Carbon Monoxide in the Cabin

M

M1tchell

Pre-Flight
Joined
Apr 12, 2017
Messages
65
Display Name
Display name:
M1tchell
I was doing my BFR the other day in a rented Cessna 150. We were enroute to the area where we were going to do the maneuvers, so we hadn't done any stalls, slow flight, any of that yet. All we did was take off, climb out, and we put the hammer down at a blistering 90 knots, because hey I don't need to be here all day, let's get this done and get outta here.

Anyway the instructor has one of the portable ADS B receivers that has a CO detector, and it started sending alerts to his iPad, saying the CO was at 75 ppm. So we opened the windows, verified that the cabin heat was off, and came back. Probably unrelated, but the radio quit working about the time we got back into the pattern, but neither of us could smell anything burning up or anything.

On the ground the mechanic took the cowl off, then the shroud around the muffler and couldn't find any holes or cracks or anything in the muffler, and the flap that controls the cabin heat seemed to be making a good seal.



Do what was likely the cause of the high CO? I could see it if we were doing slow flight or stalls or something and some exhaust made it inside, but we hadn't done that yet.

Also, I guess I will be looking for some kind of a CO detector. I have never flown with one, but I was planning on renting this plane for a several hour trip next month, and without a way of knowing there was high CO that could have been bad. Would a regular CO detector like the ones you put in your house be ok do you think? I know the real answer is get an ipad and an ADS B receiver with a detector in it, but that's not going to happen right now.

Thoughts?
 
Thanks for the link!

Edit:
I watched the video in that link about the guy who woke up in a field in his wrecked Mooney, and I ordered a sensorcon. Some guys in that thread were saying it's $111 with promo code Aircraft2021, but I used the code and now it's actually $103.20.

Thanks again!
 
Last edited:
My CO detector serves multiple purposes. I've learned to use it to lean the engine, lean until it reads zero and its at that perfect spot just before the engine gets rough. It also proved itself to be a carb ice detector this past Saturday, noticed it went from 0 up to 6ppm without changing any settings or my altitude. Then I noticed my RPM had went from 2500 down to 2400. Turned on carb heat for a minute, RPM's came back up to 2500 when I turned it off and CO dropped back to 0!
 
The CO is likely coming from where the exhaust headers mount to the cylinders, best thing is to make sure that there is no holes in the firewall and if there is to put some permatex gasket maker (or similar) in the hole to seal off any possible opening that could allow CO to enter the cabin.
 
Thanks. I'm pretty sure they won't let me get in there with a tube of RTV, and would look at me like I'm an idiot if I asked them to do it. So I'll probably scratch that plane off my list and go to Plan B for my trip coming up.
 
Bad door seals can let exhaust gases into the cabin. The 150 has two exhausts, both not far ahead of the bottoms of the doors. The swirling prop blast could carry the exhaust from the left stack past the pilot's door.
 
M1tchell said:
Also, I guess I will be looking for some kind of a CO detector. I have never flown with one, but I was planning on renting this plane for a several hour trip next month, and without a way of knowing there was high CO that could have been bad. Would a regular CO detector like the ones you put in your house be ok do you think? I know the real answer is get an ipad and an ADS B receiver with a detector in it, but that's not going to happen right now.

Thoughts?
Click to expand...

No, household or consumer CO detectors aren't practical for an airplane. Many (most?) don't provide the resolution & timing you need for a small, enclosed space such as the airplane. For quick & dirty, one of the $5 cardboard detectors is your best bet. Unless you need the ADSB receiver, there are portable detectors, around $70 or so. Here's an example. Lots more, various prices, but you definitely want one designed for aviation.

https://www.mypilotstore.com/MyPilo...sKFek3b1dOUMitXIIFBNTonFiqRNR40xoCONwQAvD_BwE
 
FredFenster said:
My CO detector serves multiple purposes. I've learned to use it to lean the engine, lean until it reads zero and its at that perfect spot just before the engine gets rough. It also proved itself to be a carb ice detector this past Saturday, noticed it went from 0 up to 6ppm without changing any settings or my altitude. Then I noticed my RPM had went from 2500 down to 2400. Turned on carb heat for a minute, RPM's came back up to 2500 when I turned it off and CO dropped back to 0!
Click to expand...

That’s interesting.

