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Getting Current: Troubleshooting a Landing Light Circuit

Getting Current: Troubleshooting a Landing Light Circuit

Finding and repairing a broken circuit is the subject of this fourth installment of A&P Jacqueline Shipe’s DIY series. 

Among the many preventive maintenance items listed in FAR 43 Appendix A that a pilot may legally perform on his or her plane is “troubleshooting and repairing a broken landing light circuit.” This specific entry is the only reference to electrical circuit troubleshooting on the list. 

Most electrical circuits for lights or pitot heat, etc. are fairly straightforward, while a wiring harness for a unit like a panel-mounted GPS can be very complex. This article will focus on the tools and expertise required to successfully troubleshoot a landing light.

Study the diagram

On any electrical circuit, the best troubleshooting tool is always the current wiring diagram pertinent to the model and serial number of the airplane. Learning how to read a wiring or system schematic can help a pilot not only in performing repairs, but also in understanding how a unit or system actually works. 

Everything electrical has to have a power source and a ground to operate. Some circuits contain numerous switches and circuitry that work in conjunction with each other to provide the needed power or ground. 

When a fault occurs, knowing how to dissect that circuit into sections—and understanding when and where voltage or a ground is supposed to be present—is essential. The wiring diagram provides all the needed information. 

There are standard symbols used on these diagrams to indicate different components in a circuit. There is always a symbology chart somewhere in the maintenance manual wiring section that lists the symbols and the components they represent. 

Some of these symbols are drawn to look somewhat like the component they represent, such as a circuit breaker. Switches, contactors and relays are generally shown on diagrams in the open (or “relaxed”) condition unless otherwise noted. 

To get familiar with a specific circuit, follow the flow of a circuit on a diagram and consult the chart when you see an unknown symbol. It doesn’t take long before the symbols all become familiar.


The parts of a circuit

Exterior light circuits are some of the most straightforward circuits on any plane, and the diagrams provide good practice for folks first learning to read a wiring schematic. Generally, it is best to start at the power source in the diagram and read down from there. 

In figure 9-35 on page 24, the Piper Comanche landing light circuit, power comes from the bus bar to a 20-amp circuit breaker, which is a shared breaker for two separate landing light circuits. The number 14 indicates that the wire size for that section is 14 gauge. 

L1A and L2A indicate the wire numbers; “L” representing a lighting circuit. (Each original wire on a plane is stamped multiple times along its entire length with the appropriate wire number, which helps tremendously.) 

Both switches are shown in the open or “off” position. Coming out of each switch is the “B” section of each wire, still 14 gauge, up to a knife connector. Past the knife connector is the “C” section of each wire up to the terminal on the bulb itself. 

The “D” section starts at the opposite terminal on the bulb and goes to ground. This section of wire is a little heavier duty as indicated by the fact that it is 12-gauge wire. 

The wire gauge refers to how big its cross-section is; the smaller the number, the fatter and heavier duty the wire is. Starter and battery wires, for example, are large and heavy duty, generally six gauge or lower. Larger diameter wires can carry much more current than smaller diameter wires.

The airframe itself is generally used to provide a ground on most planes. The exact point where a wire for a circuit is connected to the airframe for a ground is usually not too far from the electrical component itself. 

The airframe should be clean, and the wire terminal should be free of corrosion to ensure there is a resistance-free path for an electrical flow to ground. 

Check the bulb first

First, be sure that any wires being checked are not touching any other wires or the airframe. Don’t allow any metal tools to touch both a live wire and the airframe at the same time. 

In the landing light circuit, the quickest and easiest place to go to troubleshoot an inoperative bulb is to check the bulb itself. 

After removing individual bulbs from the plane, measure the resistance across the terminals with a multimeter set to ohms. This is a very simple task for landing or taxi light bulbs. 

Navigation lights and several of the interior light bulbs utilize a base that inserts into a socket. The small raised area at the bottom of the base is the positive contact point, and the base of the bulb is the ground contact point on these bulbs. A good bulb will show continuity; the resistance varies a little depending on the type of filament it has. 

