Carbon Cub EX-3 Engine Failure (N40DT)

[Neal]

The NTSB preliminary report is attached and another article linked below. For aircraft owners with Light Speed ignition systems (or any electrically driven ignition system) I think this is an important one to learn from and discuss.

Excerpt:

The airplane departed from the Thompsonville Airport (7Y2), Thompsonville, Michigan, about 0917. Shortly into the local area flight, the pilot noticed a flicker on the Garmin G3X Touch display, but a low voltage warning never appeared. The pilot then looked at the engine monitoring display which indicated the alternator was not charging and it showed 12.8 volts direct current (VDC). The pilot immediately started a climbing turn back to 7Y2. After reaching about 4,000 ft, he tried several methods to get the alternator working including cycling both the field and main alternator circuit breakers several times. He checked the voltage in the backup ignition battery which read 12.8 VDC.

Since he had about a 15-minute flight back to 7Y2, he decided to save the backup ignition battery and run off the main battery until he received a low voltage warning. The airplane was on final approach when a low voltage warning appeared on the Garmin G3X Touch display. The pilot switched over to the backup ignition battery, he watched as the voltage dropped from 12.8 VDC down to 0 VDC, and the engine sustained a total loss of engine power. The pilot noticed that the backup battery was faulty, and it only lasted about one or two minutes.

One question I have is "how old was the ignition backup battery" as this was a kit built Carbon Cub and did this battery come with the kit at time of order or was it purchased closer to date of completion as this is an item we're supposed to replace annually at condition inspections. I don't know if CubCrafters ships batteries with the kit.

I'll also add that I don't personally trust the ignition backup battery and there is a test that can be done and is supposed to be done to load test the battery which I bet 99% of us don't do. During runup this battery is tested (backup ignition switch) and it runs down fast. I hope I never have to rely on it.

My understanding is when the ignition backup battery fails it will fall through to the starter battery. I welcome any CubCrafters' owners to chime in to help all better understand the system. With that said, it seems to be your best starter battery is insurance and that's why I have the EarthX in mine. The "upgraded" (now standard) SBS-J16 has always been weak in my experience. Mike Patey did a video on how turbine failure in how the EarthX saved him and FlightChops also did a video along the same line.

So let's discuss. First off, the sad situation of a ring terminal failing. The cheapest item in the entire aircraft cost the aircraft. There are a lot of spades and terminals like this in the aircraft and from what I hear it's the most common cause of failures. I had one separate in my coil pack saga, a slight tug test can ensure they are secure. Is there such a thing as marine grade terminals and if so maybe they should be used instead instead of these cheap connectors we see as commonly used in the build. It may be wise not only to inspect the ring terminal that failed in this accident but consider replacing it with a marine grade terminal. I always use a flashlight during my preflight to look into the side vents to inspect coil pack connections. I'll now focus also on this terminal going forward.

Did the pilot do wrong by waiting for the starter battery to draw down first before going to ignition backup (ign backup battery)? When is the right time to engage the ignition backup battery? It seems the pilot's plan here may be a good one to not simultaneously draw down both batteries? Something I'd like to better understand.

In addition, I recently did a review and adjustment to the items that appear in my EIS sidebar when I realized I could add an additional one as recently posted here which added AMPS as it's a critical item to monitor in my opinion. I had my "field" breaker pop once and amps went negative and I caught it right away. I believe this was caused by plugging a USB accessory (GoPro) into my GMA 245R over the rear seat USB port.

There has been some discussion on voltage cutoff settings programmed into the G3X I believe, maybe @Cactus Charlie can help us better understand as I think this comes up when EarthX's are installed and how this is affected.

Unfortunate incident especially for a kit builder that spent years building their dream shattered by a $.25 part.

Accident Cub Crafters CCX-2000 Carbon Cub N40DT, Sunday 7 September 2025

On September 7, 2025, about 0932 eastern daylight time, a CubCrafters Carbon Cub EX airplane, N40DT, sustained substantial damage when it was involved in an accident near Thompsonville, M...

asn.flightsafety.org

 

[Cactus Charlie]

"My understanding is when the ignition backup battery fails it will fall through to the starter battery. "

There is no automatic switching of right ignition power source in an FX-3 or a standard EX-3. Right ignition is powered from MAIN bus with the emergency ignition switch "down" and by the 2 AH emergency ignition battery when the switch is "up".

I run a 10 second test of the emergency ignition battery at every run-up. I watch the rate of voltage decay and I periodically plot the data. Over 5 years of FX-3 ownership I have collected a lot of ignition battery data. I have concluded that the emergency ignition battery is unfit for purpose (crap in plain English).

Even if replaced every year the new one may not last until the next change. These batteries are typically rated for 5 years in float charge standby applications but the longest I have had one last is about 18 months.

It is essential for anyone operating an FX-3 to understand their electrical system and have a plan for load shedding and ignition management if the alternator fails. That plan will be different depending on whether the main battery is AGM or EarthX. (EarthX hard cutoff is very different from AGM voltage decay.)

I started a thread on this subject on the CubCrafters forum. It was very disturbing to see the lack of system understanding in some of the follow up discussion.

 

[Neal]

Thanks @Cactus Charlie and that's why I posted this to help better understand the system and develop a load shed plan. I would love to hear your load shed plan and how you plan to handle a scenario like this. I'm curious what the biggest loads are to be aware of.