It sounds like a good indicator of richness. Unlike automobiles, certificated planes using 100LL don’t have oxygen sensors in the exhaust pipe to detect richness, and that’s because O2 sensors are ruined by lead fouling.

What you’re doing hints of a usable workaround. If a CO sensor could be mounted where it was reliably exposed to a certain amount of exhaust, could that serve the role of an O2 sensor?
 
M1tchell said:
Would a regular CO detector like the ones you put in your house be ok do you think? I know the real answer is get an ipad and an ADS B receiver with a detector in it, but that's not going to happen right now.

Thoughts?
Click to expand...

Home CO detectors are unsuited for aviation use. A home detector has very high alarm thresholds and long dwell times (alarm delay interval) to minimize false alarms. A home detector may not alarm with a short dwell time unless CO concentrations are in the 400 ppm range (!)

For aviation use, you want an electrochemical sensor device with a low alarm threshold and short dwell time. Some vendors include Forensic Detectors and Sensorcon. They are just over $100 and have a useful lifetime of about 5 years. The anode sensor eventually gets poisoned over time, and/or the electrolyte dries up.

The color changing dots are totally useless. We won't use them in the lab, nor would I put one in the airplane. Get a good digital electrochemical monitor. I have the Forensics Detector unit and it will alarm at 9 and 25 ppm with a 10 second dwell time and at 50 ppm with zero dwell time. Mine went off in the maintenance hangar at 9 ppm due to small amounts of CO coming from the propane heaters. So it really works. I mounted mine on the center console in the supplied self-adhesive frame. I see ZERO CO detected in flight. I occasionally see single digit ppm levels intermittently when taxiing with the canopy open. (Presumably picking up some exhaust fumes swirling around the aircraft.)
 
NoHeat said:
What you’re doing hints of a usable workaround. If a CO sensor could be mounted where it was reliably exposed to a certain amount of exhaust, could that serve the role of an O2 sensor?
Click to expand...
No. CO sensors detect levels in the low parts per million. The exhaust would overwhelm it enormously.
 
I have a Foreflight Sentry that has a built in CO alarm. It regularly gives CO alerts and eventually fails. I started carrying a high grade CO detector that gives a real time digital readout in ppm. It usually stays at 0, and has never gone over 35ppm, even though the Sentry alert sometimes says the CO is in excess of 80ppm.

I don't trust Sentry's CO detector, and I'm not all that impressed with their GPS based primary flight display. If I had it to do over again, I'd get the Sentry Mini.
 
The sentry I think is what the instructor had.


So, what is the consensus on acceptable CO limits in flight? I understand altitude plays a part in it, but if I'm flying along at say 6,000 feet or less, at what point should I be thinking about landing?
 
Airplanes leak, and there is no way to seal them completely. A good sensor is (relatively) cheap insurance..
 
Remember that every CO sensor in an electronic system has a useful life - my experience was 5 years or less in an airplane. Given the extra cost of one that ties into the EFB system, I personally opted for the less expensive stand-alone unit. It also meant I could just go around the patch without the EFB on and I'd still have an alert. But everyone's calculus is different.
 
M1tchell said:
So, what is the consensus on acceptable CO limits in flight? I understand altitude plays a part in it, but if I'm flying along at say 6,000 feet or less, at what point should I be thinking about landing?
Click to expand...

There is no "acceptable" level of CO in the cockpit. Tolerable levels will vary considerably among individuals, depending on their baseline level of carboxyhemoglobin, and typical blood oxygenation levels. Symptoms usually appear when carboxyhemoglobin levels reach a critical level (typically around 15%). Smokers start with a higher baseline carboxyhemoglobin level to start with, so symptoms appear at lower CO concentrations than for nonsmokers. Levels of 50 ppm and above should certainly be considered critical, and typically produce symptoms at sea level. Consider than 50 ppm of CO has an effect on oxygen delivery approximately equivalent to adding 10,000 feet to your current altitude. That means you are already getting hypoxic at sea level. Even 25 ppm CO would be quite serious, as that adds a physiological equivalent of maybe 5000-6000 feet to your current altitude. That's where the critical alarm levels are hard-wired on my Forensics Detector unit, BTW.
 
FAA (14 CFR 23.831) wants 50 ppm, the sensorcon will alarm at 35.
 
You must log in or register to reply here.
Back
Top