Carbon Monoxide

[daddo2]

Yesterday started with hopes of a great flight.My plan was to fly from Paso Robles and do some landing practice at Santa Inez ( 2800x75') and if all felt good then on to Oceano (2300x60'). However as I passed over San Luis Obispo at 4500' I noticed my Quantum Eye CO detector had changed to a light shade of gray. This has happened before but always clears upon opening a couple fresh air vents.It's been warm so I already had all my fresh air vents open and within five minutes it went to a definative darker gray.At this point I decided it best to make a bee line to the nearest airport(SLO) and the plane is there parked in a buddies hanger.
I'm using four stacks that extend 4" below the engine cowl. I pulled them all off to check for a blown exhaust gasket and found none. I am all too aware of the dangers of CO in flight. Many years ago we blew an exhaust gasket in flight in a Cherokee 140. We had no idea what had happening untill after we landed and got out of the plane. We both were dizzy and had pounding headaches that got worse over the next hour. An examination of the aircraft revealed risidual exhaust coming out of the left side cowling just in front of the fresh air vent and 1/2 of the exhaust gasket had blown out.
I've ordered a digital Pocket CO detector so I can get a better idea of how high the CO level is but before I fly again I would welcome ideas on where to start looking for "the silent killer".........FredL

 

[eschrom]

The way I see it, there are two routes by which CO can enter the cockpit and neither one is a quick job to block off. One is from the engine compartment around the firewall and the other is from outside the a/c around the canopy skirt. This winter if I finish the other things on my list I may try to improve the firewall seal, possibly even split the cowling.

For now I am keeping CO at bay by flooding the cockpit with air from the box baffles and it's working pretty well. I can fly for an hour and a half and the detector is only beginning to change color.

Ed
2LS with four pretty short (~1") stubs

 

[EricM]

CO is a particular problem with the midwing Sonerai. It comes in the canopy and the wing root. I tried sealing all these along with the firewall and still got an unacceptable amount of CO in the cockpit. I finally extended the exhaust pipes about a foot and it cured the problem. One of the items on my "to do" list is to build an internal exhaust system which exits the lower cowl. Here's a picture of the extensions I made. They are held on to the short exhaust stubs with a pin and a clamp.

 

[Born2fly]

Wow! This is a good thread. Pretty scary though! Due to a recent experience with CO while flying, I have become a little concerned with how to avoid it in my plane when I finally get to fly it.
I will be running my exhaust down through the bottom of my cowl. So I think that will protect pretty good from CO getting in that way. However I am interested in how to seal the firewall. Would using clear RTV around it be a good idea? How have you guys done it?

 

[thomefran]

CO truly is a deadly killer. I'm wondering where to find an inexpensive detector for the inside of an a/c. I checked one site and you have to buy a 1000 minimum for a $2.49 each price. Heck that's 2500 dollars. I'm hoping that I want have this problem. I would think that as fast as we go that it wouldn't be.

 

[EricM]

The other day I flew twice for 30 minutes with three CO detectors in my Sonerai. I borrowed an expensive digital unit from a friend and also had fresh Quantum Eye and ASA stick-on detectors. While in flight with the extended exhaust pipes the digital unit registered 0 ppm but during taxi it got a whiff of CO and alarmed. It will alarm at 10 ppm and has a max readout of 100 ppm. Above that it indicates "high". The two stick-on units did not register CO during that flight.
I then removed the front pipe extensions and flew again. The digital unit started alarming almost immediately and during the flight steadily climbed to "high". The Quantum Eye turned green after about 15 minutes and was getting darker by the time I landed after 30 minutes. The ASA did nothing.
I started with the fresh air vents closed but opened them about half way through. The digital unit then showed a reduced level of CO but was still above 80 ppm. After landing, I threw the ASA in the trash and kept the Quantum Eye. Here's a link:

http://www.aircraftspruce.com/catalog/pspages/llifecodetectors.php

P.S. I will continue to fly with the exhaust extensions even if they do make my plane look like a snaggle toothed walrus.