Strobe bulbs are different; they can only be tested by an operational check. To check a strobe bulb, put the suspect bulb in a known good circuit, turn it on and see if the bulb works. If one wing strobe bulb works but the other doesn’t, switch the bulbs and you’ll immediately know if it’s the bulb or something else in the circuit that’s causing the trouble.

Check these things next

The two main checks that are required when troubleshooting a circuit are for voltage and a ground. Voltage is a measurement of the potential amount of electric power coming in to a certain point. It is not an indication of how much power is actually flowing. 

Amperage measurements give an indication of the actual amount of power (current) that is flowing. Most meters only measure voltage, but some do have amperage settings. 

Voltage is easier to measure because it can be checked anywhere in the circuit by placing the meter in parallel with the circuit. Amperage is harder to read because the meter has to be placed in series within the circuit, and the circuit has to be complete so that current is actually flowing. 


Voltage and resistance can be easily measured with an electrical multimeter. There are many different manufacturers of multimeters; even the most inexpensive ones are generally good enough to troubleshoot most electrical issues. 

Before taking any measurements, it is a good idea to set the meter to the lowest ohms setting and touch the test leads together. This tests the connection of the leads to the meter and also the continuity of the test lead wires as well as the internal resistance of the meter. The reading should be zero ideally, but in any case it shouldn’t be much over one ohm. 

When the aircraft battery is on and the landing light switches are closed (i.e., turned on), there should be 12 volts at the terminal of the L1C and L2C wires. There should also be a very low resistance path to ground, which is measured on the L1D and L2D terminals. 

In the landing light circuit, voltage can easily be measured by placing the positive probe of the multimeter on the light terminal for the L1C or L2C wire (depending on which bulb is being checked) and the negative probe on a clean, bare metal area of the airframe for a ground. (Some owner-pilots use a small alligator clip to connect the black lead off the multimeter to a spot on the airframe. —Ed.)

With the appropriate switch flipped on, the voltage reading should be approximately the same as battery voltage if the circuit is working properly.



Most of the time, a voltage measurement is all that is needed to be assured that power is being received, but it is good to know how to check amperage. 

An electric motor that is operating a little slower than normal or is on the verge of shorting out typically begins to draw an excessive current load. A high amperage reading will also be present if there is too much resistance to motion in the mechanical apparatus the motor is trying to move. 

Flaps that are binding, or a landing gear retraction or extension mechanism that is not properly adjusted will cause a motor to draw a high current load and get hot. Checking for a higher than normal amperage reading can allow you to detect a malfunction and fix it before it causes a total failure.

Amperage measurements are also useful to confirm that a component is receiving the full amount of electrical energy it needs to operate. Most of the time, voltage readings are sufficient for this, but there are some circumstances where a voltage indication can be misleading. 

A wire that is barely connected or a switch that has badly burned contacts can still make enough of a connection to show full battery voltage at points in the circuit beyond, but will not be able to actually carry enough amperage to operate different components downstream. This can cause a lot of confusion, but is a fairly rare circumstance.

If all other checks pass with proper voltage and a good ground being indicated, and a known unit that is operable still won’t function, it would be prudent to see how much amperage the unit is getting; there could very well be a poor connection in the circuit somewhere upstream, even though the voltage readings are correct. 

Ground connection

Ground connections are measured in ohms of resistance. Generally a reading of two ohms or less is indicative of a good connection to ground; readings that are five ohms or higher are cause for some concern. 

The airframe itself is used to ground most electrical circuits. The airframe often develops corrosion, which can cause excessive resistance in ground connections. Usually disconnecting a ground wire and cleaning the terminal and contacting airframe with 220 grit sandpaper or an abrasive pad (i.e., Scotch-Brite) clears it right up. 

With a little practice and persistence, pilots will be able to interpret wiring diagrams, a multimeter will become easier to use, and electrical problems will seem less complex. 

Most electrical issues can generally be traced to a problem that is fairly easy to fix. Knowing how to troubleshoot a circuit and read a schematic will save a pilot/owner both time and money in the long run. 

