Carbon Monoxide in Cockpit

[3914N]

Currently stumped, hopefully MS can provide thoughts...

On a flight a few days ago I got a carbon monoxide indication on my "orange dot" ASA detector (picture below).  Here's the sequence of events:

• Preflight & engine start are normal.  I verify the detector is still orange as part of my preflight checklist.
• I haven't used the cabin heat in months and today is cold, so I check the cabin heat is working on the ground.  Heat is on for maybe 10 seconds, then closed.
• Runup, takeoff, climb are normal.
• About 5 minutes into the flight, I glance at the CO detector and see that it has turned dark brown.  I haven't turned on the heat since being on the ground.
• I turn around and land.  No symptoms that I can perceive.

Today I go out and pull off the cowling.  No obvious exhaust leaks that I can see, but I decided not to pull off the muffler shroud (no A&P present), so a cracked muffler is not out of the question.  The only thing that I did find was that the air hose from the heater box to the defroster has come off, so I reattach it.

I then go for a quick test flight with a new CO detector.  Absolutely no detection of CO with the heater on/off or cabin air vent on/off, even holding the detector right in the warm air coming from the vent.  No exhaust smell.

Ideas?  It seems insane that the problem could have just fixed itself, and I don't know how the disconnected defroster hose could have changed anything.  I'm a bit concerned about going on longer flights before I can verify that there is really no problem.

[smwash02]

Could be a failing door seal and wind blowing it the right direction. I don't recall the delay between exposure and when it registers on the dot.

I picked up something like this for my cockpit. Pressing the 'peak detection' button will show you the level even if below 30ppm.

Hopefully others have some ideas/more experience with the 'dot's.

[carusoam]

I have the electronic CO detector.  On the ground it will give some peeps and register some numbers... never an odor to go with it.  My nose is a bit aged.  That's why I got the expensive sensor.

The chemical adsorbing dots can be a little slow to react so the actual event may have happened and cleared out before it turned dark...

You took good precautions given the type of monitor that you have.

Best regards,

-a-

[Bob_Belville]

Here's the CO Detector I use. Under $100. There may be better ones but this is reasonably priced, uses AAA batteries, and is portable. I've seen several reports that the disposable card are not adequate. 

https://www.zoro.com/uei-test-instruments-carbon-monoxide-detector-0-to-999-ppm-utlc11/i/G7609996/

[Yetti]

You could maybe take off the orange heat output hose from the muffler and look and see if there is black burning on the hose.  You can also see some of the fins on the exchanger....  Looking for soot

[tigers2007]

I'm glad someone brought this up - is there a problem using one of those battery powers home units in lieu of the ASA "eye" style? The price of one of those ASA ones shipped is almost the same price as a Kidde Nighthawk with a display. In fact I have an extra one at home that is fairly new.


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[cliffy]

Check your wheel well mouse boots to see if they are torn or missing.  Exhaust will come in there if they are compromised

When did you last have your heat muff pressure checked? and not just visually looked at? A pressure check will show leaks that don't show up visually. 

To add, those "dot" CO cards are notoriously bad and only last about 90 days from opening. Other threads are available by search on this subject. 

[Yetti]

I put one of the home units between the seats on some velcro.  Seems like it would not know that it is in a plane and would work the same.

[JohnB]

How old was the old CO detector card? If it has been longer than 18 months, or unknown, i would suspect it MIGHT be the detector card as being faulty, and I would buy a more reliable electronic CO detector in the cockpit. Even with your new card, these are insensitive to rapid changes in CO level which can be dangerous. I have found using the electronic ones, that I typically get 50ppm of CO on long taxi and hold instructions, or takeoff on cold days where i would have the cabin air vents closed which goes away once you open the vents or get out of steep climb. It seems unlikely to me that if you only had a high level on takeoff that it would change your card that quickly in 5 minutes, unless of course the level of CO in your cockpit on takeoff was VERY high, and it might.

