but a starter motor that won't stop turning can really raise havoc.

Despite the divergence of these requirements, starter motors are up to the task. Starter problems are more often traceable to starting system problems, or battery problems, or starter drive problems than motor problems.

Within the past 10 years permanent magnet technology has been incorporated into GA starters. These starters have more power, and are lighter than the Delco Remy and Prestolite starters of yesteryear. These permanent-magnet starters have another thing the old-style starters didn't have a large initial starting current draw that causes electrical system voltages to sag momentarily.

The traditional starter gets its power from a high-torque, series-wound, direct current electric motor. Current flow through heavy field windings creates magnetic lines of flux that cause motor rotation. The new lightweight starters don't have field windings high-strength permanent magnets are used instead.

Electricity 101

Current is the flow of energy (measured in amperes, or amps) through a circuit. Voltage is the amount of force that causes energy flow in a circuit. Devices that produce work, such as motors, light bulbs, and radios, are called loads, or conductors, and have resistance to current flow, which is measured in ohms. A handy reminder is: A volt pushes an amp through an ohm.

Ohm's law, which states that the current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance, helps define these values in any circuit. The values in series circuits, since they only have one path for current flow, are simple to derive if two of the three values are known. Starter circuits are series-type circuits.

The other common type of circuit is a parallel circuit, in which there is more than one path for current flow to the load. Christmas tree lights illustrate the two circuit differences perfectly. Older light sets were wired in series; if one light burned out (opened the circuit) then all the lights were out. Modern light sets are wired in parallel so when a single light burns out, the current flow to the other lights (loads) in the circuit isn't interrupted. This concept is important because any unwanted resistance in the series-type starter circuit reduces current flow through the remainder of the circuit.

When the starter motor is called to action, a tremendous amount of current is needed. It's not unusual for the old-design Delco Remy or Prestolite series-wound starter motors to draw 150 to 200 amps. Permanent magnet starters draw even more current because they're more powerful they produce one-third more starting power and develop more than two horsepower.

Starter circuits that restrict the full flow of current because of corrosion or poor contacts, or starter motors that can't effectively utilize the available current because of dirty brushes or resistive commutator segments, draw much more current than clean, well-maintained systems. So poorly maintained circuits are doubly frustrating because they can't crank the engine vigorously, and because the battery's cranking power is soon depleted.

A typical starter circuit

The typical lightplane starter circuit doesn't have circuit protection devices such as fuses or circuit breakers. These circuits require thick wire because of the high current flows required during starting.

The starter circuit in the Mooney M20K is typical since number-2 wire carries the current from the battery contactor to the starter contactor, then on to the starter motor. Number-2 wire is thick it's the diameter of a crayon.

Because the resistance of wire increases directly with its length, and inversely with its cross-sectional area, in an ideal starter system the starter and battery would be inches apart.

Yet for weight and balance purposes many GA airplanes have the battery in the tail cone this requires thicker wire than the ideal side-by-side layout. Resistance over the length of a long battery-to-starter circuit often contributes to starter problems.


The bulky starter wire runs from the battery contactor to the starter motor contactor. Contactors are used because it's impractical to route these large, bulky, stiff wires into the cockpit. In addition, the switches required to handle the current flows would be large — probably not quite as large as the switch Igor flipped to bring Frankenstein's monster to life, but you get the idea.

Contactors are remote-control switches that are designed to handle large current flows. The circuits that turn the contactors on and off are controlled with light-duty instrument panel-mounted switches. Contactors are relatively inexpensive and they shouldn't be ignored.

Here's what happened to a Cessna 185 I once worked on. The starter contactor welded itself shut and couldn't open after the engine started. The starter continued to draw a lot of current, resulting in a high-amperage battery discharge. The voltage regulator detected low system voltage and told the alternator to increase its output. The pilot finally shut the engine down when he smelled electrolyte boiling off the overheated battery. The battery, starter motor, alternator, battery contactor, and starter contactor were ruined. The battery box and surrounding aluminum structure had to be thoroughly flushed with baking soda and water to neutralize the battery acid.

This all could have been avoided with a simple "starter engaged" warning light. The new Lancair 300s have one in their annunciator panel, as does the Robinson R44 helicopter.

A very simple one-wire circuit to an instrument panel warning light is all that's required. Since I'm not aware of an STC covering this, a starter-engaged warning light circuit installation would depend on the willingness of the local FAA flight standards district office to provide a field approval. In the meantime, if the starter contactor on your airplane has been in service longer than 20 years, I'd replace it.

Remote control

When the master switch is turned on, a path for the battery-contactor control circuit is completed. This closes the high-current-capacity points in the contactor and completes the battery-to-aircraft electrical connection. A very simple starter circuit and troubleshooting diagram is online (

A similar looking control circuit closes the starter contactor. Despite being similar in appearance, the two contactors are different — the battery contactor is rated for constant duty while the starter contactor is for intermittent duty. Since opening and closing can cause arcing across the high-capacity points, resistance to current flow through contactors does increase as contactors age.

Typically, the control circuit for the battery contactor is at the airplane master switch, and the control circuit for the starter is in the magneto key circuit, although there may be a separate starter button, or even a manual pull-start T-handle like the one used on low-horsepower Continental engines.

When the starter contactor closes, the starter is connected directly to the battery, resulting in instantaneous motor rotation. How to connect, and disconnect, the high-torque starter to the engine without any damage is the next piece of the puzzle.