Eric

 

[Schmleff]

Many Sonerai fly with a negative pressure on the interior of the cockpit. This can be seen by the fabric being sucked in around the stringers. This vacuum pulls air in through any open hole whether it be one if the firewall seal or any other opening in the cowl, canopy or fabric.

One of the reasons I moved my exhaust to exit where it does is to prevent exhaust gasses from being pulled in around the canopy seams. All of my exhaust flows under the wing and I have all of the wing roots sealed to prevent this draw.

Another strategy is to get enough air into the cockpit to equalize (or close to it) the pressure inside the cockpit. If the pressure is equal or greater it is less likely to draw exhaust gasses into the cockpit.

 

[JuergenA]

another strategy is, to take the exhaust outlet down to the air outlet of the cowling, so the exhaust gases flow below the plane. Other idea of this system is, to become additional airflow inside the cowling, because the exhaust gases speeds up the cooling air (similar as Kent Paser writes it in his book "Speed with Economy"). In this wintertime (no flying), I want to increase the bottom length of my cowling about 1/2" , to become the end of the exhaust inside the cowling (about 1/4"), to increase the venturi effect.
To understand my system, have a look inside my picture gallery to exhaust2007,exhaust2008, air_outlet_2007 . Actually I take the exhaust2007 because I have no space for my exhaust2008 and also are the welding lines on ex2008 are bad
(When I have realized all the other things I want, then I want to take a new exhaust based on the ex2008 design, because this exhaust gives more engine power...)
with best regards
Juergen

 

[daddo2]

Hey Erik,
I was wondering if you ever checked the PPM with all four short stacks. My thought is that if the front and rear stacks are different lengths the flow of exhaust rearward would be diffused into two lesser concentrated streams. I was planning on lengthening my rear stacks and giving this a try but after reading your post looks like it's already been done....FredL

 

[EricM]

Fred,
I never checked the actual PPM with four short stacks but when I flew with the two rear stacks extended, the Quantum Eye took about twice as long to darken as it did with no extensions.
What PPM readings are you getting from your handheld detector?
Eric

 

[daddo2]

Hey Eric,
I haven't got the chance to fly since extending the rear stacks.Hopefully tomarrow if work and weather permits. I also sealed two small areas around the front of my canopy that might be allowing exhaust gases to enter. It's easy to see how fumes can swirl around and up from the two forward stacks and enter the cockpit.Thing is I never did detect any exhaust smell, which if it were coming in from the stacks,even though CO is oderless, I think you could detect something with your nose. If lengthening the rear stacks does not help then I'll do the fronts as you have. After that I will look at the firewall though it looks as if it is sealed real well. Next time I fly I will be taking the Quatum Eye and a digital CO detector that I plan to "aim" at different areas around the cockpit......FredL

 

[thomefran]

Thanks for the info on the quantum eye co detector. I just ordered one from sky geek.

Thanks again.

 

[daddo2]

Today I was able to get some accurate CO readings while in flight. My two front stacks extend down 3" below the cowling and the rears 5.5". I have four fresh air vents....naca vents on each side of the canopy, a vent in the plexi- glasss just in front of my left shoulder, and an underwing vent ducted into the cockpit from 3' outboard on the leading edge of the left wing.I flew with two CO detectors; a Quantum Eye on the spar carry-through in front of me, and a digital CO monitor mounted forward on the instrument panel.
Taxi canopy closed, and take off all vents open, was normal. But as I reached 1000' I got my first CO reading. I continued to climb with full throttle/mixture to 2000' and the CO reached a high of 98ppm. At 2000' I leaned the engine slightly and saw an almost immediate decrease in CO and upon reaching 3000' it had dropped to 49ppm.At this point I also noticed a color change in the Quantum Eye. I leveled off and reduced power to 3050 rpm and further leaned the engine for best performance.At this point the CO dropped rapidy to less than 10ppm (the digital CO monitor needs a reading >10ppm) and I recieved no further indications from the digital CO monitor for the remainder of the 20 min flight. I held the Quantum Eye in front of each vent and it turned (and remained) clear for the rest of the flight.
With this info I think it's easy to see that the combination high power/mixture/angle of attack during climb is allowing the CO to enter the cockpit, most likely from the short front stacks, and into the vents. I was relieved to see the level drop once in level flight. But I was surpised to see just how much mixture setting during climb affected the CO level.
Next flight I will leave the two canopy vents closed to see how this affects CO during climb......FredL

