Radio Interference of 12V iPad Charger in Cockpit

USB plug of connecting cable with ferrite core

On long flights where the iPad runs for extended periods in our Garmin GDL 39 + iPad setup, we must recharge the iPad in flight. 

That operation may cause radio interference. In this post, we analyze that problem and describe a remedy that has worked for us.

Initially, we bought a charger with the appropriate connecting cable to be kept permanently in the plane. In the cockpit, the 10W charger was plugged into the 12V receptacle.

During the first test flight, we hooked up the iPad to the charger and saw that the iPad was properly charged. We concluded that all was well. 

As it turned out, we were quite wrong.

Interference Problem

The evaluation changed on our first long trip with the GdL 39 + iPad, where on the first day, coping with significant weather, we were more than 7hr in the air and used the iPad extensively.

When the iPad charge became less than 80%, we decided to hook up the charger. 

After a few seconds, we were surprised by a strong interference signal coming from the radio. It seemed that we were receiving something like an open mike on the tuned-in frequency. 

But changing to another frequency did not help. Finally, we suspected that the charging process caused the interference, and unplugged the charger. Golden silence!

Remedy Attempt 1: Routing, Reconnecting

During that flight, we tried to eliminate the interference by rerouting the connecting cable and moving the iPad. This did not help. 

Then we briefly unplugged the iPad and/or the charger several times, and reconnected. But whatever we tried, after seconds or sometimes minutes the interference returned. 

Eventually, we turned down the radio since recharging the iPad was a must, and unplugged the charger when we used the radio.

Remedy Attempt 2: Apple Cable

We conjectured that the cable caused the problem since it was not made by Apple. 

But a test of the Apple cable in the plane produced disappointment: The interference signal was back. 

In fact, with the Apple cable we could produce it reliably: When we held the cable with both hands, we had the interference, guaranteed.

Remedy Attempt 3: Ferrite Core

Jack, the aircraft guru at our airport, advised installation of a ferrite core on the cable.  

Sure enough, when we added a ferrite core near the USB plug on the cable, the annoying signal was filtered out and the noise stopped. 

The low-cost ferrite core, also known as ferrite choke, is readily available on the Internet. The above photo shows the cable with the ferrite core.

We concluded that the charger emitted the interference signal, and the ferrite core eliminated that problem. 

However, our conclusion wasn't quite right, as we shall see in a moment.

Connection Problem

Besides radio interference, the plugged-in charger had an additional problem: In flight, vibration sometimes caused the charger to lose electrical connection in the 12V receptacle, and we unintentionally ran down the iPad while we believed that it was being charged. Not good.

Solution Part 1: Permanent Charger Installation

The remedy was installation of a charger with a USB port that is permanently wired into the 12V system of the plane. 

For this purpose, we selected a high-quality 10W charger produced by Blue Sea Systems for marine or specialized vehicle applications. 

The charger has excellent reviews and is readily available on the Internet; for example, from Amazon. It has a cap that covers the charger when not in use, and has only 15mA parasite current draw. 

The charger is compliant with FCC Part 15, subpart B, which imposes a strict limit on radio frequency emission. 

We installed the charger between the seats so that the USB plug of the connecting cable is protected by the seat cushions.

There was tense anticipation when we used the installed charger for the first time.  Would it produce interference when the Apple cable, which has no ferrite core, was used? 

Jack, our guru, predicted, "It will."

He was right. There was nonstop interference. But even the cable with ferrite core produced occasional interference. 

And we had thought the ferrite core had licked that problem!

USB plug with ferrite core connected to 12V charger

Solution Part 2: Routing

Then we had the right idea. We routed the cable so that, from the ferrite core onward, it ran between two seat cushions along the metal seat back for 9 inches or so. 

As soon as this was done, the interference disappeared no matter how we moved the remainder of the cable or positioned the iPad.

To confirm that both ferrite core and routing were needed, we used the Apple cable and routed it exactly as described above. 

The result was strong radio inference. Thus, both changes were required.

Jack explained why: The ferrite core eliminates broadcast of high frequencies, while the routing reduces broadcast of low frequencies.

Solution Part 3: Yet another ferrite core

In late 2014, Duffy, a pilot in California, emailed us that he had eliminated the interference problem entirely by use of two ferrite cores, one near the charger and the other one near his Nexus tablet. 

So we added a second ferrite core near the iPad. Testing showed that interference was only produced when we touched the cable.

Duffy tried many different variations of cables and ferrites, and ultimately determined that the charger was the weak link.  

He tried several versions from Fry's at under $10 each, until one finally did the trick.  He's since replaced the Android cables to his Nexus phone and Nexus tablet several times because of connector issues, but those cheap cables never produced interference.  

He's convinced it's the charger, and not the cable. He suggests buying several chargers and keeping the receipts until you find one that works.  Cost or "aviation specific" doesn't necessarily reflect success in your setup. 

Solution Part 4: USB extension cable

With the USB connection between the seats, the charging cable was a bit short when we passed the iPad between pilot and copilot. Often, the plug of the charging cable would pull partially out of the iPad. 

So in Spring 2015, we decided to add a 1 meter USB extension cable that is plugged into the charger and then connects with the iPad charge cable. 

With the extension, the iPad could be easily moved around the cockpit. But at the same time, the interference problem was back. 

To suppress the interference, we installed two ferrite cores with 5 mm ID at the ends of the USB extension cable. Lo and behold, the interference was gone, and different routing in any way or touching or whatever couldn't produce it.  

But in later flights, the interference problem occurred again sporadically. We would reroute the cable this way and  that way, and typically managed to get rid of the problem. 

So, we continued to search for a real solution.

