In a recent column, my friend Paul McBride, shared some important information on pre-heating an aircraft engine (Should I preheat the oil in my aircraft engine during the winter?)
There was quite a bit of discussion on the column, so I thought I would give some background on several of the recommendations.
For example, one method discussed was draining the oil and keeping it in a warm area overnight and then adding it back to the engine just prior to starting.
This was standard operating procedure for large radial engines during World War II. Many of the airfields in northern Europe and Alaska did not have pre-heating facilities, so they would drain the oil after the last flight of the day. They would mix the oil with a prescribed amount of avgas and place it in a heated area until the next flight.
It’s important to remember that this was before the introduction of Aeroshell Oil W in 1958, so all they had was straight mineral oil qualified against the Mil L 6082 specification.
In fact, that specification used to have a minimum requirement for a diluted viscosity that measured the thickness of the oil with a specified amount of avgas diluent.
Just prior to the next flight the crew would fill the crankcase with the warm diluted oil. Then they would pull the prop through to force the oil to all areas and to check against a hydro-locked cylinder. They would then fire the engine up and let it warm up. This helped to evaporate most of the fuel in the oil prior to takeoff.
This procedure could be used in an opposed engine in an emergency.
However, there is another problem with opposed engines in very cold weather.
In an opposed engine, there is an aluminum crankcase that holds plane bearings against an iron crankcase. Unfortunately, the aluminum crankcase has a higher coefficient of thermal expansion than the iron crankshaft.
Since bearing clearances are set at room temperature, this means that the colder the ambient temperature, the lower the bearing clearance and the less room for the lubricant to reach the critical bearings to crankshaft interface.
The bottom line is pre-heating your engine is not just to thin out the oil, but also to normalize the clearances for the engine itself.
This brings us to the subject of electrical pre-heaters and the practice of leaving them plugged in all of the time.
Years ago, Tanis Aircraft Products developed a system for preheating an aircraft engine electrically.
The system had a small heating pad attached to the oil pan and then individual heaters for each cylinder, either in the thermal couple spot or attached to the cylinder with a steel band. This worked well because it not only heated the oil, but also the engine assembly.
Tanis then added cowl plugs and a thermal blanket to keep the entire engine assembly warm and, in some cases, they even placed a small vent fan on the oil breather to draw out the moisture.
With this system company officials claim that it is OK to plug the system in and let it operate continuously.
Unfortunately, there are several other companies that went the cheap route and just attached a larger heating pad to the bottom of the crank case.
This caused two problems.
First the heating pad was large enough that it raised the contact spot on the inside of the pan to over 300°F in some applications. This caused coking in that area and could even break down the oil. This coke could wash off and plug oil passages or the pickup screen, especially if the system is left on continually.
The other real problem with a single spot heater is that it will vaporize any moisture in the oil, which will rise until it meets the cold camshaft at the top of your Lycoming engine. This will greatly increase the moisture and subsequent rusting of the cam.
I have seen numerous ruined camshafts from engines that used single spot heaters and left them on continuously.
So, what are the best recommendations for pre-heating an aircraft engine?
- Do not drain the oil and just heat it prior to startup unless it is an emergency.
- Pre-heat your engine, not just the oil, to ensure proper clearances.
- Do not leave your electronic heater going all the time if you only have just a heater on the oil pan.
- And always check your oil temperature during cruise flight to ensure that it is in the 180°F range so that you boil off any moisture in the oil during flight.
Good article Ben. But actually, the stainless steel band heating element around the cylinder is not a product of Tanis. It’s ours. I have a couple patents on it, and have been producing it for over 30 yrs, since 1992. It’s installed on about 20,000 aircraft.
Reiff Preheat Systems
http://www.ReiffPreheat.com
Does the molecular structure of oil change over time-?
That’s a question for Ben V., the oil expert.!!
Oil sump pad heater on IO-520BB. Reiff Thermostat controller. When temps are predicted to go below 40 deg F, after flight our process:
1) Open oil dipstick spout, pull up dipstick about 1 inch, allows steam to escape from engine. Dip Stick is red flagged to close before flight (easy check because we always check oil level before each flight so never going to be forgotten, red flag is there just in case someone gets complacent).
2) Plug in heater via Reiff Thermostat, with thermostat set to 70 deg F, thermostat is placed over right hand bank of cylinders through cowl opening.
3) Cover cowling with two thick blankets.
4) Engine cools down to 70 deg F, and oil sump heater pad keeps the entire engine at 70 deg F via conduction of heat into metal and oil.
