Acid trip


Name withheld by request

A recreational pilot realises he should have trusted his nose for trouble

Autumn mornings often present great flying weather in south-east Queensland and so it was on this morning as I prepared my aircraft, a 35-year-old amateur built timber and fabric two-seater, for a 90 nautical mile flight to a breakfast fly-in at Murgon. Conditions in the air were as good as they looked from the ground and the trip up was one of those early morning flights that remind me why I love flying.

After two or three hours on the ground having a great breakfast, catching up with friends and meeting new people who also love flying, it was time to perform a quick check of the aircraft, climb in, buckle up, start up, radio call, taxi to the end of the strip, line up, throttle forward and once again I’m in the air and bound for home.

Not long after departing the circuit area and while on climb to the planned altitude of 5500 feet, the red alarm light on the engine monitor came on. A quick scan confirmed it wasn’t a temperature alarm. A more careful look revealed an over-voltage alarm indicating 16.1 volts. ‘A bit high, but not the end of the world,’ I thought. The flight continued.

Ten minutes or so later I noticed a faint strange smell. It wasn’t fuel and it wasn’t smoke (I could relax a bit). I couldn’t identify what it was but I was flying past a coal mine and power station at the time and I wondered if perhaps that’s what I was smelling—the smell of burning coal did seem a good match. Another part of my mind said that at about 4000 feet AGL it was unlikely I’d be smelling anything on the ground, but I didn’t have any other explanation.

While wondering what the smell was I was also contemplating what to do. I was about 40 minutes from home and about five minutes past Kingaroy. There were a number of other fields a good deal closer than home. Should I turn back to Kingaroy? Divert to somewhere ahead but closer than home? I decided not to decide but to keep a close eye on things for a bit.

A minute or two later the smell had decreased to the point where I wasn’t sure if I could smell it or not. The over-voltage alarm was still there, but everything else appeared normal. Satisfied the engine wasn’t going to stop I continued towards home, a bit more relaxed but still alert for anything unusual.

A few minutes from home I noticed the smell again. Once again it was faint but definitely there and definitely not fuel or oil or melted wiring. It was then the idea struck me that perhaps the battery might not be happy with being charged at 16.1 volts. I resolved to inspect it as soon as I landed.

After landing, taxiing back to the hangar and shutting down, I opened the hatch into the rear fuselage where the battery is located and was greeted with a choking cloud of sulphuric acid fumes. I had to ventilate the fuselage for a few minutes before I could get to the battery. When I removed the lid of the battery box the underside was covered in yellow droplets of acid condensation. The battery itself was hot—I needed gloves to remove it. Then I noticed the Styrofoam packing around the battery, which was there to stop the battery from moving within the battery box. It had begun to melt—not from exposure to battery acid but from heat.

So, what did I learn? A few things:

  1. Just because a particular fault (a failed voltage regulator in this case) won’t stop the engine doesn’t mean the flight should continue. While I didn’t actually ignore the over-voltage alarm, I didn’t take it seriously enough. I should have taken the first opportunity to put the plane on the ground and investigate.
  2. What you don’t know can hurt you. I’m well aware of the dangers of exposing lithium ion batteries to over-voltage conditions. Now I know lead acid batteries also don’t like sustained over-voltage. The results might take longer to manifest and may not be as dramatic, but had my journey home been an hour longer (or perhaps even less) I may well have had a fire on board and I have no doubt that if that had happened, I would likely not have survived.
  3. The advent of relatively inexpensive and highly accurate digital instruments can give pilots a far greater awareness of the performance of their aircraft’s systems in flight, but you have to know what to do with the information presented. In my case I made some decisions that could have had disastrous consequences because I didn’t know what to do with the knowledge that the electrical system was sitting at 16.1 volts.
  4. Time and cost pressures definitely drive sub-optimal decision making. The fact I’d promised my wife I’d be home by lunchtime and that I had a heap of things to do that afternoon definitely contributed to my decision to continue the flight. Potential costs if I’d had to leave the aircraft parked somewhere also crossed my mind.

My aircraft has been repaired and I’ve enjoyed a number of great flights since. But I’m sure I dodged a bullet that day and it was by good luck rather than good management. I’d like to think I’ll make better decisions next time I encounter a fault in flight. Time will tell.


  1. Good lesson there. After near 40 yrs driving planes I’ve always treated flying as a temporary experience, if it doesn’t look, feel, sound or smell right I end my experience early.:-)

  2. Did you have an alternator switch that you could have turned off? Sixteen volts represents a total failure of the voltage regulation part of the alternator, but cannot expect everyone to know that. Lucky escape.

  3. You will need to keep a close eye on anywhere the fumes might have reached. Acid fumes are as corrosive as the acid itself, it just takes longer to manifest..

    • I second the advice of Tony and just want to add that materials other than metals might have been affected by the acid fumes so it’s not just corrosion you want to keep an eye on.

  4. Thanks. Very good story. This can happen in a certificated aircraft as well. I’ve discovered that, like most people, the maintenance people are usually really good at some things, passably good at other things, and not so good at some things. FWIW, the “not so good at” stuff seems to often be electron related. Several years ago I had repeated minor OV problems in my C172 that manifested as corrosion in my battery box. I mentioned it to the very experienced fellow who cared for my aircraft. “Not to worry”, he said. So I just kept rebuilding the box at every annual. Then one day I landed several hundred miles from home. The maintenance shop who helped me out with my small mechanical (grounding) problem also noticed my regulator was sending a lot of juice to the battery. A simple adjustment and now, years later everything is still happy. No more corrosion from acid bubbling over. Battery life is good. (and I’ve got a new maintainer).

  5. The smell does not necessarily dissipate. Your nose ‘gets used to it’ for want of a better term so the dissipation of odour is not a reliable indicator that the problem has been resolved.