Had a pretty traumatic experience today while I was trying to get an APM 2.0 working on an RTF "trainer," when I turned off the transmitter and the ESC burst into flames. I was hoping the community might be able to point me in the direction of a solution.

I was ground-testing an electric RC (a Hobbico NexStar electric) with an APM 2.0. The APM was completely stock, and had been loaded with firmware from the Mission Planner without any modification. Controls were calibrated on the Mission Planner as instructed in the wiki, with CH5 on the transmitter dedicated to switching between manual and "stabilize." This was using a Futaba 6ch controller.

The jumper was connected on JP1, in the stock configuration on the APM 2.0. I was supplying power to the output rail via 4.8v NiCd power source, which I checked beforehand actually was supplying about 5V. A standard feature of this trainer-style airplane, an external on/off switch was between the power source and the output rail. The 50A ESC used in the RC airplane did not have a BEC. A previous test without APM 2.0 proved the servos, ESC, motor, RX and TX were all working as designed.

With the APM installed in the airframe and connectors attached as per the wiki, I turned on the transmitter and then the power source to the APM. In manual mode, all controls responded to inputs as was expected. I switched CH5 to "stabilize," and servos and control surfaces responded to yaw, pitch and roll, albeit in the reverse, and potentially with not enough input (I chalked this up to a calibration issue I hoped to resolve after shutting everything down).

I switched CH5 on the transmitter (CH8 on the APM 2.0 input rail) back to "manual," and then turned off the transmitter. The motor turned quickly as if responding to input, then stopped, and smoke rolled out of the electronics bay, followed promptly by a fire coming from the ESC. This fire got approximately 6 inches tall from the ESC and began to burn away the aircraft's external coating and lighted the wood frame of the fuselage. I yelled to my wife to run and get the fire extinguisher, which she ripped off the wall and passed to me, and I applied the extinguisher directly to the electronics bay. This put out the flame and saved the house, and I carried out the airplane to the back yard for safety. By this time, there was no more smoke.

An inspection after the fire showed that a section of external coating was burned, as was some of the balsa frame. Several wires leading to the ESC (two motor wires and battery positive wire) were melted right through. Although the LiPo batteries were mounted next to where the fire took place, they appeared at least on visual inspection to be unharmed by the fire. APM and receiver seemed unharmed from a visual inspection, but I'm not certain at this time whether the chemical flame retardant from the extinguisher has altered the function of those electronics.

At this point, I'm real hesitant to test this particular APM 2.0 on a new ESC to investigate the problem, and I'm really concerned about this as a safety issue. Does anyone know of a condition whereby the autopilot would supply max current to the ESC if the connection between TX and RX is severed? Thank you.



TX: Futaba 6EXA 6ch
RX: Futaba R168DF
ESC: Hobbico SS-50D 50A (w/o BEC)
Motor: RimFireTM 42-50-800 Outrunner Brushless Motor

More pics:

3689468643?profile=originalLeft side of aircraft, showing damage to external coating.

3689468847?profile=originalElectronics bay shortly after the event. In front are dual LiPo batteries, wrapped in protective foam. The 4.8v RX/APM power source can be seen hanging over the edge of the aircraft on the right of the photo. Yellow residue is flame supressant.


A shot of the APM and RX upon extraction. Both were coated in a thick layer of frame suppressing powder, until some of it was vacuumed off.

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  • At least there is no doubt exactly what happened and hopefully everyone feels safer knowing in no way the APM had anything to do with the incident. Further, it should be a warning to everyone to check the ESC max cell rating long before you order batteries. Putting the 2 each 3 cell packs in series was enough to blow it (might take a few times of trying but it would likely fail) but 4+3 was garanteed to cause an instant failure. Just a guess but I bet the Caps are rated at 25 volts. You want to be well under that assuming the highest possible voltages for the cells. 6 cells *3.7 volts nominal is 22.2V close but may not blow the cap the first try. 7* 3.7V +25.9 the magic number to blow the Cap sky high. I know that seems trivial to be 0.9 volts over and blow, but you do not mess around with Caps and voltage ratings. Take computer parts for example. On a 3.3 volt rail, they use 6 volt rated caps. On the 5 volt rail, they typically use at least 10 and maybe 16 volts caps. They try to use 25 volt rated caps on the 12 volt rails. Supplies that blow usually used 16 volt rated caps on the 12 volt rail.

    Inside the ESC, the FETS likely are minimum 40 volt rated but the caps are always the weak point in most designs.

  • The ESC is fried, I removed the stock ESC went to a 6 cell ESC on my setup and normally use a single 4 cell pack, I didn't like the 5 cell setup using a 2 and 3 cell pack in series.  This is a bad setup for a trainer and the lessons learned can be disasterous.

