Engine Preheat

Parked alongside my Lancair in a Toronto Buttonville hangar are two Cirri. Today I noticed that the SR22 was plugged in, and a sign in the cabin instructed the airport staff to plug the plane in when the aircraft is put into the hangar. So the engine heater would be on all the time when the plane is not flying. The SR20 was plugged in too.

The C300 has the same engine as the SR22 and my manual states:

“Do not leave an engine-mounted preheater system on for more than twenty four hours prior to flight. Continuous operation of engine-mounted preheater systems may result in aggressive corrosive attack internal to the engine.”

For this reason, I use a timer to turn the preheater on 5 hours before flight. It turns off again if I choose not to fly. I didn’t see any timer or similar device on the Cirri today and noted the engines were warm.

Am I missing something here, or should this practice of constantly heating the engine be reviewed?

Steve

lots of debate about that.
One way to look at it is that it’s not the heating, but the cooling, which gets you
into trouble. The moisture which is driven out during heating condenses elsewhere.
One way that might get you is if you have a sump heater only, you are then
continually heating the oil pan, water vapor is then going up and condensing on
the cool top end of the engine. That, people have suggested, may be bad.
If however you have a sump and bands system, the whole engine gets hot. There
really isn’t anywhere for the vapor to re-condense. So, the argument goes, you are
ok. When it’s running your motor is making a lot of steam, but you don’t worry about
that because the thing is hot all over. It may be a good idea to open the oil cap to
allow any moisture to get right out of the plane.
So. If you have a ‘whole engine’ preheater, it may be better to keep it on all the
time than let it cycle, which would promote a series of drive-out-moisture and then
let-it-recondense.

I personally have a bands and sump solution, a high-power one, and I use a
comforter over the engine compartment and open the oil filler cap. Once the heater
is on, it stays on until the plane is flown. Preheat for me is 12 hours minimum
especially in the cold weather we had this winter.

I think that having the heater go on, and then off again and not fly every now and
again … no problem at all. I wouldn’t myself have a solution which some people
have where the heater turns on and off with ambient air temperature, I want to avoid
heating and cooling cycles as much as I can.

I have one of those Reiff sump and cylinder band systems. If I remember correctly, it thermostatically cuts off at about 70 degrees. I think that the latter may be right because no matter how long I turn it on or what the temp is, the cht’s/oil temp rarely get above 75(even with a cowl blanket). Therefore, i would be surprised that any significant moisture in the crankcase/oil would be evaporated off with only an ambient crankcase temp of 75 from modest heating. Perhaps a bit simplistic, but Fe oxidation(rust) doesn’t need condensation of actual water droplets on metal to occur. Simple moisture in the air will do it and the warmer the temp the faster the chemical reaction, all other variables kept constant. Ergo, in theory, the least amount of rust would be with a recently flown hot crankcase that has blown off most excess crankcase moisture, and then a cold engine temp as long as possible to SLOW down the oxidation process. I don’t preheat except just long enough to get the temps up right before engine start. Perhaps as a corollary, throw a piece of metal in constant temp cold vs warm water,the warm water will rust it faster. Simplistic, but that is how I do it.

The discussion about condensation/cooling makes a lot of sense and would argue in favour of continuous preheating.

But are there factors other than moisture at play? Does a thin layer of protective oil tend to get thinner or disappear from internal surfaces that are hot? Are acids liberated from warmed oil to attack these surfaces? Oxidation requires oxygen, but not necessarily moisure.

I too have the Reiff Turbo XP system. The advantage is that an acceptable level of heat can be accomplished in 5 hours - over 100°F CHT on a -10°C day. I suspect (but don’t really know) that it may be too hot to safely use all the time.

One further question: Does anyone preheat when it’s warm outside? Sure the engine starts easily without it, but a hotter engine would have less of the steep temperature climb immediately after start - already being closer to operating temperature. If it’s the rapid temperature rise that contributes to post-start wear, then a warmer start might be better.

Right about now a PHD in chemistry, thermodynamics, metalurgy and mechanics would come in handy. My brain at least, is a bit rusty…

In reply to:


I have one of those Reiff sump and cylinder band systems. If I remember correctly, it thermostatically cuts off at about 70 degrees.


150 degrees. That’s what it says on the website and that’s what I was getting this winter after leaving it on for hours with a cowl blanket. The oil started in the green.

thanks for the info…I have a three years old model…mine for certain doesn’t get even close to 150 degrees.

In reply to:


thanks for the info…I have a three years old model…mine for certain doesn’t get even close to 150 degrees.


150F would be about right for the Turbo XP System he has. Since you are getting much less it sounds like you have the Standard System, which is half the wattage of the Turbo XP.