Know your FAR/AIM and check with your mechanic before starting any work.


Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Voltmeter and multimeter tools
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Aircraft Spruce & Specialty Co.
Chief Aircraft
Aircraft Battery Care: "Learning your ABCs"

Aircraft Battery Care: "Learning your ABCs"

Jacqueline Shipe, A&P, explains the technology and preventive maintenance for aviation batteries in her sixth DIY article targeted to owner-pilots. 

The bulk of the items listed in FAR 43 Appendix A, paragraph (c) that an owner may legally perform on his or her owned aircraft are primarily maintenance tasks that have to be performed on a fairly regular basis. 

This is definitely true concerning aircraft battery maintenance, and “servicing or replacement of aircraft batteries” is included on the list of 31 preventive maintenance items. 

All batteries begin to degrade in performance from the moment they are placed in service. The constant chemical reactions that take place cause an ever-increasing lack of efficiency within the battery. This is especially true of batteries that are allowed to run down and remain in a low or depleted state.

Lead-acid batteries are the type used in almost all General Aviation planes and are becoming more common for turbines employed in low-cyclic applications like medevac. (Turbine powered planes in high-cyclic applications (i.e., airliners) often have nickel cadmium or “NiCad” batteries installed. These batteries are costly, and the servicing requirements are much more complex than for the lead-acid batteries. NiCad batteries should only be serviced by a professional.) 

Anatomy of a battery

A lead-acid “flooded” battery consists of multiple cells enclosed in a plastic case. Each cell consists of alternating sets of lead plates. 

Half the plates contain lead oxide, and the other half of the plates contain soft spongy lead. The plates are set in an alternating arrangement; each lead oxide plate is next to, but not touching, each spongy lead plate. 

The plates are immersed in an electrolyte solution of sulfuric acid and water. Removable caps allow an owner to inspect and adjust the electrolyte level of the battery. 

Each battery cell produces roughly two volts of electric power. A 12-volt battery has six cells (and six caps) and a 24-volt battery has 12 cells (and 12 caps).  


The chemical reaction

Sulfuric acid produces a chemical reaction between the opposing plates, causing the lead oxide plates to become positively charged and the spongy lead plates to become negatively charged. 

As a battery discharges, the sulfuric acid in the electrolyte solution is converted into lead sulfate on both the positive and negative plates. Lead sulfate is not conductive. As it grows on the plates, covering more and more of the surface area, it reduces the efficiency and output of the battery. 

The discharge process also makes the electrolyte far more watery as the sulfuric acid is depleted. Batteries not only discharge under an electrical load, but they also self-discharge when not being maintained in a fully charged state.

If a battery is left for a prolonged length of time in an uncharged state, it will eventually completely discharge once the plates become so coated in lead sulfate that no more exchanges of electrons or ions can take place.  

The charging process

During the charging process, the chemical process is reversed: the lead sulfate on the plates is converted back into sulfuric acid; lead oxide is redeposited back on the positive plates; and pure lead is deposited back on the negative plates. 

A battery which remains in a depleted state of charge for a prolonged period of time forms lead sulfate that eventually hardens and crystallizes on the plates to the point that it can’t be converted back into its original components of lead oxide, pure lead and sulfuric acid—no matter how long the battery is left on a charger. 


Maintenance-free batteries

Maintenance-free or “sealed” batteries have non-removable covers and the electrolyte level cannot be adjusted. These sealed batteries go by a variety of names: RG, or recombinant gas; AGM, or absorbed glass mat; and VRLA, or valve regulated lead acid. 

These batteries use a fireproof glass mat separator between the positive and negative plates. The glass mat is saturated with electrolyte and the mat’s microporosity allows the hydrogen and oxygen to recombine. 

VRLA batteries are designed to recombine the gases generated during the charge-discharge process and to maintain electrolyte throughout the lifespan of the battery, which makes them maintenance-free for the aircraft owner. 

