In the early days of aviation, before there were gas turbine engines, piston engines were king. A magneto was used to start that engine. Because it requires no battery or other source of energy, the magneto is a compact and reliable self-contained ignition system, which is why it remains in use in many general aviation applications today.
Since the beginning of World War I in 1914, magneto-equipped aircraft engines have typically been dual plugged, whereby each cylinder has two spark plugs, with each plug having a separate magneto system. Dual plugs provide both redundancy should a magneto fail and better engine performance (through enhanced combustion). Twin sparks provide two flame fronts within the cylinder. These two flame fronts decrease the time needed for the fuel charge to burn, thereby burning more of the fuel at a lower temperature and pressure.
Around 1930, electrical systems began to be installed on aircraft. Powered by a battery and/or a small wind-driven generator, the systems were initially not powerful enough to drive starter motors. The introduction of electric starter motors for aircraft engines increased convenience at the expense of extra weight and complexity. Today, helicopter piston engines use an electric starter motor to get the engine running. Let’s look at some tips and tricks.
Keeping Your Piston Engine Helicopter’s Starting System Healthy
Whether you are in charge of maintaining one piston-powered helicopter or a fleet of 100, you are continually faced with trying to decode what a pilot means when he or she writes “starter problem” on the helicopter’s squawk sheet.
The problem is that few pilots really understand that the engine’s starter doesn’t actually start the aircraft (not by itself anyway). The starter is actually the last part of a sophisticated, multi-component starting system and issues with any of the parts, whether environmental, mechanical or operator induced, will show up as problems with starting the helicopter.
Starting System Components
The starting system is comprised of several inter-related components. Beginning at the battery, the system progresses through electrical connectors, electrical conductors and switching devices and finally ends at the starter itself. The health of the entire system must be well maintained in order to achieve consistent engine starting performance.
In addition to individual performance issues with these components, if the engine is misadjusted or has a poorly operating fuel system, the helicopter will also be difficult to start. Nothing’s easy on a helicopter. To that end, let’s start with a refresher on each of the five components and highlight their individual roles and their impact on how the starting system operates.
• The battery: Batteries used on piston helicopters can vary in size and mounting location, either of which can have an effect on the performance of the starting system.
• Electrical connectors: There are more connectors in a typical system than any other component. They serve as the termination points for the electrical conductors that interconnect all of the starting system’s various components.
• Electrical Conductors: Typically these are highly flexible insulated copper or aluminum cables. The length and size of each has a significant impact on the system’s performance.
• Switching devices: Their primary use is to control the flow of electrical power throughout the starting system.
• The starter: This is the actual unit that converts the electrical power to mechanical energy in the form of torque, which is used to physically rotate the engine to initiate the starting process. No matter what the cause or reason, if any of the system’s components are not working properly, the results can run from poor starter performance to outright damage to the starter itself.
Common Starter System Component Problems and Troubleshooting Tips
As you can see, the components that make up a helicopter’s starting system are diverse. Because all the components are daisy chained together, a small issue with any of the upstream units then travels downstream until it manifests itself as a seemingly major issue with the starter’s performance. All the pilot knows is that “there’s a problem with the starter.”
The next section gives a brief introduction to a few common problems with each of the components, along with some proven basic troubleshooting tips. Remember that every component in the system can be affected by numerous problems and solutions, so always refer to the manufacturer’s maintenance instructions before undertaking any troubleshooting or repair activities.
Common Battery Problems
While the typical lead acid or NiCad helicopter battery is a rugged unit, it is not immune to problems in its strenuous environment. On a piston helicopter in particular, it’s exposed to heat, cold and moisture extremes, as well as high levels of vibration due to engine, transmission and rotor blade rotation.
The battery’s health has a direct impact on the performance of the entire starting system. For example, a weak battery results in poor starter performance, which requires the starter to remain engaged for a longer period of time to affect a successful start. This extended engagement causes greater heat buildup in the contactor and starter motor, shortening the service life of these critical components.
Battery Troubleshooting Tips
Follow the manufacturer’s guidelines for ongoing battery inspections, capacity testing and maintenance. Include checking the battery terminals and fittings for corrosion. Even a well-maintained battery can lose 2.5 percent of its charge in a week. If the battery should go unused for more than a month, the best course of action is to remove it from the helicopter and top off its charge.
Common Connector Problems
Even though they form critical electrical interconnection links between components, connectors are perhaps the most overlooked component in the starting system. Their simplicity belies their susceptibility to damage from wear, mishandling and corrosion.
