This was not part of the plan for maiden day. A simple mistake wound up costing me dearly.
Text, photos, video and fire by Tom Hintz
Posted – 10-24-2018
Since returning to RC flying I have been using the modern LiPo batteries, both motor packs in my electric planes and helicopters and as in this case, receiver packs that run the radio gear. Though fully aware of the fires other people were having I convinced myself that by using quality LiPo packs, a very good charger and low charging rates, I could avoid these electrical disasters. While that may still be true I was forgetting the one variable that nobody can fully escape – ourselves. I know it’s hard to fathom but it is looking like I screwed up. I’ll give you a moment to catch your breath.
I had taken my new Hangar 9 RV-4 out for its maiden day. Clear blue Carolina skies, light winds and tolerable temperatures promised a good day for sorting out a new plane. During the maiden flight, it was apparent that I had a CG (center of gravity) problem so I landed to move the battery packs.
After taking off the canopy/cockpit which fully exposes the interior of the fuselage I unplugged and moved one of the 7.4V, 3600mAh LiPo packs forward in the fuselage. When I tried to move the tie wrap on the second pack to make it easier to cut I noticed a wisp of smoke come from the bottom of the pack. That was quickly followed by small sparks shooting out of the same area of the pack. Within seconds the smoke and sparks grew substantially, and it was clear I had a huge problem – in a brand-new plane.
I had a pliers in hand and tried using them to snatch the now burning pack from the plane. The heat surrounding the pack was surprisingly intense and a huge amount of smoke made it impossible to see anything inside of the fuselage. My fancy new giant scale plane was in mortal danger.
A friend at the field hosed the Hangar 9 RV-4 off with a dry chemical fire extinguisher and that knocked most of the fire down. We had to give the smoldering battery pack a couple more squirts, but further damage was averted. It is possible that the pack simply finished burning on its own. When the smoke cleared there was literally nothing remaining of the battery but paper-like ash leaves that used to be its insides. All that debris was laying on the starting stand below what used to be the bottom of the fuselage.
As the smoke cleared a little it was obvious the fuselage was a gonner. The forward part of the tail section was burned out through the bottom and sides. The metal pushrods going to the tail control surfaces were now badly distorted, evidence of the fire’s intensity. Even the steel wire pull-pull cables to the rudder were so burned up that I pulled them apart with my fingers.
The wings appeared to be spared serious damage. The covering on both wing panels will need some replacing but the structure and servos appeared to be undamaged. The nylon wing bolts located in the bottom of the fuselage cavity were now covered with rock-hard chemical residue. I was able to chip some of the chemical residue away and get the bolts out, but they also need replacing as the heads are badly melted.
Much of the wiring surrounding the now ash-based battery pack was reduced to fine, unrecognizable strands. Because much of the wiring is hidden below the radio gear deck within the fuse I would have to get more of the carcass apart to get a good look at most of the wiring.
It looked as though I had trapped one of the battery
balance leads between the battery and framework. That
wore through the wire insulation and the fire was on.
I was hopeful that because the radio gear was shut off when the fire erupted (a truly descriptive term for a LiPo fire) it would be spared damage. Here again, I would wait to get all of it out of the carcass, cleaned up and closely inspected before plugging it in to check its’ function.
After checking and re checking the radio gear I found that the receiver survived though one satellite receiver was toast along with its cable. Three servos are gonners though might be saved with new cables soldered in. I’ll have to determine if sending them in for repair is cost effective or I might replace them myself. Of course, the one battery pack is now ashes as are various cables and extensions.
Everything ahead of the radio bay in the fuselage was untouched except by the fire suppressant chemical so I spent some time cleaning that up. Considering the intensity of a fire amidships in the fuselage, I think I got off pretty cheap.
I impose a “warts and all” mandate on myself when producing content for my site. As much as I would like to be thought of as infallible, that even sounds dumb to me. The “warts” in this case appear to be very large and all on me.
When I moved the battery packs rearward while setting the CG, looks like I pinched the balance leads of the now burned up pack between the battery and fuse structure. For many years I wrapped my battery packs (regardless of chemistry) in foam rubber. For some reason I went to putting a foam pad under the packs and then strapping them down with tie wraps. In this case, I had left the foam out and simply strapped the pack to the structure. In doing so it looks like I trapped the balance leads between the pack and the edges of the structure. The vibrations of running the engine in the yard and then through the maiden flight must have compromised the balance lead insulation. When I tried to reposition the tie wrap to make it easier to cut, that bit of movement apparently completed the short and the pack lit up. After the fire I found the burned balance leads pressed into the wood structure directly behind where that pack had been strapped.
I do not have a legitimate reason for not encasing any air borne battery pack in foam rubber. I also do not have an even remotely-viable excuse for not checking where the wires were when strapping the pack down.
The simple fact is that I was in a hurry, over-confident or (more likely) a bit of both. A dose of dumb can also be safely factored in. There just is no way for me to excuse myself from this incident. I know better, I have done better, and I am the one who set the stage for this battery pack to burn up.
LiPo packs do burn with an intensity I had not fully appreciated before seeing it happen within my plane. That is a liability of that chemistry. However. I have seen Ni-Cad and other packs burn when shorted out and our airframes are not capable of withstanding the heat they generate either.
Changing battery chemistry is not going to prevent this kind of electrical fire if I don’t change how I do things. Thousands and thousands of our RC brethren fly LiPo packs forever without a problem. I could be among that number today if I had not screwed up a couple days ago.
As so often happens, it is the humans involved that cause the majority of the problems. In my case, while rebuilding a brand-new plane I will be ordering a supply of foam rubber and perhaps taping it to my forehead as a reminder to employ this existing “cushioning technology” to help prevent future Tom-based disasters.
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