Extending battery life

The best thing any owner can do to extend the life of his or her battery is to keep it fully charged. The alternator or generator on a plane that is regularly flown helps to keep the battery in a good state of charge. 

A plane that sits for extended periods, however, needs an external charging source to keep the battery maintained in good shape and prevent permanent sulfating of the plates. The Achilles’ heel on any battery is to allow it to completely discharge, especially if the discharge occurs slowly over a long period of time.

Handling a vented aviation battery

Battery acid is harmful to the skin and eyes, so rubber gloves and safety glasses should be worn any time you are charging or servicing the battery in your aircraft. 

To prevent electric shock, ensure that any metal tool that is in contact with the positive battery terminal is not allowed to touch any metal structure on the battery box or airframe.

Anytime the battery is charged or serviced, the best thing to do is to completely remove it from its compartment. 

This can be difficult to do depending on the location of the battery, and all batteries are heavy and can be tough to lift out of the box. The 24-volt batteries are particularly cumbersome. 

The straps that are occasionally installed on the tops of the batteries are only there to aid in the removal from and installation into the battery box. 

Once it is out of the aircraft, the
battery should be supported from underneath; very often the plastic or rope-like straps weaken over time and can easily break. 

Taking care of the battery box

The complete removal of a vented battery from the airplane not only makes it easier to service, but also allows the battery box to be cleaned and inspected. 

A solution of baking soda and water will neutralize any acid residue in the box. 

The drain line should be inspected to be sure it is still attached properly and is clear of any clogs. 

Any corrosion should be thoroughly cleaned off, and the box should be painted with either a zinc chromate primer topped by a good quality epoxy paint or with a bituminous or acid proof paint that is specially made for battery boxes. (Battery box modifications for Piper aircraft are available by STC from Bogert Aviation. —Ed.)


Adjusting electrolyte levels

In addition to charging the battery, the electrolyte level should be inspected on flooded batteries. The electrolyte will be low if the battery is in a discharged state and will increase as the battery is being charged; therefore, the final adjustments of the electrolyte level should take place once the charging process is complete. 

Most service manuals recommend adding only distilled water to cells that are low on electrolyte after the battery is fully charged. 

During initial servicing of a new battery, however, only aviation electrolyte should be used and the cells should not be diluted with water. The specific gravity of the electrolyte on a charged battery is 1.285 while electrolyte for an automotive battery has a specific gravity of 1.265. 

When adding fluid, a syringe or a bulb-type battery filler works well so that fluid can be removed if too much is added. 

Any spills can be cleaned and neutralized with a little baking soda and water, but only do so after the battery caps are reinstalled and tightened. Care should be taken to make sure none of the baking soda enters the battery.

Upon reinstallation, be sure not to overtighten the battery terminals. The terminals on a sealed battery require a relatively low torque, and overtightening can cause them to leak.


External charging of a battery

When using an external charger to charge a battery, it is best to use an aviation-specific charger. Always charge the battery to the manufacturer’s specifications. 

Aircraft batteries have thinner plates than automotive batteries and are more susceptible to damage from overcharge. They also require lower charging voltages than automotive batteries. This is also true of float chargers that are typically left plugged in any time a battery is not in use.

Teledyne Battery Products, the company that makes Gill batteries, lists four chargers for its various battery products on its website; these are available through Gill distributors.

The charger recommended by Concorde for use on its batteries is the Battery MINDer brand. This company has aviation-specific float chargers for aircraft batteries that are temperature compensated voltage regulated. These chargers provide a higher charge rate in colder temperatures and a greatly reduced rate of charge as temperature increases, preventing an overcharge. 

Once the battery reaches a fully charged state, the charger shuts itself off. Battery MINDer also has some solar powered versions for planes that are parked out on the ramp.

Float chargers are nice and lots of folks permanently install them on the battery. If you do, be sure to use FAA approved components like those available from Audio Authority Corp. that are designed for aviation use. 


Not designed to last forever

Even with the best care, batteries by design have a fairly short lifespan of usefulness. Periodic replacement is a given—around five years if unmaintained and up to 10 years if properly maintained. When choosing a new battery, pick a high quality product. 