Because of all the forces working against good connector performance, it is critical that each connector in the helicopter’s starter system (and the entire electrical system, for that matter) be routinely inspected for signs of fatigue, distress and/or corrosion.
Connector Troubleshooting Tips
Every connector inspection includes a close look for thermal discoloration, loose connections, physical damage and the buildup of corrosion. A commonly overlooked point of corrosion is the engine-bonding strap. Both ends of this critical component should receive routine inspections.
The system should also be checked for electrical integrity using a Volt-Ohm meter (VOM). A maximum of 0.2 ohms of resistance at any bonding/ground connection is the borderline limit.
Any connector exhibiting corrosion should be removed and cleaned thoroughly. Discoloration is a symptom of a problem that cannot be ignored, and might be a sign of overheating caused by an increase in resistance. Worn or damaged connectors should always be replaced and torqued to proper values to reduce chances of recurring damage. The over-tightening of any connector will cause excessive stress on the connector and lead to permanent mechanical damage and joint failure.
Common Conductor Problems
The primary goal of the conductors (interconnects) is to provide a low-resistance path for electrical current traveling from the battery to the starter. It looks simple — but in the world of piston helicopters, looks can be deceiving. While the typical conductor will last for years, it can be damaged easily through abuse or incorrect installation.
The insulation jacket itself can be damaged as the result of chafing or overheating. Even slight damage to the conductor’s insulation jacket can allow moisture or other metal parts to contact its metal stranding and effectively short the electrical path. An extreme short could create a high electrical energy surge that can damage critical helicopter components. Electrical arcing can occur when the insulation is heavily damaged. This can be extremely hazardous to the helicopter.
Conductor Troubleshooting Tips
Routine visual inspection is critical to maintaining the overall health of the helicopter’s conductors. The leading cause of chafing is when conductors actually rub against other components and compromise the insulation’s effectiveness.
It is also important to inspect the ends of the conductor for fraying at the connection point. Conductors with gaps between the insulation and connector or with excessive fraying should be replaced immediately.
Common Switch/Contactor Problems
The most common problem associated with the switch/contactor is corrosion on the connecting terminals and the wearing out of the internal conductor. The continued opening and closing of the contactor causes electrical arcing between the movable and the stationary contact. This erosion will increase the circuit’s resistance over time and can lead to a total loss of functionality.
Due to its mechanical nature and constant erosion of contact material, the switch/contactor is a common cause of poor “starter” performance. A worn switch/contactor can drop several volts of electrical potential and rob the starter of the power it needs to crank the engine properly.
Switch/Contactor Troubleshooting Tips
Check the switch/contactor’s external electrical connectors for signs of corrosion and wear. Any corrosion should be cleaned properly and parts exhibiting wear should be replaced. Inspect for signs of thermal distress, which is an indicator of unit wear or that the control coil might be failing. If thermal distress is in evidence, the unit should be replaced.
With the switch/contactor under load (both mags grounded and fuel off), use a VOM to measure the voltage drop across the switched terminals. Average drop should be in the order of 100 mv (0.1v). Higher voltage drops might indicate that the contacts are worn.
A drop of 0.5v or higher at any point in the system (excluding the actual starter) is cause for further investigation.
Common Starter Problems
The starter itself is by far the most complex component found in the starting system. It is made up of three primary sections: the motor, the gear reduction system and the engagement system.
The brushes are the first component that will require maintenance for the typical motor. They erode through under activity, mechanical abrasion or electrical arcing. Any of these instances will require that the brushes be cleaned. Next would be the motor’s commutator, which will often have excessive carbon film buildup, causing poor brush contact. If this is the case, the commutator will need to be cleaned per the manufacturer’s recommendations.
While insufficient lubrication can damage the gear reduction system, the primary cause of failure is the result of engine kickback. Kickback occurs during the starting process when the engine’s crankshaft changes rotational direction abruptly. A significant kickback can displace the crank as much as 90 degrees in 33 milliseconds and cause significant damage to the starter’s gear reduction unit. In extreme instances, kickback can actually break the starter’s mounting pad away from the engine.
Later generation starters installed on engines with a ring gear have a solenoid-type retraction/extension system that allows the starter’s pinion gear to engage the ring gear to crank the engine, and then disengage the unit after ignition. Older starter designs utilize a mechanical Bendix type engagement system, which serves the same purpose.