Also be aware that the oil temp gauge does not necessarily give a good indication of the oil temp achieved with the preheat system, since the sender is on the rear accessory case and is not actually in the oil sump. When I measure oil temp I drop a sensor down the dipstick tube into the oil so as to get an accurate reading.

In reply to:


I too have the Reiff Turbo XP system. The advantage is that an acceptable level of heat can be accomplished in 5 hours - over 100°F CHT on a -10°C day. I suspect (but don’t really know) that it may be too hot to safely use all the time.


I asked them about that, I have this one too, and they say it’s fine to leave it on. The way I think about it, 150F isn’t that hot for an engine. Would I intentionally leave the pre-heat on for weeks at a time, probably not but I have left it on for a couple of days with no qualms at all. The engine was a nice toasty 150F, and everything else was warm too under the cowl, which is a good thing IMHO.

I know people who preheat up to about 50F ambient. I do too if I get the time, arrive at the hangar, throw the blanket over the cowl, switch on and go pump the tires or whatever, even that 30 minutes adds a few welcome degrees.

I had to scrape some rust off my brain and some dust off my college chem books to remember the iron oxidation process.
For all practical purposes, water IS necessary since it is the electrolyte (catalyst) that gets the rust process going. Simply, water contacts iron(directly or vapor) and is a good electrolyte. It combines with carbon dioxide to form a weak acid which is a better electrolyte. Carbonic acid chews a wee bit of the iron away into solution. Water breaks apart into free anion/cations…hydrogen/oxygen. Dissolved free oxygen and iron combine and form iron oxide and voila’ you get the rusty stuff. Extrapolate this to the crankcase of any engine. There may not be much carbon dioxide in a crankcase, but combustion acids are probably more potent and are certainly around. You can only “cook” so much moisture out of a crankcase with acids, since by definition an acid unless extremely pure and kept in a chemically anhydrous environment will have water in it(e.g. look at battery acid solution…only a modest strength sulfuric acid…has plenty of water in it). It is hard to separate water out of most acids to make that acid “pure”. Anything that makes that “electrolyte” stronger(more acid, salt) will enhance rust formation. Temperature definitely has an influence on this basic electrochemical reaction. The higher the temp the faster the reaction. Who hasn’t noticed the anecdotal experience of watching something rust right before your eyes in a salt laden, moist, warm environment…as opposed to a dry, no salt, colder than a well digger’s a… environment? So, there may be a fine line between preheating just enough to warm the engine so that cyhlinders aren’t scored and the oil is flowing nicely…and perhaps warming that electrochemical reaction a bit too lengthy a time unless the temp is really high to substantially really cook off the moisture. To decrease chances of rust it makes sense to me to: use LOP as much as possible to lessen blowbly and crankcase acid buildup, change oil frequently to keep the amt of acid in the crankcase to a minimum, heat off as much water from the crankcase with flying and don’t heat unnecessarily long unless you can really cook it. You’d think that with all the high tech knowledge and gadgets out there that someone would have come to some difinitive conclusions on all this by now. You can borescope cylinders…anyway of borescoping the crankcase through the oil filler neck to actually look for rust???(probably stupid question). Thanks for listening, just some of my musings.

Witrakw wins the scientific post of the week award. Makes sense too.

So here’s the practice until more science comes to light: Pre-heat to as high a temperature as possible, 5 or so hours before start (depending on your heating system’s power), then go. After flight, lift the oil filer cap 1/2 inch to allow excess moisture to vent away as engine cools. When not planning to fly, let the engine sit cold. Fly often. Some pre-heating in warmer temperatures is a good idea too.

As a postscript, I think the Buttonville Cirri owners are lurking. Those planes are not plugged in any more and there’s no longer a sign telling line staff to keep then plugged in.

Steve

The Buttonville Cirrus C-FFFW is still plugged in.

The other one FJSH is probably not plugged in because the sign is not in the window.

You might notice that most of the hangared aircraft at YKZ are plugged in all winter.

There are MANY theories on pre-heating times and strategies. The one about heating the water out of the oil and it condensing on the rest of the engine is illogical on its face as it cannot condense on surfaces that are 50 F above ambient.

Witrakw has posted the most compelling argument I’ve heard so far and I’ve been digesting it for a day now.

While I’m not one given to accepting anecdotal evidence as gospel, I have always equipped my planes with Tanis pre-heaters (and now Reiff) and left them plugged in all winter. I also am religious about oil analysis. Never had any high FE readings so don’t really know just how rusty or not things have gotten over the years and planes. Never had an early overhaul nor top, nor failed cam. I do strongly believe in preheat that I simply won’t start any aircraft engine withuot real preheat (to the core) on a below freezing day.