I have both a battery operated CO micro detector (which sometimes the battery contact can come apart if the unit is bumped accidentally) and an in panel one by Guardian, so I can adjust any CO leak well before it becomes even a concern as you will not smell CO alone. But this can be real, so I wouldn't ignore this and get an electronic one (not the home depot version) or borrow one from someone and go fly.

 

John

[JohnB]

15 hours ago, Bob_Belville said:

Here's the CO Detector I use. Under $100. There may be better ones but this is reasonably priced, uses AAA batteries, and is portable. I've seen several reports that the disposable card are not adequate. 

https://www.zoro.com/uei-test-instruments-carbon-monoxide-detector-0-to-999-ppm-utlc11/i/G7609996/

Agree with you totally Bob! I was holding back how I really feel about the ASA cards, which I have disposed of them all, as in my opinion, should never be relied upon to detect CO in your cockpit for several reasons as I've mentioned some above. However, a relatively new ASA card in your cockpit can be very useful to an NTSB investigator though to determine a likely cause of a crash though if you get my drift.

[captainglen]

A small exhaust leak or even some lower cowl back draft could have caused the detached hose to draw in exhaust gas. Back draft occurs when the cowl flaps are closed which explains your normal taxi, run up and climb. Larger exhaust. Small leaks usually occur around fittings; to find them wrap the fittings with the thinnest aluminum foil you can find and do an extended high power ground run the leak will burn away the foil. Cracks in the muffler/heater can usually be found with a detailed visual inspection.

[3914N]

Thank you everyone for your feedback.  I bit the bullet and bought an electronic CO detector from Aircraft Spruce ($140 after shipping).  It shows a PPM readout every 2 seconds and has an alarm for dangerous CO levels.

Went for a flight today to test it out.  After a prolonged taxi and runup (with heat off), the device registered 45 PPM.  Holy cow!  I even hit 60 PPM when I opened the cabin door.  I had no idea that I was getting so much CO in the cockpit under those conditions.

After takeoff, the level drops and levels off around 12 PPM.  That's on the higher end, but not an immediate concern.  I get as low as 6 PPM on the descent, right before landing.

It is certainly not an exhaust leak.  If I turn on the cabin heat and hold the detector directly in the vent airstream, it registers exactly ZERO.  Good to know.

I checked the main gear "mouse boots", and they are in terrible condition... completely separated from the bulkhead.  They effectively block nothing.  The CO levels I was seeing today would be consistent with exhaust leaking in through there.  I will order new boots from LASAR and see how they affect the levels and report back.

On a side note, I can't believe how much CO leaks in through an open door while taxiing.  Maybe time to invest in one of those air scoops for the storm window...

[takair]

I would personally be surprised that you would get that much through the rat boot.  They are rather outboard of the airflow in flight.  You may want to check for an exhaust leak outside of the heat muff.  You can get leaks at each joint and at the cylinders.  Also check the firewall for leaks around harnesses and other feedthroughs.  Also, the nose gear doors not closing could be a source.  Not saying not to change the rat boots, but I'm afraid you may have another source of leakage.

[JohnB]

13 minutes ago, 3914N said:

Thank you everyone for your feedback.  I bit the bullet and bought an electronic CO detector from Aircraft Spruce ($140 after shipping).  It shows a PPM readout every 2 seconds and has an alarm for dangerous CO levels.

Went for a flight today to test it out.  After a prolonged taxi and runup (with heat off), the device registered 45 PPM.  Holy cow!  I even hit 60 PPM when I opened the cabin door.  I had no idea that I was getting so much CO in the cockpit under those conditions.

 

Good call. Sounds like you're on the right track. I found a faulty exhaust clamp after buying my CO detector and after replacing, my levels are pretty low, but not always zero.  I asked my mechanic about those little CO leaks on taxi now that I can detect them, he said he has spend several days on planes trying to find sources of these small leaks, replacing all sorts of hoses with no change in levels. If its cold, and I don't open the cabin air vents before takeoff, I'll often get up to 50ppm CO on takeoff which then goes down once I open the overhead vents. I get 0 PPM once in level flight or descent, so it sounds like you might have a small leak somewhere.  Ive found that these all keep my CO minimal (now that we can detect it at very low levels)

• Keep overhead vents open even on cold days on taxi/ takeoff
• side air scoop open on window helps on taxi. (unless you're near jet traffic in which case I close it)

[3914N]

Thanks John and Rob.  I'll inspect the heat muff along with the boots.  Would be great to get that 12 ppm in cruise down closer to zero.