Some folks like flooded-style batteries best, some prefer VRLA. Flooded batteries are typically messier than sealed batteries and cause corrosion, but they are slightly more forgiving of being overcharged since electrolyte levels can be adjusted. Flooded batteries are also less expensive.

Sealed batteries are less corrosive, and they self-discharge at a slower rate than flooded batteries. Sealed batteries typically cost more than flooded batteries.

With either style, the best thing an owner can do to extend the life of his or her battery is to keep it fully charged. With the improved chargers on the market today, that is becoming easier to do. 


Aviation batteries
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Concorde Battery Corp.
Teledyne Battery Products
(Gill batteries)
Replacement battery boxes
Bogert Aviation
– PFA supporter
Temperature compensated voltage regulated chargers

Battery MINDer
Airframe interface kits and accessories
Audio Authority Corp.
No Appointment Necessary: DIY Oil Change

No Appointment Necessary: DIY Oil Change

A&P Jacqueline Shipe details the process of changing the oil and filter on an aircraft in this fifth installment in Piper Flyer’s DIY series. 

One of the items that is labeled as preventive maintenance by the FAA that a pilot may perform on his or her own airplane is the cleaning or replacing fuel and oil strainers or filter elements.

An oil change performed at a regular interval is one of the best things that can be done to prolong engine life. Clean oil lubricates and carries away harmful deposits better than dirty oil; plus, inspecting the contents of the removed filter or screen often helps to detect a malfunction before it becomes catastrophic. 

Draining the oil

Warm oil drains faster than cold oil, so it is nice to run the engine a little before beginning the oil change to cut down on the amount of time it takes to drain. (If you choose to do this, remember that the exhaust components will be especially hot. Use care.) 

The first step in the oil change process is draining the old oil. The container the oil is being drained into needs to be large enough to accommodate it. An old five-gallon bucket works well. 

Most planes have a drain valve on the bottom of the sump to facilitate oil changes. There are two primary types of valves: the style that pushes straight up to lock in place, and the type that has to be pushed up and turned. 

If it is possible to reach the valve through an opening in the lower cowling, an old rubber hose that fits snugly over the drain end of the valve works great to port the old oil into a bucket and saves having to remove the entire cowling. (It makes a huge mess if the hose pops off the drain valve midstream, so be sure it is secure.)

If the valve is inaccessible, the lower cowling will have to be removed, which isn’t such a bad thing because it allows access to give the engine a good looking-over. 


Removing the filter

Filter removal begins with cutting the old safety wire. The wire should be cut in the loop that goes through the safety hole on the engine, never pulled through. 

If the wire is pulled, it will cut through the soft, very thin tab on the engine and the tab will then be useless. Once this happens, the safety wire has to be attached at another point on the engine. 

After the safety wire is removed, the filter is loosened by using a one-inch size wrench on the end adapter to remove the filter. It is best if the wrench is the six-point style because the adapters are fairly thin; if the filter is stuck on the engine from being overtightened the time before, a lot of torque will be required to break it loose. The ears on the adapter can be easily rounded off if this happens. 

The space between the filter and the other parts on the accessory case is very limited on some engines; a short wrench may be required in these situations. 

The filter usually drains a little oil as it is removed. An empty oil container turned sideways with the top cut off can catch any oil that dribbles out as the filter comes off. Once the old filter is off, it can be placed aside and allowed to drain.


Installing the new filter

The new filter needs to be double-checked to be sure it has the correct part number. 

Some folks jot the tail number and tach time on the side of the filter using a permanent marker when it’s installed. This helps to determine when the oil was last changed without having to drag out the logbooks. It also ensures that the old filter won’t get mixed up with one that came off another engine or airplane. 

The new filter also needs to be inspected to be sure there are no leftover pieces of packaging or debris laying in it. If the filter has any dents or other signs of mishandling, it should be discarded or returned. 

After the filter is inspected, the gasket should be lubricated to protect it during installation. Some mechanics also partially fill the filter with clean oil before it is installed to help prevent a dry startup. 