Your procedure of venting the cap, then closing and putting the engine into cold storage, then preheating for five hours before starting is a good one though. Won’t hurt anything and there is no downside if you can manage to plug it in ahead of time. However, I’d think that once plugged in, you would want to leave it plugged in until you fly next rather than unplugging if the proposed flight gets scrubbed. The on-off will cause the condensation that we worry about.

In reply to:


After flight, lift the oil filer cap 1/2 inch to allow excess moisture to vent away as engine cools.


Steve: I do that out here in California, where it is down to 53F tonight and everybody is complaining about the cold.

If the plane is hangared, I don’t see any reason why you should not leave the oil cap 1/2" up with the oil access door with one latch closed as a reminder that the cap is up. Nothing is going to get in there, and a path for air to flow through the engine is going to keep it dryer. In fact, I saw a device made out of a plastic bottle and and air pump with the bottle filled with desiccant gel to pump dry air through the engine. I don’t think there is a great advantage in using such a device since the natural heat of the engine is going to remove most of the moisture.

In reply to:


If the plane is hangared, I don’t see any reason why you should not leave the oil cap 1/2" up


mostly neither do I. However, remember to tell people about it. I got a call from my avionics guy after he reinstalled my PFD and the conversation went a little like this


AG: "Which idiot left your oil filler cap off?"
Me: "I did, I always leave it off, lets the moisture out"
AG: "Well it's a good thing I checked before I started the engine"

I then told him about the rusting filler caps/necks.

Not that I’m sure what happens if you start an engine with that cap off. Anyone care to speculate?

Oh note of course that I had all the sets of keys for the plane in my bag, so I didn’t really expect anyone to be starting it anyway, but it seems cirri are as easy to start with a paperclip as any other plane in the fleet.

In reply to:


Not that I’m sure what happens if you start an engine with that cap off. Anyone care to speculate?


Not much. At least if you notice it right away, stop the engine, and put the cap back on. Doesn’t appear that any oil is lost or thrown out or anything. AMHIK. [:)]

Anyone care to speculate on what happens if you actually bring the engine to high RPM or fly with the oil cap off? Luckily I can say that I don’t know what happens in that case!

I typically take off the cap after shutdown and always put it back on b4 flight- except once!

At cruise altitude I “felt” I hadn’t put it back on so I pulled power and dead sticked to an airport in range and landed fine.

Cap was off after all- no leakage though and there were not any readings to indicatede any troubles.

Still recommend putting it on b4 flight!!!

In reply to:


Anyone care to speculate on what happens if you actually bring the engine to high RPM or fly with the oil cap off?


Someone at the G2 showing in LA told me that they had indeed done this - flown with the cap off. Apparently some oil, but not a lot, was blown out due to increased crankcase pressure but the engine compartment was clean, so it apparently didn’t splash out through the filler. It was not a catastrophic event by any means.

However, I’m not planning on testing this!

I don’t have to speculate- unfortunately I know. My Skyhawk lost about 3 - qts of oil on a two 1/2 hour flight.

John
N468JP SR20 #1261

In reply to:


I don’t have to speculate- unfortunately I know. My Skyhawk lost about 3 - qts of oil on a two 1/2 hour flight.
John
N468JP SR20 #1261


that’s not so bad, I mean it’s not GREAT but not the nightmare oil-pumping-all-over-the-place scenario my avionics guy seemed to be concerned about.

In reply to:


I don’t have to speculate- unfortunately I know. My Skyhawk lost about 3 - qts of oil on a two 1/2 hour flight.


In reply to:


that’s not so bad, I mean it’s not GREAT but not the nightmare oil-pumping-all-over-the-place scenario my avionics guy seemed to be concerned about.


Roland,
Hummm… was your avionics guy worried about the avionics, or your safety?

My pre-Cirrus airplane was a 182RG that I owned with a couple of partners. One day one of them added a quart of oil, and forgot one important step. He became “artificially IMC” on takeoff. It was a struggle for him to see enough to land the airplane. It was worse than John’s experience – more than half the oil was on the outside of the airplane, and the engine compartment was a BIG mess.

  • M.

In reply to:


Hummm… was your avionics guy worried about the avionics, or your safety?


He was concerned that had he not checked before starting the engine there
would have been a lot of oil jumping out of the engine and perhaps some
major damage.

Honestly I wouldn’t expect any A&P or other aircraft maintenance guy to start
a plane without checking the oil level, even if they were just doing it as part of
a ground check for the avionics. Nevertheless I’m pleased this particular one
did, he just never heard of anyone leaving the oil filler cap off before.