Carbon Monoxide and the Fly Baby

Posted May 2006

It all started about a month ago, when it was time for my annual.

Fly Babies have an inspection panel across the bottom of the fuselage, just aft of the firewall, for access to the rudder pedals and the brake cylinders.  Usual attachment is just a couple of wood screws into the bottom longerons.  The panel is a tad awkward to get at, since the cross-bracing wires of the landing gear are just below this area and the brake lines run out of it.

N500F's panel was made out of 0.025 aluminum.  Taking it out wasn't hard, as your could remove the screws, then bend and twist the panel to take it clear of the brake lines and the cross-wires.

The guy who did my airplane...well, he up-sized the thickness of just about everything.  I have't put a micrometer to the panel on my airplane, but I'm guessing it's at least 0.050".  I can't *bend* the thing, by hand.  Years ago, just after buying the airplane, I actually cut it down the center and added a set of anchor nuts so I could remove the thing in two parts.

Even so, working it past the brake lines was a problem.  The brake line came out of the panel about three inches forward of the back edge, and about two inches from the side.  The original builder had drilled a hole for the brake line, then cut a narrower slot to the side edge so one could move the panel into place.  The panel would be moved close, the brake line slit through the slot into the hole, then the panel slipped past the brake-wires into position.  When screwed down, a cut grommet would be fed into the hole around the brake line to help protect the soft aluminum line.

It worked just as good as it *sounds* like it would.  The grommet was always getting lost, and the edge of the panel resting on the brake line.

So, a month ago, I was taking off the panels for the inspection...and one brake line started oozing hydraulic fluid. And of course this was 9 PM the night before the A&P was to show up to do the annual.

Fortunately, when I had replaced the line on the other side when changing gear legs, I'd bought twice the tube and fittings that I needed.  I did a desperate call to a buddy with a flaring tool.  He dropped it off at the airport on his way into work the next morning, and when the A&P arrived, I was cheerfully bending up a new brake line.

Got the annual signed off (after botching one flare so that the brake line bled all over the place when I tested it in front of the A&P...), and it was time to re-install those damn panels.

Yes, I know:  I should just replace the pair with a 0.025 aluminum piece.  But my transponder antenna was now installed on one side, and I was loath to mount it on a lighter piece of aluminum.

I compromised, using a band saw:  I cut out the corner of the panels where the brake line hole was.  Instead of a hole and a slot, I now had a 3" x 2" notch on the corner.  I slit some rubber tubing down one side, RTV'd it to edge of the notches, and quickly re-installed the panels.

One problem.  There was now, of course, a bit of open area around the brake lines.  I figured I wouldn't get much of a breeze...but the one on the right was somewhat close to the exhaust pipe.

I'm a Carbon Monoxide chicken.  I've actually *been* poisoned...years ago, I was a passenger in a Mooney that had holes rusted in its cabin heat muff.  I keeled over in the terminal at the destination.

(As an aside:  In a small airport in the midwest during the "drug years" of the 60s and 70s, a rent-a-cop can think of only ONE reason a 17-year-old kid might collapse in public.  I was treated appropriately...)

Anyway, I cautious about Carbon Monoxide.

Some of you might grin and say, "In an open-cockpit plane, Ron?"

Heck, yes.  As I've posted before, the Fly Baby cockpit is a *low pressure area*.  It's certainly possible for exhaust gases to enter the cabin, flow up the pilot's torso, and be inhaled before shooting out the top.  A friend of mine got a bad case of CO poisoning in a Starduster.

After getting the plane back together, I worried a bit about the CO levels.  Was I smelling exhaust more?  Was I just imagining it?

So...I decided to do a little testing.  I'd had a pair of Sears gift cards burning a hole in my wallet since Christmas, so I went by there last night and found a nice battery-operated Kidde CO Detector for about $40.  It wasn't merely an alarm, it has an LCD display of the PPM levels for CO.  I threaded a piece of safety wire though the mounting holes, twisted the wires around the master turnbuckle, and went flying.

To start with, let's look at what the safety levels are.  From :

CO Concentration
in Parts Per 
Million (PPM)
Toxic level for healthy adult over eight-hour period
Slight headache within 2-3 hours
Frontal headache within 1-2 hours, becoming widespread in 3 hours
Dizziness, nausea, convulsions within 45 minutes, insensible in 2 hours

Other sites give different exposure levels.  For instance, from :

"For healthy adults, CO becomes toxic when it reaches a level higher than 50 ppm (parts per million) with continuous exposure over an eight hour period.. When the level of CO becomes higher than that, a person will suffer from symptoms of exposure. Mild exposure over a few hours (a CO level between 70 ppm and 100 ppm) include flu-like symptoms such as headaches, sore eyes and a runny nose. Medium exposure (a CO level between 150 ppm to 300 ppm) will produce dizziness, drowsiness and vomiting. Extreme exposure (a CO level of 400 ppm and higher) will result in unconsciousness, brain damage and death."

The CO detector I bought displayed the PPM, but also had an alarm rigged to go off at 200 PPM.

I cranked up the engine, and sat idling for a few minutes.  The CO Detector read zero.

I started taxiing downwind to the runup pad at the opposite end.  As I rolled along, I caught a waft of exhaust smell.  I casually flipped up the Detector to check the readout.

100 PPM.

Gulp.  I kept rolling, and checked a bit later.  Down to about 58.

At the runup pad, the Detector read about 45.

I goosed it for takeoff.  After climbing a bit, I checked the readout.  Down to 38 ppm.  Climbing to 2,000 feet, it bounced back and forth from the mid-30s to the mid-40s.

At altitude, configure for cruise.  Get everything set up, stabilize speed, read the meter.

0 PPM.

Hmmmm.  Broken?  I cruised around for a while, checking the usual sites, then headed back home for touch-and-goes.

Still zero, all the way on downwind.  I cut the power, turned base, and slowed up.  Caught a waft of exhaust, checked the gauge, still zero.

Turned final, smell was stronger, gauge was indicating 45.

After the touch-and-go, it read about 35-40 on climbout.

Back to zero on downwind.

Anyway, that's how things went. The most reassuring thing is that the levels at cruise are low...below the detectability range of the meter. I never saw a reading between zero and 30; I wonder if the gauge is even able to differentiate levels that low.  I'm also suspecting there's a ~10 second lag in the readings as well.

Levels rise when the plane slows other words, when the angle of attack increases.  When the angle of attack goes up, the slipstream is more likely to push the exhaust gasses into the belly.  Or swirl up the side of the fuselage.

The high taxi levels?  Not sure, but it could just be that the plane is slow enough that I'm catching the hot exhaust gasses as they rise.

The upshot:  Nominally, I don't have a problem.  Long-duration exposure levels are less than 30 ppm, most higher levels are short duration and still below the 50 ppm danger level.

In reality?  I'm going to see what I can do to reduce the levels.  The stuff just ain't good for you.  Besides, it was still located 15 inches or so from my may not be indicating the environment I'm actually experiencing. I might hang it on a neck strap, next time.

Two main suspects, right now.  The first is the modified bottom panel just aft of the firewall.  There's a hole there, big enough to stick a finger or two in.  A bit of duct tape will temporarily seal that; if it helps, I come up with some small removable plugs.

The second suspect...the belly panel.  The edges don't make a seal on the side of the longerons. However, I *was* smelling exhaust before I put the belly panel in.  I'll slap some temporary duct tape on it, after sealing the front panel, to see if there's any benefit.  If there is, I'll look at coming up with a better seal.

Anyway, I've got my project for the next couple of months.... :-)

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