I just read Paul McBride’s response to John Folinsbee in the April 22, 2005, issue (What led to camshaft damage?) I know this is a huge subject with no easy answers.
May I give some information about engines and preheaters that we have observed and which might help your readers understand? Contrary to what you wrote, “a preheater left on simply becomes a giant condensation generator resulting in internal engine corrosion,” Tanis preheaters do not create moisture.
Water vapor gets into the crankcase mainly because it is a byproduct of combustion which enters from ring blow-by. Since aircraft engines do not have positive crankcase venting as our cars do, this vapor has nowhere to go. As the engine cools down below the dew point it is deposited somewhere inside that case.
I have measured the relative humidity in many engine crankcases immediately after they return from a flight (summer or winter) and it is almost always near 98% when the engine is hot. At 150°F our local University of Minnesota-Morris Chemistry Department calculated that would be equal to one-quarter cup water if the case is 2 cubic feet in size and cooled to 35°. I suggest that as the engine cools, water vapor forms a very thin film on any exposed parts inside the case like a cup of cold water in the summer time. As the film thickens, droplets may form and sink to the bottom of the oil pan. It has been our experience when doing oil changes that at times we have drained nearly 2 oz. of water from the oil sump before the oil begins to drain. This is no problem when immediately after flight all parts are coated and protected with oil.
According to their literature, oil companies’ standard test for corrosion protection is to spray oil on a shiny clean metal plate, then spray it with salt water and place it in a standard humidity chamber at 98% humidity/70°F. They often illustrate these results on a graph showing “our” oil protected from rust for a longer time than “theirs.” Wisely no one puts elapsed time numbers on these graphs. It is accepted that rust will happen.
I have talked to several old-time mechanics who say the rust problem was much worse years ago, they just thought nothing of it. They assume oils are getting better. We also have had reports from pilots with rusty cam issues who blame their preheater for the problem, only to discover they have never had a preheater…hmm.
Daryl Bolduc, of Bolduc Aviation Specialized Services, Blaine, Minn., our area aircraft engine rebuilder-machine shop, made an interesting observation from another perspective. He suspects that there may be a quality control problem with some of the cam followers and has seen low time cam followers that have failed with no evidence of rust on the part. He will resurface a used one that has stood the test of time rather than replace with new.
Let me pose this question: Can you explain the difference between pushing your aircraft into a hangar as soon as you land after flying when it’s 80° outside for weeks versus flying at 20°, then covering the engine with an insulated engine cover and plugging in your Tanis heater, designed to keep the engine temperature 60° above the outside air temperature?
There is no doubt that heat will thin out the oil protection film faster than no heat. I suppose the best engine storage would be at about 10° or lower.
One other note: Because Tanis Preheaters are designed to heat the entire engine, our systems do not need to focus on raising the oil temperatures in an effort to heat the remainder of the engine. Raising oil temperatures too high forces water out of the oil, which could be an issue with “sump only” preheaters.
The dilemma remains, how should aircraft engines be preheated?
Our standard recommendation is:
– If you fly once a week, and get your oil temperature up to 180°F for approximately 45 minutes, leave the plane plugged in with an insulated engine cover on it.
– If you do not fly regularly, plug your preheater in four or five hours or the night prior to flight and use an insulated engine cover.
Tips to prevent engine corrosion damage:
– Fly often. Get those shiny metal parts re-oiled regularly. It is a great excuse to fly!
– Change oil often. Every 25 hours? Along with water vapors come corrosive acids that accumulate in the oil and are circulated by the oil pump and oil splash.
– Remove the water vapor. Corrosion doesn’t happen in a dry desert. I have watched the humidity level go from high 90s to 20% or less overnight by removing the oil fill cap (replace before flight) with the Tanis engine preheater plugged in. Air exchange from a “chimney effect” sucks in cool dry replacement air.
– Even better – use a dehydrator to lower humidity to near zero. However, I do not know if such equipment is available. The importance of low humidity in rust development is illustrated by our tests with steel wool washed and dipped in salt water show no visible rust in 5% humidity chamber even weeks later.
– Pickle your engine if long-term storage is expected. Both major engine manufacturers have Service Bulletins with recommended procedure. (Tanis has Pickle Kits available).
Tanis Aircraft Services