Here's the text and diagrams/photos from the pdf
Recent observations from Kjeller airport, Norway is that a few aerobatic aircraft with Lycoming Engines
type AEIO 360 and the Christen Inverted Oil system have experienced water accumulation in the oil
system. One to two cups of water has been drained from the lowest hose connecting the Christen 803 oil
separator during 50 hour service. See figure 1 below. This water is also believed to have caused
premature internal engine corrosion on engines with less than 3 years since new/overhaul.
Ice in the hoses connecting the Christen separator has also caused an engine failure in 2010, see
preliminary aibn report SL2010/01P:
In this case, water accumulation in the hose connecting the 802 Ball valve and Breather Tee 806 had frozen
solid and caused loss of oil pressure during inverted flight that subsequent lead to an engine failure.
Fortunately, the airplane landed safely without anyone being hurt.
We believe that a possible explanation to this is that water vapor from the hot oil condenses in the
breather line during normal operation, as this line has a lower temperature than the engine itself,
especially in cold weather. Droplets of condensed water then trickles down in the Breather Tee and
accumulates in the hose above the 803 ball valve (see figure 1). When the aircraft is inverted, this hose
becomes the suction hose to the oil pump and the accumulated water enters the oil system. When the
aircraft is upright again, the cycle repeats, and when the flight is ended, there is some water in this hose
that may freeze under sub-zero conditions in the hangar.
Other aircraft with a different breather line configuration, see figure 2 below, have seen no water
accumulation and no corrosion under very similar operating conditions. With this hose configuration,
there is no transport of humid gases in the breather line that becomes the oil suction hose when
inverted, hence less risk of water accumulation other places than in the separator vessel itself.
The number of aircraft we have compared is of course very small, but the observations are consistent.
We are therefore interested to hear if this matches other operators experience, and if so, what can be
done to reduce or eliminate the problem. Clearly to change the breather line confirmation to what is
shown in figure 2 would work, but a recommendations should be gives as to what engine connection
that should be used on various engine models. A temporary fix could be to re-orient the breather line
Tee so that the branch line points upwards (if space/hose lengths allows) to avoid gravity flow of water
into the hose.
Aircraft seen with figure 1 configuration and corrosion problem are CAP 10 and Decathlon. The figure 2
configuration has been observed on Extra 230, Slick 360 and Christen Eagle.
The valve and the hose was brought inside a warm hangar and heated up slowly. Oil and ice came
out of the ports of the valve and the balls became gradually released (see figure 2). Figure 3 and 4
show the amount of ice that was trapped inside the hose.
It is likely that the following caused the loss of oil pressure. The mentioned 4-point roll was the first
time in nearly two flight hours where manoeuvres involving negative g were flown. During these
two hours a lot of humidity from the crank case ventilation probably condensated at the inner walls
of the hose leading from the crank case to the oil separator. This drained down from the t-fitting
into the “blue hose”, filling it up with water. As no negative g-manoeuvres had been flown last two
flight hours, the water was not flushed out of the hose. It is likely that all components at the fire
wall had temperatures below freezing level during the night prior to the flight 8 January. Further it
is likely that the area between the engine baffle plates and the fire wall stayed below freezing
temperature during the entire flight. Engine oil at about 60 °C is likely to have been the only heat
source in that area. This would heat the lower portion of the Christen 802 valve and the two
associated hoses. When the airplane became inverted it is likely that ice and probably water from
the “blue hose” entered the Christen 802 valve. This blocked the oil from entering the oil pump and
the oil pressure dropped. When the airplane completed the roll, the bottom ball (during normal
flight) was probably kept on its seat by suction produced by the turning oil pump. It is also a
possibility that the balls were jammed by ice.
The AFM contains no limitation regarding minimum operation temperature or any warnings
regarding the inverted oil system and cold weather operations.
Regardless of type of airplane, it is likely that all inverted oil systems of this type might be affected
by sub zero temperatures in a similar way
...they have been known to stick shut due to varnished or dirty internals. It used to be recommended to take the shuttle valve apart and clean it every so often.