Carbon Cub EX-3 Engine Failure (N40DT)

[Cactus Charlie]

One option for Lycoming O-360 and IO-360 engines is to fit a standby alternator on the accessory housing vacuum pump pad.

That option may not be available on the CC363i without significant engine modification.

"for example the latest CC363i rear accessory case is currently made from Magnesium and does not include installation of typical 360 style rear accessory drive gears (as weight reduction)."

ref - CC 363i crankcase - CubCrafters Forums

The CC363i parts catalog is confusing as it shows the pad in some drawings but not in others.

 

[Neal]

So here we are, very cold winter in VA. But those affected by winter weather, snow, etc. will fly less frequently. So your plane sits for a month, temps never get out of the 20's. For those that now are educated about this setup know that if you have to rely on your $29 ignition backup battery that is weak from the cold and not being charged for a month or more you might as well not have one. Combine that with a weak starter battery such as the SBS-J16, which hopefully has been on a charger, it's just not a good situation IMHO.

Education and community share is vital and learning from scenarios such as this thread has taught many of us. Knowledge is key.

 

[ve6yeq]

One option for Lycoming O-360 and IO-360 engines is to fit a standby alternator on the accessory housing vacuum pump pad.

My 2024 Factory Carbon Cub EX-3 has a Lycoming assembled CC363i (YIO-360-EXP128) engine, and I can confirm that the vacuum pump accessory pad has not been milled out. Both magneto pads have been milled and have covers installed. The propeller governor pad has also been milled, but there is almost no space between that and the firewall.

I just got off the phone with Monkworx, and it appears that the gearing behind the magneto pads is very slow, and the propeller governor has a very small gear, so the current MZ-30 will not work. However, he did mention that he is working on a dual belt-driven generator solution, so it might be possible to replace the current 40 Amp alternator with a single belt / dual generator solution. He is going to send me the dimensions so I can see if it will all fit under the carbon cub cowling.

 

[ve6yeq]

if you have to rely on your $29 ignition backup battery that is weak from the cold and not being charged for a month or more you might as well not have one.

I normally fly for a couple of hours once a week, and now that I am paying more attention to the ignition backup battery, it is clear that the AGM PowerSonic battery has been designed for continuous float charging, not the usage pattern of my cub. During the XC from Toronto to Florida (16+ hours in 2.5 days) the battery definitely performed stronger during my run-up ignition battery tests on the final legs than it did during the early legs. Down here in Florida, I have been refueling at a local grass strip 20 min from where I am based, and the run-up battery voltage test is ~0.4 Vdc higher after a couple of hours of flying, so it is clear that the lack of continuous float charging is not good for the battery condition. Rather than try to rig up a solar float charger for the AGM, I think I will replace it with the EarthX EBBS once I have worked out the options for power generation...

 

[Tsquared]

Final NTSB report published for N40DT. I have no doubt this is going to generate more discussion.

NTSB Docket - Docket Management System

data.ntsb.gov

Aviation Investigation Final Report

Location: Thompsonville, Michigan Accident Number: CEN25LA401

Date & Time: September 7, 2025, 09:32 Local Registration: N40DT

Aircraft: CubCrafters CCK-2000 Aircraft Damage: Substantial

Defining Event: Loss of engine power (total) Injuries: 2 None

Flight Conducted Under: Part 91: General aviation - Personal

Analysis

Shortly into the local area flight with the experimental amateur-built airplane, the pilot noticed a flicker on the avionics 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) on the main battery. The pilot immediately started a climbing turn back to the departure airport. After reaching about 4,000 ft, he tried several methods to get the alternator working including cycling both the field and the main alternator circuit breakers several times. He checked the voltage for the ignition backup battery, which read 12.8 VDC.

Since he had about a 15-minute flight back to the departure airport, he decided to save the ignition backup 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 display. The pilot switched over to the ignition backup battery and 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 reported that the ignition backup battery only lasted about one or two minutes before the voltage dropped to 0 VDC.

While on short final, about 300 ft agl, the pilot performed a forced landing near trees and a grass field. The airplane came to rest nose down in the trees and sustained substantial damage to both wings and the wing struts.

Postaccident engine examination revealed a fractured ring terminal for the 12-volt alternator. It is likely that the ring terminal fractured from vibrations while the engine was operating. It is also likely that there was too much tension and not enough relief on the wire that the ring terminal was connected during the installation to the alternator.

The airplane’s engine was built to use a dual electronic ignition system that operated from the airplane’s electrical system, which included the main battery, the alternator to maintain the main battery’s charge, and the ignition backup battery. Once the ring terminal fractured, the ignition system relied completely on the main battery as the source of ignition power. Once the main battery power was depleted and the ignition backup battery power was depleted, the ignition system could not operate, and the engine sustained a total loss of power due to the lack of ignition.

The airplane did not accumulate enough time to require its first annual inspection for the airframe and the engine. According to the airplane maintenance manual, the ignition backup battery is to be changed at each annual inspection. The pilot expressed concern that with the ignition backup battery that came with the kit, as it may take a builder some time to complete
the building of the airplane, and that could easily go over a one-year timeline for example.

The pilot would not have been able to detect the fractured ring terminal without removing the engine cowl, which is not normally part of the pilot’s preflight inspection.

Probable Cause and Findings

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

A total loss of engine power due to a lack of ignition resulting from a fractured alternator ring terminal and the builder’s incorrect installation of the wire connecting to the alternator.

Contributing to the total loss of engine power was the failure of the ignition backup battery.

 

[Neal]

Thanks for the info @Tsquared. I hope you'll post your recent experience with the ignition backup battery in a separate thread.

 

[Cactus Charlie]

Final NTSB report published for N40DT. I have no doubt this is going to generate more discussion.

The only thing I learned was the reluctance of CubCrafters to participate in the investigation.

There are two very simple things that CubCrafters could do to reduce the risk exposure. I have mentioned both before and I may have made some converts.

1. Revise BAT current alerting so it gives immediate warning of alternator failure.
2. Revise pre-takeoff checklist to require at least a 10 second test of the emergency ignition battery.