[takair]

21 minutes ago, 3914N said:

Thanks John and Rob.  I'll inspect the heat muff along with the boots.  Would be great to get that 12 ppm in cruise down closer to zero.

When you have the cowl off, look for telltale signs of a leak.  You will typically see a white discoloration.  Look at each exhaust stack junction and especially where it is bolted to the cylinders.  Also, where the tailpipe connects to the muffler.  I am thinking you have a leak in one of those areas.

[kortopates]

I have a Guardian CO unit in my panel, it replaced my clock. I personally think serious CO protection is really important for turbo planes that fly high like a few of us. Our pulse oximeters sure can't tell between CO and O2 and we're along ways from the ground if we need to get down. So an alarming or alerting device that you can't fail to notice like on a panel mounted device provides peace of mind. At least we have unpolluted supplemental O2 onboard.

The only time I have ever had it alarm (50ppm) was on the ground in the run-up area with door open. My unit doesn't display anything till a minimum of 10 ppm. There is a thread from probably a couple years ago here on MS discussing what might be typical background amounts of CO and what was safe. You might try searching for that. But I would think that normal background should be in the sub 10 ppm range.

[DonMuncy]

It is probably not the case here, but after a period of time (3 years or so), I have had Guardian CO detectors start giving false positive alerts.

[ijs12fly]

On 1/3/2017 at 9:32 PM, cliffy said:

Check your wheel well mouse boots to see if they are torn or missing.  Exhaust will come in there if they are compromised

When did you last have your heat muff pressure checked? and not just visually looked at? A pressure check will show leaks that don't show up visually. 

To add, those "dot" CO cards are notoriously bad and only last about 90 days from opening. Other threads are available by search on this subject. 

@cliffy  If the mouse boots are torn, do you have any idea how to you repair and with what material?  It looks to be a green / brown burlap-ish type fabric.  

[cliffy]

If they are torn the fabric is probably old and weak and will just keep tearing

Replacing the boots is easy by cutting new ones with a long open side and then hand stitching the closure in place with heavy thread. Fold the small end double and sew it for a more sturdy small end. 

Fabric can be most any durable tightly woven fabric, canvas, sail cloth 

[Shiny moose]

I once read the ASA dots are to be replaced every 30 days as they are not reliable after that.  not sure if that still holds.

With an electronic detector on the ground with a tail wind you could get a high reading, but inflight you would want a very low number, the closer you are to 0 the better you are, pressure test your exhaust system, look visually for the leak, small cracks turn into bigger ones, loose connections get looser. Its there,  you just gotta find it before it becomes a number that  puts you to sleep 

An old friend of mine and his wife took off and both woke up in a field after the crash several miles form the airport( strange but upright, both survived, no major injuries) He did not remember anything after starting the take off. 

[carusoam]

We have an MSer... @DanM20C with a similar CO survival story....

Nothing more serious than finding the source of the CO and putting a stop to it...

Some CO leaks start mild... and tear open in one flight...

CO is easily absorbed by the blood and takes several hours to leave it....

PP thoughts only...

Best regards,

-a-

On 5/14/2021 at 9:18 AM, Shiny moose said:

I once read the ASA dots are to be replaced every 30 days as they are not reliable after that.  not sure if that still holds.

With an electronic detector on the ground with a tail wind you could get a high reading, but inflight you would want a very low number, the closer you are to 0 the better you are, pressure test your exhaust system, look visually for the leak, small cracks turn into bigger ones, loose connections get looser. Its there,  you just gotta find it before it becomes a number that  puts you to sleep 

An old friend of mine and his wife took off and both woke up in a field after the crash several miles form the airport( strange but upright, both survived, no major injuries) He did not remember anything after starting the take off.