From Chas:
I went out to my plane (a23-24) today in the hopes of running the engine to charge the battery and clear out any crankcase condensation. The OAT was 17 degrees Fahrenheit. I have summer weight oil in it (20 W 50), as I could not get the oil changed before the weather changed. Anyway, I preheated the engine for 30 minutes, and started the engine running it at 900 to 1,000 rpm. Oil pressure on my antique Beech indicator was normal.
Attempting to keep the prop-wash down, and to keep any ice and snow from hitting the prop, I pulled out the knob on the CS prop a few turns. Five minutes into running the engine, I noticed that the oil pressure had dropped into the upper area of the red arc. I have never seen it in that area before . After shut-down I touched the oil on the dipstick, which was at the 6 quart level, and it was barely lukewarm.
Is it possible the lower oil pressure is due to the oil’s viscosity, and that the oil cooler was cooling the temp faster than the engine heat could warm it up? I am concerned that I could have done some internal damage to the engine, even though it still sounds normal. What guarantees do I have that the old oil pressure gauge is accurate? Is there any upgrade to the pressure gauge, that I can do?
This is a reposting from MML.
You probably have not damaged the engine (yet) from the low oil pressure indication. An engine running at no load idle (or virtually no load above idle) only needs about 5 PSI per thousand RPM to prevent damage.
Your oil pressure almost certainly went into the red because you tried to cycle the CS prop on cold oil. When you pulled the prop pitch knob, the prop governor sucked oil from the engine pressure gallery faster than the oil pump could replenish it, because of the cold and thick oil.
Ground run-ups to “dry out an engine” are usually a bad idea, especially in winter. You cannot get the oil temp up to the required 180 degrees, because you cannot load the engine enough while running it on the ground. All you are doing is adding yet more condensation to the oil. If the oil doesn’t get hot enough to burn you, it doesn’t get hot enough to dry out. You cannot get the engine load (horsepower output) high enough to achieve this, on the ground. Wait for a break in the weather, then take it up for a few laps, despite the uncomfortable conditions.
Others with more experience with cold weather starts may have differing opinions. My cold weather experience is primarily travel-related (Yellowstone, Canada, etc.), and to a lesser extent from the periods when we are up in the South Carolina Piedmont Region in winter (18 degrees F there last Monday AM). Here are my thoughts:
1. It takes a lot of preheat to get an engine reasonably warm, especially outside (not hangered). If the plane is hangered, has cowl plugs, and has a blanket over the cowling, a small and cheap 1500 watt electric heater will keep it plenty warm most of the time, even when set on low heat. You can rig a little heater like this so that it isn’t right under the plane, by using metal flex duct from Lowes or Home Depot. If the plane is put away hot (dry oil), the little heater will prevent the temperature fluctuations that allow condensation to form in the crankcase. As long as the crankcase never goes below the dewpoint, it can’t collect moisture inside. I would use preheat any time it is available when the OAT is subfreezing. I would avoid starting the engine until it feels like it is at or above 40 degrees anywhere I touch it, except the far edges. This usually takes much more than 30 minutes, with the small heating equipment I have available. I seem to recall that Aviation Consumer or LPM did a good write-up on pre-heat equipment some time ago. One thing I took away from the article was that it takes a lot of heat, and quite a while, to get a valid pre-heat.
2. A small Coleman-type propane lantern puts out about 15,000 BTU, and the small radiant heaters put out the same or more. They can be very dangerous around airplanes if you aren’t very careful, and they must not be left unattended, but they can also do a great job of preheat out on a ramp (including inside the cabin after the engine is warm). I have watched steam come off my entire cabin area many a time, as it defrosted itself while I wiped heavy frost and light snow off the wings and tail, while my wife sat inside with a propane unit in her hands (grinning at me and my frozen hands, through the window). If you aren’t comfortable with the safety aspects of this, just don’t do it; don’t bother to fuss at me about it (!). I still believe in personal responsibility for my choices, and I won’t blame anyone else if they go awry. I think most pilots are like that, out of necessity (a history of self-preservation?).
3. 20W-50 oil should not be a severe problem at 17F, if preheated above 32F. That’s what I always put in before winter travel (rather than straight 50). I don’t put in thinner oil because I load the engine close to normal temps when traveling high in the mountains, despite the low OAT.
4. The engine has to be run at “high idle” (1,000-1,200) until the oil temp needle at least comes off the peg, at about 60 degrees. You should not try to cycle the prop until the oil is above 60 degrees. You should not use higher RPM, or try to take off, until the oil is above 85-90 degrees. Otherwise small passages (like in the lifters and rocker arms) may be starved for oil, due to slow flow at high viscosities.
5. If you cannot achieve these oil temps due to OAT conditions, you should make a plug for the oil cooler inlet. This is for use during ground warm-up prior to run-up and takeoff, unless you are in really extreme low temps. Just make darn sure it can’t be left in place during flight (like by having a red streamer that goes back into the storm window).
6. Do not block off all the cooling inlets to the cylinders, other than by use of factory-designed winterization kits that restrict (but don’t totally block) cylinder cooling air. Cylinders will rapidly develop severe hot spots if they do not have adequate air flow, even in very low OATs.
Hope this helps, and that others find it consistent with their experience.