Dow Corning DC-4 is the lubricant most filter companies recommend for greasing the gasket on the filter prior to installation. The lube helps with removal the next time, but mainly it keeps the rubber seal from being broken loose from the filter during installation It usually costs around twenty bucks for a tube of this lubricant, and one tube can last for several years.

The oil filter adapters are designed to provide a place to grab the filter with a wrench and are spot-welded on during the manufacturing process. If the new filter is overtightened during installation, one or more of these spot welds may break loose, which can cause a severe leak initially or later on down the road. 

Some mechanics never use the wrench adapter to install a filter because of this. At any rate, care should be taken to not over-torque the new filter. 

Once the new filter has been installed and properly tightened, it is ready for safety wire. Wherever the new wire is routed, it should be situated so that it can’t chafe into the filter or into anything else. Some mechanics slip a piece of heat shrink or something similar over the new wire to help prevent it from rubbing into the filter. 

The wire should have a positive pull on the filter and be fairly taut. 


Service the oil screen

Oil screens are removed in a similar fashion as an oil filter. The screen is small (compared to a filter) and generally has a larger area for access. The screen is safety-wired in the same manner as a filter. 

A copper crush washer is used as the seal on the oil screen housing, and the crush washer should be replaced at each oil change. 

Engines that didn’t originally come with an oil filter as standard equipment and have an aftermarket oil filter kit installed often will have the original screen as part of the oil system. 

The oil screen should be regularly removed and cleaned in addition to replacing the filter on these engines because the dirty oil goes through the screen before it reaches the filter. If it’s neglected, the screen can actually become clogged with debris and restrict oil flow to the engine.


Perform a leak check

Changing the oil and filter is fairly straightforward, but there are some precautions to take. The oil pressure at the point where it flows through the filter or screen is very high; a leak in a gasket can quickly lower the oil level if the airplane is flown with a leaking filter or crush washer. 

Any time the oil filter is replaced or the oil screen is cleaned, the engine should be run on the ground afterward and then checked for leaks. After the proper oil amount is added, the engine is ready for a ground run and leak check. 

Inspecting the oil filter and/or screen

The contents of a removed screen or old filter should always be thoroughly inspected for any metal. A clean white paper towel works well to catch the residue off a screen as it is cleaned. Most mechanics use a solvent or parts cleaner to flush a screen. 

An old filter should be cut open using a filter cutter so the inner part of the filter can be removed. The paper element needs to be completely cut out of the inner metal housing and unfolded so it can be inspected. 

When the pleats are still wet with oil it can be hard to see very small particles embedded in the paper. Some folks let the filter drain overnight because of this; the element can be folded sideways and compressed in a vice if a person is in more of a hurry. A vice will squeeze out the old oil but leave any contaminants in the pleats. 

A magnet gently drug across the filter pleats or the paper towel will pick up any steel particles. Finding any metal is cause for concern, but finding steel contaminants is a major concern. The source of the steel should be tracked down and remedied if there is more than a trace amount. 

Aluminum flakes or slivers are generally caused by wear on one or more of the piston pin plugs in the cylinders. These plugs are made of aluminum in most engines; some are now made of brass. 

They contain the free-floating piston pins on either side and keep them from contacting the cylinder walls. Aluminum and brass are relatively soft compared to steel and are designed to wear without damaging the cylinder wall. 

If just a few particles of aluminum or brass are found, it is best to change the oil at an earlier interval next time in order to inspect the filter contents. Generally the problem goes away on its own because the plug wears itself in a little and stops, but not always. 

Increasing amounts, or large initial amounts, of any metal is grounds for disassembling the cylinders—and possibly the entire engine—to discover the cause. It is best to let an experienced mechanic inspect oil filter contents if any metal is present to get a second opinion about what action, if any, should be taken.

Regular oil changes will save money in the long run. Owners that change the oil, filter and/or screens themselves will not only save money on labor costs, but also have a good understanding of the health of their airplane engine.


Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks. Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

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