I agree the ignition backup battery is more of a placeholder than a functional backup. The question lies is there a better option? It seems to just be a 12V power source. I wonder if there is a LiFePO4 alternative now in 2025?

 

[Cactus Charlie]

From the accident report -

"He checked the voltage in the backup ignition battery which read 12.8 VDC"

The backup ignition voltage scale reads MAIN BUS voltage when the emergency ignition is not active. The only way to read the battery voltage is to switch to the battery! That is why monitoring ignition battery voltage during the emergency ignition test is essential.

If the report says what main battery was in use I must have missed it. For the standard SBS-J16 AGM battery - shed non essential loads, force reversion to IBBS, run ignition on main battery until engine runs rough or main voltage drops to about 6.5 V.

What I don't yet understand is why the engine stopped. If ignition battery was bad and zero volts the engine should still run on the left ignition until main battery was exhausted. Was this an EarthX battery and engine stopped when the battery cut off?

I would very much like to see the SD card log for this flight.

Revised Edit - "The backup ignition voltage scale reads MAIN BUS voltage when the emergency ignition is not active. " MAIN BUS is connected through a diode to the ignition battery. The voltage monitor point is the junction of the diode and the battery positive terminal. With the ignition switch in the normal (down) position the indicated voltage will be MAIN bus voltage less the diode drop. This voltage does not provide any indication of the state of battery charge unless MAIN bus volts is lower than the battery terminal voltage. When the switch is moved to the emergency (up) position MAIN bus is disconnected and no longer influences the voltage reading. Indicated voltage is now battery voltage and does provide some indication of the battery charge state.

 

[Cactus Charlie]

EarthX now has a "backup" battery that is not rated for cranking. I had considered fitting this but the voltage drop in the blocking diode would likely prevent the battery reaching full charge.

I plan to fit a right ignition IBBS this winter. I have bench tested my design and am now working on the implementation details.

Fitting the EarthX backup battery would be much easier to do as it would require no wiring changes.

 

[Cactus Charlie]

Link to the new EarthX backup battery -

ETX104 Back Up Battery

The ETX104 back up battery provides power for critical loads with current limiting built in to protect from excessive discharge/charge current.

earthxbatteries.com

 

[Neal]

Link to the new EarthX backup battery -

ETX104 Back Up Battery

The ETX104 back up battery provides power for critical loads with current limiting built in to protect from excessive discharge/charge current.

earthxbatteries.com

I like it. Would be worth experimenting with.

 

[Neal]

Link to the new EarthX backup battery -

ETX104 Back Up Battery

The ETX104 back up battery provides power for critical loads with current limiting built in to protect from excessive discharge/charge current.

earthxbatteries.com

It seems this battery is not new. I googled it and there are discussions from quite a while ago, maybe 2020? Why won't this work? Sure seems like a far better option than what we have now. I'll have to check with Pete and see if CC has looked into this although they are slower to change as we know.

 

[Cactus Charlie]

I had not seen this battery before when browsing the EarthX site. Thanks for the correction on its newness.

My only reservation about using this battery is that it likely will not reach full charge. I would still expect the useful duration to be far more than the 2 AH AGM battery even if that was in good condition.

If I did not have a near new IBBS waiting to be put to good use I would probably try the EarthX option. The mod is as simple as removing the existing battery and carrier, fabricating a mount (3D printed tray with strap?), and fabricating a short adapter cable. Probably also should wire the warning LED.

Completely reversible if results are not as expected.

 

[Neal]

I ordered one this morning as I intend to try it and design a 3D printed mount. I'm not sure why it would not charge to 100% as the BMS controls the charge and discharge. There was mention on the Van's forum I believe about a diode setup needed. I'm not smart on that, not sure if that's valid or not. Seems if there is charge current provided and nothing is cutting that off then it should go to full until the BMS stops the charge. Correct me if I'm wrong.

I also saw discussion of using the same battery as the IBBS uses, the TCW battery. Only problem is it's far more expensive and I'd prefer LiFePO4 due to the longer discharge capability. While you could piggy back the two on the same battery it was discussed a dedicated battery should be used for ignition backup. Imagine a higher capacity TCW battery which I think you just bought from someone or what's used in the XCub?

While people will say "We've used this ignition backup battery for years and never had an issue" - the problem is rarely has anyone needed to rely on it. If things go south where I am now, it's not so bad but in hostile terrain like Idaho, you have a problem.

I don't trust the current battery in use to be running my engine. I'm surprised it hasn't been improved in all this time, hence, I ordered the EarthX this morning.

 

[Cactus Charlie]

The diode is required for the existing AGM battery and is already present. I described the required modification.

I have not measured the diode voltage drop. I do not know if CubCrafters used the diode recommended by Lightspeed and I have not studied the data sheet for that diode.

Any diode will have some forward voltage drop and this reduces the voltage available for charging the battery.

Some internet sources say LiFePO4 requires 14.6 V for full charge. I could not find a specific reference on the EarthX site. MAIN bus voltage is normally no more than 14.2 V and the charging voltage will be reduced by the diode.

We do know that 14.2 V is enough for the EarthX main battery. What I don't know is the diode voltage drop.

I simply do not know if the battery will reach full charge. I do not say that it will not.