Even today, the Bendix unit is unquestionably the leading cause of the majority of mechanical starter problems. It is highly subject to wear and will fail and become locked in either the extended or retracted position over time.
Failure in the retracted position will prevent engagement with the ring gear, leaving the starter free spinning. When the Bendix unit fails in the extended position, it results in starter overheating and in extreme conditions that can lead to catastrophic failure of the Bendix and the armature, along with damaging the ring gear.
Another common problem area is the starter drive pinion gear. Because it rotates to mesh with the flywheel, mechanical wear is common. Over time, the pinion gear can become worn to the point where engagement is difficult and can result in damage to the flywheel.
Flywheel Ring Gears on Lycoming Engines
In addition to inspecting the pinion gear on Lycoming engines, you also have to pay attention to the flywheel ring gear. The ring gear wears a bit with every engagement, which leads to poor starter engagement performance and eventual damage to the starter’s drive pinion.
Ring Gear Troubleshooting Tips
Flywheel ring gears require frequent inspection to look for signs of damage that might require replacement between scheduled overhauls.
Starter Overheating Problems
Overheating is another common cause of a variety of starter problems. In most instances, overheating is the result of excessive starter cranking without allowing enough time for the starter to cool between starting attempts. This is known as the starter’s duty cycle rating.
While the recommended duty cycle varies by the starter, it consists of a 10-second engagement followed by a 30-second cool down on average. This cycle can usually be repeated up to three times. After that, the starter needs to rest for at least one minute.
Some newer starters have shortened duty cycles to allow for a 10-second engagement followed by a 20-second rest with an indefinite number of starting attempts.
Most pilots are not aware that if they violate the starter’s duty cycle just a couple of times, they can do irreparable damage to the starter’s contactor, armature and brushes, dramatically reducing the starter’s useful life.
Starter Troubleshooting Tips
Perform routine inspections of the starter’s electrical power delivery system for damage, contact security, corrosion buildup and signs of overheating (usually in the form of discoloration around the conductors). Again, you are looking for a 0.5v maximum drop and 0.2 ohms of maximum resistance. Worn connectors are an issue that is often misdiagnosed as a starter problem.
If solenoid failure is suspected, follow the manufacturer’s maintenance instructions for the troubleshooting and repair of the failed solenoid.
Periodically inspect the starter’s pinion gear for signs of wear, dings, nicks, cracks or gouges. If any of these conditions are present, follow the manufacturer’s maintenance instructions.
Kickback issues can often be resolved by adjustments to the engine’s ignition and fuel systems or through the pilot’s “modification” of engine starting techniques. Always follow the engine OEM’s instructions when making changes to the system’s settings or starting procedures.
New-Generation Starters from Hartzell Engine Technologies
While the functionality of starters hasn’t changed in decades, Hartzell Engine Technologies (HET) has recently introduced an array of new-generation starters, led by its newest lightweight E-Drive and X-Drive units.
HET took a different approach when it began to develop these new starters. The HET E-Drive and X-Drive were the first starters to be designed and manufactured with absolutely no automotive aftermarket parts. These new starters are totally optimized to take into account the unique workloads that they would be facing in fixed-wing and helicopter applications.
Both units represent significant advancements in starter design, manufacturing and performance.
Highlighting the E-Drive’s many improvements is HET’s immunity to the kickback problems that plague prior-generation starters. Instead of using a shear pin that can easily break increasing maintenance downtime and costs, the E-Drive introduces a proprietary drive system that allows the starter’s pinion to rotate in the non-standard direction once the torque threshold is reached.
This proprietary drive system also introduces a new tuned detent clutch mechanism that permits the mechanical release of the stressful energy from the kickback event without damaging the starter. The system immediately re-engages when the torque falls below the normal threshold level. HET’s kickback immunity technology not only makes the gear engagement system more durable, it makes the entire starter more robust and damage tolerant.
To further ensure that these new starters would provide years of service, HET did static and dynamic load testing on the unit’s housing and mounting pad. The tests show that the X-Drive is 30 percent stronger than the competitive unit from a structural standpoint.
The E-Drive’s kickback immunity system’s performance was verified through extensive engine and laboratory simulation testing. HET is the first manufacturer to bring this system/technology into the aviation industry.
For more information about HET’s complete selection of starters, contact your local dealer or visit www.hartzellenginetech.com
Tim Gauntt is the director of product support for Hartzell Engine Technologies LLC. He has been in the aviation industry for more than 30 years as a general aviation mechanic, IA, AMT Part 147 instructor and in other technical support-related positions.