Starter clutches

Starters for Lycoming engines are mounted on the lower-left-front part of the engine. These starters have a pinion gear that engages a series of teeth that are cut into a circular ring gear. This ring gear has a set of specially designed teeth and is shrunk onto the outer circumference of the ring-gear support assembly, which also has a pulley groove for the alternator or generator drive belt. The support assembly is sandwiched between the propeller mounting flange and the crankshaft flange. Rotation of the starter motor is transferred to the crankshaft through this arrangement.

The standard connection between the starter motor and the engine on Lycoming installations has been a centrifugal, or Bendix, starter drive. The Bendix drive extends to mesh its pinion-gear teeth with the ring-gear teeth when the starter motor begins to rotate, and retracts as the teeth on the ring gear accelerate during engine start. One of the drawbacks of the Bendix-type drive is that it's more exposed to weather and dirt than the system used on Continental engines. Preventive maintenance in the form of keeping the shaft clean and lubricated by the regular application of silicon spray will smooth Bendix operation.

Although Bendix drives were phased out of the automotive world in the mid-1950s, they are still standard equipment on some new airplane starters. However, the B&C Specialty Products starter uses a linear actuator, and the Sky-Tec starter uses an electromechanical actuator to replace the Bendix drive. If the engine doesn't start, the Bendix stays extended, meshing tooth-to-tooth with the ring gear. An extended Bendix can create problems if hand-propping is required.

Continental clutches

The starters on smaller Continental engines (early 145-horsepower and smaller models) are mounted on the back of the engine accessory case. Between the starter and the engine is a clutch assembly. There are two types of these starters — the pull type and the key-start type. The pinion gear on a pull-type clutch is mechanically moved into engagement with a gear in the engine accessory section by pulling on a T-handle. If the T-handle pull cable is rigged correctly, after full gear engagement, additional pulling depresses a switch that completes the electrical circuit from the battery to the motor, and the engine rotates.

When the engine starts, the T-handle is released and the pinion gear retracts. The key to this type of system is to ensure that the rigging is correct and that the T-handle and cable move smoothly. Rigging instructions are in the aircraft service manual.

The key-start clutch assembly has a one-way clutch that engages when the starter is driving the airplane engine and freewheels when the engine is running. Neither of these clutches is regarded as a starter part, and both are sold separately. Neither of these systems uses a starter contactor.

Starters for big-bore Continental starter motors also have a clutchlike arrangement to absorb and transfer the torque from the starter to the engine. Starting with 145-hp six-cylinder O-300D engines, a 90-degree starter adapter was installed on the upper-right-rear quarter of all TCM engines. A spring between the starter motor and an accessory gear tightens up as the starter rotates. After a couple of revolutions, the spring tightens its grip on a drumlike section of a shaft and this transfers rotation to the engine.

These starter adapters are very dependable but there is some evidence that some of the new permanent magnet starters, especially the starters that achieve their power through internal planetary gear-reduction devices, don't rotate freely after electrical power has been removed. This inability of these starters to "unwind" prevents or delays the complete release of the spring-drum connection. This can cause damage that ranges from broken starter-adapter springs (bad noises and zero rotation) to rapid wear of the adapter components with accompanying metal contamination of the oil system.

For more information on starter and starter-adapter compatibility, visit the Niagara Air Parts Web site (

The new starters

Within the past five years, TCM, Electrosystems, Lamar, and Sky-Tec have introduced new starters that substitute powerful rare-earth permanent magnets for the heavy field windings.

Electrosystems (MagnaFlite), Teledyne Continental (Iskra), and Sky-Tec are selling permanent magnet, reduction-gear-type starters. The Sky-Tec HT starter is a reduction-gear, series-wound starter. Lamar uses permanent-magnet technology in a direct-drive application (with a Bendix drive). B&C Specialty Products also makes a lightweight starter but continues to use series-wound construction. The weight reduction in the B&C starter is accomplished through modern close-tolerance manufacturing techniques.

Starter-system woes

Whenever slow engine rotation occurs during starting, the primary suspects are a weak battery, resistance in the wiring or contactors, or dirty or worn starter internal components.

The average 25- or 35-amp-hour lead-acid flooded-cell aircraft battery must be kept clean and serviced with distilled water for good service. A better alternative to the old flooded-cell technology can be found in recombinant gas, or RG, batteries. In addition to being able to supply more cranking power, the batteries don't leak and won't off-gas, so the possibility of corrosion around the battery box is greatly reduced.

Voltage measurements are simple and help locate corrosion in the connections, cables, and contactors between the battery and starter. A well-maintained system delivers more than 10 volts to the starter with the starter energized. Taking the time to find and remove resistance in the system helps to keep the starter performing.

Resistance to current flow occurs between the crimped or soldered terminals and the wires, especially the aluminum battery cables used on some light airplanes. Often, cutting or sawing off the old terminals, cleaning the wires, and crimping on new terminals eliminates high-resistance connections. Bogert Aviation (, of Kennewick, Washington, sells ready-to-install STCed copper replacement cables.

High resistance occurs within contactors. The high-voltage contacts arc every time the contactors open and close. Over the years this lessens the point contact area, increasing resistance. A test comparing the voltage at one side of the contactor to the voltage at the other side (with the starter engaged) is a good test of contactor internal resistance. If the voltages vary by more than one volt, replace the contactor.

If the cable connections are resistance-free, the contactors don't cause an excessive voltage drop, the battery is healthy, and the starter is still weak or slows to a stop when each piston moves up on the compression stroke, it's time to get a new or rebuilt starter.

Proper preventive maintenance of the storage battery, the wires that connect the starter and the battery, and the contactors make your starting system work with vigor and cut down on the anxiety of wondering if your airplane will start when you turn the key.