Carbon Monoxide in the Cockpit

[Romeo Romeo]

I fly a Cessna 150 Aerobat. We've got a CO meter which displays the parts per million of CO. On the ground with the engine off it reads 0ppm, but after flying for a while the reading slowly builds up to about 70ppm. We've also got one of those spot-things that turns black with the presence of CO and that does darken slightly whilst flying. So we're definitely getting CO in the cockpit.

The obvious place to look is the exhaust, but we had it pressure tested a while ago and it was OK then. Also it doesn't seem to make a difference whether the heater is on or not. One of the nose-wheel steering boots is slightly damaged (and is going to be replaced soon), so it's possible it might be getting in there.

Anyone got any ideas where this CO is getting in and what tests we should do?

[High Wing Drifter]

Check the door seals. Fumes can be sucked, however little, it will build up. I believe this is something my group found with our Tri-pacer.

[funfly]

Had the same problem, we had a long silencer fitted which vented centrally but at a point level with the flaps at the back of the (low) wing - had fumes in the cockpit up to 80ppm at times.

Removal of the silencer allowing the exhaust to f*art out just below the engine and in front of the wing leading edge cured the problem.(so far anyway)

There is every possibility that the airflow can go forward from the trailing edge of a low wing along the fuselage almost to the leading edge and that is often the point where the fresh air vent is (it was in our case).

80ppm won't kill you (very quickly) and the danger limit seems to be about 100 ppm. However 80ppm will give you a headache after a while and the effect of CO is cummulative.

May help you.

[Romeo Romeo]

Our door seals are very drafty so that might be where it's coming in. Getting these sorted might make a difference but certainly will make it warmer! So, thanks for that HWD.

Interesting information, funfly. I assumed that once outside the aeroplane the exhaust would just be blown away, but it would appear not.

[Morane]

Hi Romeo,

on my Morane 880B with the O200 engine, I discovered that the lower left part of the firewall (directly behind the exhaust pipe) was black. Although the end of the exhaust pipe is about 10cm below the fuselage, it seems that the exaust gases can hit the firewall. I guess this will be the case while flying with a high angle of attack. If some gases are directed behind the cowling, they will be pressed into the cockpit thru every little gap in the firewall.
In my aircraft, the exhaust pipe has allready been modified by one of the previous owners. They welded an additional piece of pipe to the exhaust outlet to lengthening it. With the original silencer (short exhaust pipe) the problem must have bin even worser.

Hope it could help.

Joe