Solution Part 4: A Different Charger

We knew from Duffy's experience that some chargers work flawlessly. But Duffy could not identify the manufacturer of the charger. 

But then we were lucky. My son complained that the 12 V charger he used in his car produced interference in the car radio. Yet, a 12 V charger we used in our car never caused that problem. 

So we took that charger out to the airport, plugged it in, and lo and behold: There was not interference when we used a cable without any ferrite cores. 

When we held the cable right up against the display panel of the radio, we did have interference. But this stopped immediately when we moved two inches away. 

On the other hand, when we used that same cable with the installed 12 V charger, we had interference no matter how far the cable was removed from the radio.

Aha, we had found the right charger!! It is produced by Skiva. So we removed the current charger from the airplane and replaced it with the Skiva charger, knowing that the interference problem had finally been solved.

Since we needed a replacement charger for the car, we bought another Skiva charger at Amazon. It has two USB ports, each producing 2.1 A. 

We tested the new charger in the airplane, too. It was reassuring to see that the new charger did not produce interference, either. So, maybe, Skiva in general is the brand to go to for airplane 12 V chargers. 

Solution Part 5:

We now connect iPad and Skiva charger with a braided 6 ft cable of a set of four cables of different lengths, with ferrite cores added at both ends. There is no radio interference whatsoever.

A Word of Caution

The Skiva charger has at knurled nut that retains the fuse and connecting pin. 

Skiva charger, with knurled nut and connecting pin at one end

Over time, the nut may become loose and eventually fall out, together with the connecting pin. This happened to us. Fortunately, we weren't flying and could locate the nut and pin after careful search.

If this troublesome event occurs while you are in the air, you likely won't be able to locate the nut and pin.

So for the remainder of the trip you won't be able charge any equipment, a potentially troublesome situation.

There is an easy way to prevent the problem: Before installing the charger, remove the knurled nut, add a bit of glue to the thread, and reassemble.

Solution Part 6: Yet Another Charger

In 2020, we had difficulty purchasing another Skiva charger. Amazon no longer listed it, and the company showed it as "Sold Out." We searched the Internet for a replacement, a difficult process. 

There are dozens of chargers offered, all of them not as good as the Skiva charger. In particular, the Skiva charger achieves a total of 4.2A, while other dual chargers typically max out at 3A or less. On top, the Skiva charger has an integrated fuse to protect circuitry.

If you have to buy another charger, select one with a rating of at least 4.5 stars out of 5.  Make sure that the circuit of the power supply is well protected by adding a fuse just before the socket for the plug-in connection. Chargers have been known to overheat, produce smoke, and blow fuses. Just read the 1-star ratings to check for such flaws. 

  

If the charger has a nominal charge rate of 3A for two connections, expect that the charger cannot always keep up with the iPad. This happens when the iPad is exposed to sunlight and brightness of the screen is turned up to the max value.

Solution Part 7: Yet another USB Port

In late 2020 we became aware of USB port model 1045 by Blue Sea Systems. It has 

- two USB ports

- maximum output current 4.8A, so 2.4A for each port

- input voltage 9-32V DC

- parasitic current draw 1mA

- overload protection 

- short circuit protection

- reverse polarity protection

- cutout dimension 1-1/8in. (29mm) matching the usual socket dia.

Blue Sea Systems dual USB charger. Photo of Amazon website

The charger is claimed to have reduced electronic interference. We have tested the charger in our plane:  When used with our cables, there is no interference whatsoever. And when we tried it with a new cable without any ferrite cores, there wasn't any interference either! 

The charge rate is excellent: The charger no only keeps up with the iPad, but actually increases the charge even when the screen is set at maximum brightness.

A Word of Caution

The iPad has become a critical component of safe cross-country flying for us. Since we may spend seven hours or longer in the air on any given day, some backup charging system is needed.

If the charger fails in flight, we will insert a spare plug-in charger into the 12V receptacle of the panel and work around any radio interference during the remainder of the flight.

If you also rely on the iPad for long cross-country flights, you may want to prepare for such alternate charging as well.

Conclusions 

1. Even quality 12V chargers for the iPad compliant with FCC Part 15 subpart B may cause radio interference in  the cockpit. This was proved by our initial choice.

2. A ferrite core, installed next to the USB plug, likely helps to reduce the problem, but may not eliminate it entirely. A second ferrite core near the iPad seems to help as well.

3. A USB extension cable with two additional ferrite cores may also be used. But here, too, interference may occasionally be encountered.

4. In 2013 we determined that an older Skiva charger works perfectly. We  used it till 2020. Unfortunately the model was discontinued. But another Skiva model purchased in 2017 worked just as well.

5. We have achieved the same results with a single braided cable connecting the iPad with the Skiva charger.

6. If you have to make a compromise and buy a charger not made by Skiva, insist on a rating of at least 4.5 stars out of 5, check for overheating problems by looking at 1-star ratings, and make sure the circuit of the the power supply is well protected. Among the choices, pick one with highest charge rate.

7. We switched to USB port model 1045 by Blue Sea Systems in 2020. It produces no radio interference when used with cables shielded with ferrite cores. It has sufficient charge capacity so that the iPad is charged even when the screen is set at maximum brightness.

Suggestions

If you buy a Skiva charger and find that it completely eliminates the interference problem, please add a comment to this post so others see your report of success. 

Make sure to add the exact model number or URL of purchase. 

If you find that some other charger achieves the same positive result, please post that, too, with exact model number or URL.

Please also send comments for any USB port model that turns out to work without radio interference and is able to keep up with the iPad.


Have any questions or feedback about radio interference of the iPad charger in the cockpit? Please share your thoughts in the comments.