We finished flying off our 40 hour initial flight
restrictions this week end. The most difficult
problem during this testing has been carbon
monoxide in the cabin. We installed the AIM
CO detector (as recommended in AVWEB)
before first flight.
During flight testing we found the CO levels
were about 60 ppm at cruise. There were
two sources for this CO;
1. The exhaust entered the rear cockpit thru
the flap drive slot, and then made its way into
the cabin, which is lower pressure than the
rear fuselage (on our plane). We tried a
number of fixes, but finally ended up
riveting cowl seal to the bottom, inboard
edge of the flaps, sealing up the drive slot
when the flaps are up. There is also the added
benefit of sealing the flap to fuselage gap,
which may help reduce drag. This fix dropped
the CO levels to about 30 ppm.
2. The second source of CO was under the
cowl. The exhaust system shipped with our
912S package used slip rings to seal the
muffler / header pipe interface. These rings
do not make a perfect seal. Some exhaust
gets into the engine compartment, exiting
thru the louvers. It then came into the cockpit
thru the fresh air vents. When we installed the
new exhaust system from Europa the CO levels
dropped to about 16 ppm, which is acceptable.
The new system uses ball joints (vs slip joints),
which seal tightly, keeping CO out of the engine
compartment.
It is not obvious what is an exceptable level for
CO. Although some Government agencies suggest
50 ppm is OK, the writer of the article in AVWEB
believes anything over 35 ppm should be reason for
an emergency landing, and I tend to agree with him.
After one two hour flight at 10,000 feet with 60 ppm
CO, I had difficulty landing, and needed a calculator to
subtract one small number from another. These low levels
may not give classic symptoms (headache, nausea, etc.),
but may still reduce your ability to fly the airplane.
I highly recommend buying, or at least borrowing, a
HIGH quality CO detector and checking out your
plane. Note, the article on CO detectors is at;
www.avweb.com/articles/codetect/
Terry Seaver
Pleasanton, CA
A135 / N135TD
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