Learn from my mistakes.. 12/24V Jump Starts

[Z W]

On a return trip from a great vacation last month, full family on board, I stopped for fuel, halfway home after a 2.5-hour flight.  After gassing up the tanks, and hitting the restroom, we all got back in the plane.  I turned the key, and the prop ticked, but wouldn't turn over.  I suspected either the starter or the ignition switch.  The battery was replaced last April, and I had just landed without any low voltage warnings, so low battery seemed unlikely, but I asked if they would give me a jump anyways, because I figured that would be the easiest solution if it happened to work, and I only needed one more start to get home where my usual shop could take a look at it.

A helpful lineman took me to the maintenance hangar to pick up this beast:

Although I knew I had a 14-volt electric system, I did not notice the 28V sticker on this unit, and it did not occur to me that the unit would not be suitable for jump-starting a 14-volt plane.  You may be able to guess what happened next.  He fired up the diesel motor on the GPU and plugged it into the plane.  I hopped in to see if the engine would fire.  After about 5 attempts to crank over the motor, my lead-acid battery exploded with an audible thump, leaking acid through the drain hole installed for this purpose in the bottom of the battery box and belly of the plane:

This ill-advised course of action destroyed the battery and all three relays between the battery and the starter motor.  The starter motor was also toast, although it probably was before that.  I had to fly all of us home commercial, and return 3 weeks later after they had finally chased down all of the problems to pick up the plane. 

The FBO involved, to their credit, fixed the damage and charged us only for the parts on the starter .  The lineman apologized profusely to me, and said that he had recently used that cart to jump-start another Mooney that had a 24-volt electrical system.  There was a "12-volt" placard on the plane, but it was inside the flapper door that covers the external power plug, and I guess he didn't see it.  I did not even know the placard was there before this.

Lessons I learned:

1.  Understand your plane, and never assume a stranger on a ramp knows what they're doing.

2. Always verify your power source for a jump.  12-volt to 12-volt and 24-volt to 24- volt.  No mixing and matching.  Consider that Mooneys come in both varieties.

3. Continue my practice of planning fuel stops at locations with commercial airline service and on-field maintenance shops when possible.  This is the first time I've had to use it, but I was very glad not to be at a small airport 2 hours from the nearest commercial ride home.  It also was good that at least there were people there to help with the problems on a Saturday morning, instead of an unattended field with a self-service pump.

4. I have purchased a 12-volt DC external power placard and installed it on the exterior of the plane where it is more visible.  Every plane with an external power plug should probably have a placard, in my opinion.

While the lineman played a role, I bear some of the blame for this.  I watched him do it all, and I was in the pilot's seat turning the key.  I will not make that mistake again.  I was aware of the issues of jump-starting 12-volt and 24-volt cars, but the issue was never brought up during my private pilot training with respect to planes, or at any other seminar or event I've attended since.

I hope sharing this might help prevent one or more of you from having the same bad experience.  Fly safe my friends.

Edited September 27, 2021 by Zane Williams

[N201MKTurbo]

I hate to be one of those guys, but you can’t trust line guys/gals with anything. Before I get a jump from them, I make the prove it is hooked to 12V.

[EricJ]

Sorry you had to learn the hard-ish way, and it probably doesn't help, but some have lost their entire panels/avionics this way, too.

It's unfortunate that the 12V and 24V systems use the same plug.   Usually these sort of compatibility issues are solved by using incompatible plugs, but that's not the case.   As much attention as is paid to such things in aviation, the ball has been dropped pretty hard like this in several areas, and folks like you are still paying the price from time to time.

I'm glad it got sorted out and the FBO stepped up for you.

[carusoam]

uuuuugh!
 

Voltage… 12/24.

Fuel… 100LL/Jet A.

Refrigerant… expect to find variations here as well…. If you have AC.

Essentially, any input to our plane can be accidentally switched, at any time, for any reason, even after the discussion or written directions…. No matter how important the mistake can be or not be….

 

What impresses me the most…

Zane’s really cool explanation of what and how it happened…. Mellow, steady…. Ownership….

Way to go Zane!

That wins the MS Cool as a cucumber award! 

Thanks for sharing all of the detail… good luck with the next steps…

Best regards,

-a-

[Z W]

In case this inspires anyone else, here is the placard I now have on the outside of the plane:

https://www.ebay.com/itm/123241539555?chn=ps&_trkparms=ispr%3D1&amdata=enc%3A1anv6JMzAQ0Ged6JNGJg6vQ22&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=123241539555&targetid=1262376588856&device=c&mktype=pla&googleloc=1020526&poi=&campaignid=12873834640&mkgroupid=120270032814&rlsatarget=aud-622027676548:pla-1262376588856&abcId=9300536&merchantid=112038953&gclid=Cj0KCQjw18WKBhCUARIsAFiW7JzsYKUh-O6q8qsqwesVZneJPEDrGVN_6BP7K5sbWOCXMdIyB46X8yYaAt5rEALw_wcB

Aircraft Spruce has a slightly different version:

https://www.aircraftspruce.com/catalog/cspages/extpower12vdcplac.php?clickkey=8443

A low-cost way to maybe avoid a very expensive mistake.

[LANCECASPER]

Thanks for sharing that. That is one first class FBO. If someone does something to my airplane when I'm not there that's on them, but if, as pilot in commend, I'm standing there watching and even encouraging them do it, I have to know more about my airplane than a lineman does.

For them to step up and cover the expenses without a major hassle, is very commendable. I would go out of my way to give them my business. 

(One other thought . . . I am not a fan of getting a jump and taking off on what's probably a close-to-dead battery, alternator or something else in the electrical system. I'd rather figure it out on the ground than in the air. An engine failure in the air is pretty straight forward . .  land, a fire is another thing completely.)

[carusoam]

Aircraft charging systems are pretty good at keeping up….

But they are highly taxed when having to charge a dead battery…

Some charging systems don’t survive the recharge a dead battery in flight exercise…. The next step is hunting down the dead electrical components…

PP thoughts only, not a mechanic…

Best regards,

-a-