5) Next flight, without fail, via digital engine monitor, oil temp, CHT’s and EGT’s all read 70 deg F. Ergo, the entire engine is at 70 deg F. Engine start is as easy as on a summer day.
A question unrelated to anything: was there ever a time between overhauls on the radios such as the 2800 3350? I flew with East for Eastern Airlines, and Days when they had the round engines, it seemed like they were constantly over-rebuilding both the 3350s and 2800s, and they were continually breaking down. They have never heard of one going to an overhaul time.
Jim Hamilton
North Pole, AK (Fairbanks “suburb”): Parked my 53 C-170B outside year round, down to -40F +/-. Stored battery at home between flights; used a 12-volt Red Dragon propane heater, ducted hot air up from underneath into Bruce’s Custom Covered cowling. Took a while, some days, but worked like a champ.
I think the data published by Tanis is pretty convincing that it is a superior system for preheating aircraft engines, as Ben points out here. Of course, this system requires a source of electricity, which may not be readily available out on a frozen lake or remote cabin. For most of us, though, have an electrical outlet into which the Tanis can be plugged. There is a joke about pilots…Q: What is the cheapest part of the airplane? A: The pilot. It amazes me how many pilots will make compromises that are likely to contribute to poor performance or problems with their aircraft.
Using a pilot produced parts for warming the engine is being frugal, as long as they meet the required function.
Our engines are air cooled, so I see no problem using air heating.!!!
Since a heated air unit is not attached to the aircraft , there are no STCs or 337s required.
I put an oil heater under the cowling near the exhaust pipe (and opening). I have plugs in the front intakes as well. It doesn’t get very hot, but keeps it from freezing. I leave it on continuously. My hangar is not heated and drafty and the temperature gets in the low 20’s at night sometimes. I live in a dry area with low humidity. Could this be damaging to my engine?
I actually heated a quart of oil in a microwave once and it worked!
Get a Switchbox for heater control, it activates up to two outlets. A Reiff system/oil pan arrangement will function just fine, just give them enough time. An O-470 is 400+ lbs. of metal and nothing happens quickly, for example. Fan heater also works. Preheat if hangar temp < 40F. Helpful info in Teledyne Engine Manual or Lycoming equivalent. This is what we Montanans/Alaskans/Minnesotans must do for a large part of the year. Works well and preserves your expensive engine.
Actually, back in the day of radial engines operating in the Arctic, pilots did indeed drain the engine oil to a bucket, then took the bucket into the roadhouse, and placed it near the wood stove to keep it nice and hot. No need to add gasoline to dilute. Then, when prepping for takeoff the next day, they placed a tent over the engine, and lit fire pots inside the tent. An hour or two later, the engine was nice and warm, the pilot poured the warm oil into the oil tank, fired up, and went flying.
Oil dilution was a purely last ditch effort.
But NOTHING, beats the warm air from a BT-400-10 on a 20 below sub arctic morning!
Better known as a Herman Nelson. And you can heat the Cabin at the same time.
I’ve been using a Reiff standard heating system, which includes a sump heater, band heaters around each cylinder, and a small pad heater on the oil cooler. I think I’ve had it for about 19 years. My engine is a custom built O-360 Lycoming. I have tried remote switches, but each has failed. So for the last 5 or so years, I just leave the system plugged in during the winter. The entire engine is slightly warm to the touch although my hangar is unheated. I can’t imagine that my engine is suffering in any way, any more than it does just sitting during the summertime. And it starts readily, just as it does in the summer.
Best explanation I’ve heard so far regarding how the moisture rising from a single spot heater contributes to cam corrosion.
I’m using a fan-forced electric heater that circulates warm air through the engine compartment, and I usually turn it on remotely an hour or two before starting. I get the impression that it might not be have enough time to warm up the oil and the deeper components of the engine, so I’m curious if maybe switching it on the night before would be a better option, but I’m not sure about the possibility of that causing moisture-related corrosion. Thoughts?
I use a similar 1,500 watt small electric heater on my Cessna. I use 4 inch aluminum ducts to blow the hot air up the lower cowl opening, avoiding blowing it on the carburetor. I put a sleeping bag over the upper cowl, also covering the inlets with it.
I use a timer and run it for 4 hours before my arrival.
The wx in Northern California is not very cold, about 30 degrees overnight.
The heater gets the oil and cylinders to about 90 degrees F, so my engine start is easy and I only have to do a short run up/ mag check…no waiting for the oil temp to get to 100 degF.!
Sounds like we have about the same setup. Thanks!
Yup, and it only cost me less than $100 in parts.!!
[ I already had the sleeping bag.!]