Loud backfire noise in the battery compartment area when the power switch is turned off (engine not running and cold).

[Steve Parker]

Help! 2006 M-20R Ovation hangered at Hermann, MO.  During an airport open house on a warm day (temp in the 70's) I turned on the master switch to show an interested pilot my G1000 system. There's no avionics master switch, just the master power. The engine was cold, mags off, the keys were not in the mags keyhole.

Then, finished with the G1000 demo, I turned off the master power switch and "BOOM", there was an instant, loud 'backfire' noise from behind and below me in the region of the battery compartment.  I wondered if it was some relay or the batteries making the backfire noise, but it's loud!

I knew it wasn't an engine 'backfire' since the sound was in the region behind and below me, not from the exhaust pipes under and forward of my feet. Besides, the engine hadn't been run in three months and was not involved. I instantly suspected something to do with one or both of the batteries, which were always on a trickle charger. 

I disconnected the trickle chargers and a couple hours later, again turned on the master power switch on to again demonstrate the G1000 display. When I turned off the master switch it again, instantly had a loud "BOOM" 'backfire-type sound below and behind the passenger compartment. 

My avionics guy said he'd look at it but he's been working out of the area and I'm in a rural area in Hermann, MO. I'm hesitant to even run the engine or turn on the power. It's back on the trickle chargers to keep the batteries topped up. I haven't turned on the power since as I don't want to risk a fire. 

Suggestions? Ideas? Advice on what to do or how to troubleshoot or who to call?  

Any help is appreciated. Have a safe, successful, happy, healthy New Year.

Edited January 1 by Steve Parker

[N201MKTurbo]

The battery can release hydrogen gas. There could have been a buildup of hydrogen gas in the battery box. The master relay will make an arc when it turns off. It is possible that the hydrogen gas got into the master relay and was ignited when the relay opened. It is possible the flames from the relay ignited the gas in the battery box. A stoichiometric  mixture of hydrogen and air will make a loud report when ignited. 
 

I would inspect the battery box and master relay for damage.

[Ragsf15e]

The “clunk” from the master solenoid is pretty loud in a quiet hangar.  Maybe open the battery access panel and stick your head back there while someone else turns it off?

[Steve Parker]

44 minutes ago, N201MKTurbo said:

The battery can release hydrogen gas. There could have been a buildup of hydrogen gas in the battery box. The master relay will make an arc when it turns off. It is possible that the hydrogen gas got into the master relay and was ignited when the relay opened. It is possible the flames from the relay ignited the gas in the battery box. A stoichiometric  mixture of hydrogen and air will make a loud report when ignited. 

I would inspect the battery box and master relay for damage.

Thanks for the response. I had a vague idea that it might be gasses built up over time, but wouldn't the first explosion have burned up the built-up hydrogen gas? The second backfire was two hours or so after the initial backfire and sounded the same loud "Boom!"  I disconnected the trickle chargers after the first 'backfire'.

[Steve Parker]

24 minutes ago, Ragsf15e said:

The “clunk” from the master solenoid is pretty loud in a quiet hangar.  Maybe open the battery access panel and stick your head back there while someone else turns it off?

This wasn't the normal 'klunk', but sounded like a loud backfire, almost like a large firecracker or an M-80 artillery simulator.

[Ragsf15e]

18 minutes ago, Steve Parker said:

This wasn't the normal 'klunk', but sounded like a loud backfire, almost like a large firecracker or an M-80 artillery simulator.

Oh, well the hydrogen theory is probably better then.  Maybe not stick your head in there to test that one…

[PT20J]

If the batteries are giving off that much hydrogen perhaps the trickle charger is overcharging them. I’d measure open circuit and float voltages.

[M20F-1968]

Try this:

The speed brakes were deployed when the master was turned off.  The klunk was the speed brakes retracting when the power was turned off.

John Breda

[PT20J]

9 minutes ago, M20F-1968 said:

Try this:

The speed brakes were deployed when the master was turned off.  The klunk was the speed brakes retracting when the power was turned off.

John Breda

Good point. I always forget how loud those are and it matches the scenario: yoke mounted switch gets bumped when fooling around with the G1000 deploying brakes. Master off and brakes free fall closed with a bang. Turn the master on again later and the logic remembers the brakes were deployed and redeploys them. Turn the master off and the again free fall with a bang.

[LANCECASPER]

6 hours ago, Steve Parker said:

Help! 2006 M-20R Acclaim hangered at Hermann, MO. 

 

M20R is an Ovation, not an Acclaim (M20TN). N903DS looks like a 60th Anniversary Ovation.

 

I agree that the loud noise was most likely the Speed Brakes if the master is turned off when the brakes are deployed.

[buddy]

5 hours ago, N201MKTurbo said:

The battery can release hydrogen gas. There could have been a buildup of hydrogen gas in the battery box. The master relay will make an arc when it turns off. It is possible that the hydrogen gas got into the master relay and was ignited when the relay opened. It is possible the flames from the relay ignited the gas in the battery box. A stoichiometric  mixture of hydrogen and air will make a loud report when ignited. 
 

I would inspect the battery box and master relay for damage.

The Ovation does not have battery boxes.

[N201MKTurbo]

17 minutes ago, buddy said:

The Ovation does not have battery boxes.

Ok, probably not a battery gas bang.

[blaine beaven]

@Steve Parker was it the speed brakes, or a battery issue?

[Steve Parker]

On 12/29/2023 at 11:16 PM, blaine beaven said:

@Steve Parker was it the speed brakes, or a battery issue?

Speed brakes. 

[Steve Parker]

You guys are great. I had an avionics buddy drop by and we identified that was the cause.  When I showed the G1000 off at the airport open house, the visitor must have deployed the speed brakes with the tiny yoke button.  Turn off power and the speed brakes slam down.

Happy New Year.

[M20F-1968]

On 12/28/2023 at 3:22 PM, PT20J said:

Good point. I always forget how loud those are and it matches the scenario: yoke mounted switch gets bumped when fooling around with the G1000 deploying brakes. Master off and brakes free fall closed with a bang. Turn the master on again later and the logic remembers the brakes were deployed and redeploys them. Turn the master off and the again free fall with a bang.

The other opportunity for this to happen is with landing in a crosswind.  Some of my best landings have been done with speed brakes and when completed, I sometimes forget they have been deployed.

John Breda