    Most of my multirotors are small and I use 10 to 12 amp ESC's which are rated at 3 cell, on ocassion I use a 4 cell pack with no problems. 

    Good luck!

  • SO if that's the case, you did overvolt it, that ESC is probably OK (I wouldn't use it but it likely still works). What burned was the electrolytic capacitor on the outside, and once it internally shorted dead short across the batteries, then it burned the power wire. Caps are the thign that probably had the lowest voltage tolleracne and that's what broke down. Being it is in parallel across the battery at the input to the ESC, it would cause the solder joint and wire to fail.

  • Oh boy. Is my face red.

    Here's the latest. What happened was the manual for the RTF suggested either a 3S + 2S in series, to essentially make a 5S, or two 8-cell NiMH packs in series to make 19.2V. The manual didn't actually sate the limits of the ESC. But I didn't have a 2-cell LiPo. I had two 3 cells and one 4 cell.

    No problem, I thought. Clearly, this ESC could handle more voltage than what it was suggesting for LiPo. And for whatever reason, I did the math wrong (in terms of cells times voltage), and somehow figured 4S +3S was close to 19.2V; the limit they were suggesting. I was running a 4S and a 3S in series, which as I know now was almost 30 volts, into an ESC designed to handle at most 5 LiPo cells, or 18.5 volts. But I didn't know that until I did some research and find the actual specs of the ESC; the manual failed to state the limits of the ESC or any mention of a safe operable voltage. Still, should have known better.

    So, yeah, there you have it. I continued to think this was a non-issue, because in the short time that I tested the motor on this configuration, nothing happened. Perhaps something was happening then, but I couldn't tell it.

  • I went back and looked at the photos, it is very clear the upper pack is 4 cells (5 wires on the equilizer plug), I can't see the equilizer plug and wires on the lower pack but I don't think it is a single cell pack. 

    The ESC is rated at 5 cells and I don't see a jumper on either of the ESC plugs to bypass a pack for single pack operation instead of the series configeration it is set up for.  If the other pack is anything above single cell mystery solved.  If the other pack is 4 cells then that accounts for the 6 inch flame, WOW!!! 

  • So I would rinse it with charcoal lighter (hexathane aka electronics cleaner) while scrubbing it with a tooth brush; then I'd dry it with a heat gun at low heat.

    I'd also suspect the overvoltage. ESC's blow when they have an internal fault with causes both the high side and the low side drivers to engage simultaneously. You needn't look upstream for the cause; no signal combination can blow up a proper H bridge; not the APM, not the receiver etc. Yes - something happened when the signal quit, but this is normal and every ESC should remain within reasonable behaviors regardless of the input signal.

    Sometime just a little water can cause one side to lock up (i blew a number of esc in boating rc).

    Clean it up and get back on the horse. It's obviously rare, and you might plug things up outside - since you can't fly inside anyway.

    I'd also recommend a very small battery for testing things indoors.

  • I agree, your ESC failed.  Your reciever can be cleaned and reused as it has that plastic case on it.  Take it all the way apart and use standard electronics cleaner.  Inspect it for defects with a magnifying glass, retest it.  Because it was covered I'd reuse it.

    The APM was exposed to corrosive extinguisher chemicals, in the navy we had a lot of training on such things and the standard procedure for such a thing would be total replacement, if it was a combat situation we would rig it as required to work, but I doubt that is the case for you.

    ESCs failing is not common enough for people to complain everywhere but YES they do fail like this, absolutely.

    I'm sooooo sorry for your loss, my condolences.

  • I have this plane, the first thing I did even before connecting a battery (batteries) was to replace the ESC.  I did not like the 5 cell setup, a three and a two cell in series?  Too many things to go wrong.  I converted the power system to 6 cell before doing anything.  I also recall looking for literature on this ESC and could not find anything usefull.

    I see no mention of the battery combination and voltage used in this incident.  Critical factor.

    I agree with John, ESC's seem to fail for no reason.  The fire and smoke usualy go away when the components burn through or the solder melts.  I get nervous every time I plug in the main battery and get the spark and pop and wait for things to equalize.  Lots of amps looking for some place to go.

  • Ive had a couple ESCs let the smoke out for seemingly no reason, of course theres always a reason but there was no APM involved.

  • Have you cleaned off the APM2 yet?  I'd get some electronics parts cleaner and hose them off good to get the powder off. I believe electronics cleaner is also the same thing as brake cleaner.  And I think you can use 99% rubbing alcohol if you can't get either of those.

    I have cleaned out electronics using brake cleaner after getting them wet, and they still work years later so...  I think it's OK.

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