 

[EricM]

Fred,
When I was testing for CO with short stacks, the CO level increased with the vents closed, probably due to the negative cockpit pressure schmleff described. I tried weatherstripping the canopy skirt, taping the wing root and taping the cowl piano hinge. These all helped but did not cure the problem. I never thought to tape the joint between the cowl and the fuselage.
I am very interested in your underwing vent. Can you post some pictures.

Eric

 

[thomefran]

I'm wondering if the underwing vent could be your problem. Have you tried to close it and see if it helps?

 

[EricM]

I think the underwing vent would not be a problem, especially if it is three feet from the wing root. When I was flying with the short exhaust stacks, the exhaust stains were along the fuselage above and below the wing root. With my extensions which are angled away from the fuselage, there are no exhaust stains on the plane at all.

 

[daddo2]

Just got home from a great flight around Morro Bay. The weather was great and air smooth smooth smooth.
On climb out today I left all my fresh air vents closed except for the underwing vent. I checked to see how good the ventilation is by cracking one of the other vents to see which way the air flows and with just the wing vent the cockpit has positive ventilation as I was able to detect air flowing out.
With mixture and throttle at full there was only a slight momentary CO reading on climb to 3000' (31ppm) and the Quantum Eye remained clear. Slight leaning during climb brought the CO level to <10ppm and remained there for the remainder of the flight. Again I think the combination high mixture/power/AOA during climb induces high CO into the fresh air vents and should be carefully monitored during a long extended climb.Erik is right about the underwing NACA vent........no CO and it is very effective.I will post photos soon......FredL

 

[daddo2]

Spent the morning scraping the muck off my plane ( stop laughing ) and took a couple photos of my underwing vent.

 

[Bil4381]

The first 3 years I flew my SII, the card CO detector changed color. Twice it turned black They are $5 at most FBO's.
I changed the exhaust gaskets twice, a year apart. I don't think that was it.

The first thing to do is to fly straight. Get the ball centered and keep it centered. Otherwise the exhaust slips up around the side and comes in the canopy vents. I installed a 5" ball.
The second thing I've found that oil leaks onto the exhaust will produce a hot smell, and that's one time the CO det changes color. Seal the oil leaks. Keep the exhaust system oil free.
Thirdly I installed a 3" high X 14"wide deflector \ to force the airflow over the lower cowling down. It helps a lot,... if I fly straight.
This year my new CO detector never changed Color once. What's different? I laid up epoxy/glas wing root fairings say 1.5" X 1.5" last winter. They don't look that great and I didn't think the seal was perfect but this year they probably made a big difference. I used foam and modelling clay to get the shape, used food wrap as a separator, and did the layup, 2 layers of cloth. Parting film was used to improve the finish. 1/8" foam seals the fairings to the skins and fabric.

If you can smell exhaust in the cockpit, sometimes "closing" the canopy vents stops the CO ingress.
I'm still thinking of installing exhaust pipe extensions. But they need to go back as far as the canopy air vents. On my SII that's a lot of SS pipe and would need to be supported. I hate adding weight.
Bill

 

[daddo2]

If I were in the process of building I would put one of these vents in each wing. Just one has enough airflow to adequatly ventilate the cockpit and it is far enough removed from the exhaust that there is no chance of CO contamination. Using this single underwing vent as my primary seems to have solved my CO problem......FredL.