Amazing, isnt it? Aircraft owners accept the notion that theyll have to pump a million BTUs into an engine just to go flying during the winter yet theyre steamed if the car heater isnt blasting 100-degree heat by the end of the driveway. Ironic or not, we seem to be stuck with the need to warm up cold engines before starting. Over the years, all sorts of schemes – some bordering on the lunatic – have been devised to tend to the heating chore. But the mainstream commercial products havent changed much.
These devices heat the oil or the cylinders-or both-either with electric heating elements or the fiery blast of a propane heater. Setting aside the propane, two philosophical schools have emerged: Get the oil very toasty with a hot pad on the sump or heat the cylinders with electric probes or bands or perhaps some combination of the two. All of these methods work, of course. But which is best? Which delivers the biggest BTU blast for the buck? We set out to find out this winter, as part of an on going long-term test of pre-heat products.
Whats Out There
Of late, the pre-heater market has been ruled by two companies, in promotional profile if not sales. Minnesota-based Tanis Aircraft Services is well known for its line of premium electric pre-heat products while Reiff Corp. has made inroads with less expensive sump-type heaters and heated bands for the cylinders.
Two other companies, McFarlane Aviation and Symtec, Inc. make a line of pad heaters. Kennon Aircraft Covers-primarily a cover maker-also sells a heated cover for pre-heat purposes. And, of course, Flame Engineering is still out there with the venerable Red Dragon propane heater, a device pilots have come to both swear by and swear at.
The point of pre-heating, of course, is to get the engine to a temperature that will both allow it to start without excessive cranking and continue to run without doing damage to cylinders and bearings with fits shrunken by cold. Lycoming and Continental both recommend pre-heat at temperatures 20 degrees F or colder. Nonetheless, many owners use the more conservative 30 degrees, reasoning that it wont hurt.
How We Tested
To measure pre-heat temperature rise, we installed a series of thermocouples on the cylinders and crankcase and a temperature probe in the oil sump.
We allowed each of our subject engines to cold soak in an unheated hangar and, with the exception of the Red Dragon test, all of our trials were done in a hangar, but without an insulated engine cover. We did, however, use cowl plugs.
An assumption: We were able to measure only the surface temperature of the engine, not the insides of the cylinders or the crankcase and bearing surfaces. We think its reasonable to assume, however, that surface temperature is a good indicator of interior temperatures as well, especially with sump heaters, which heat from the inside out.
Sump Heat Only
For our first test, we heated a cold engine-the IO-360 in our Mooney-with only a sump pad, the Safe-Heet model from McFarlane Aviation. Weve had the pad on this engine for five years.
From a cold start in 25-degree weather, we noted an oil temperature of 70 degrees after just an hour or 45 degrees above ambient. Not bad. However, not much of this heat found its way to the cylinders. Our instruments indicated the jugs were uniformly between 30 and 32 degrees, for a rise of 5 to 7 degrees above ambient. Not much help but probably okay for starting.
At two hours, the oil was 58 degrees above ambient (85 degrees F) and the cylinders were about 7 to 9 degrees warmer than the outside temperature. Left on overnight, the sump heater finally drove the oil temperature to 101 degrees and the cylinders to near 45 degrees or about 10 to 15 degrees above ambient.
In our experience, thats about as good as it gets for a pad-type heater used without a cover. Conclusion: If 30 degrees is your benchmark starting temperature, a pad alone will hack it down to 15 degrees or so. Invest in an insulated cover and youll see temperature rises of 40 degrees above ambient, meaning the engine will be acceptably well heated down to perhaps -10 degrees F. The size of the pad obviously matters. The Safe-Heet consumes about 300 watts. When we last tested pads, a somewhat smaller 160-watt Reiff HotPadd wasnt quite as effective. Pad size is dictated by sump design and by the STC covering the engine type. If theres a choice, we recommend getting the pad with the highest possible wattage.
Worth noting here is another pad-type heater we tested, the Symtec pad heater. Unlike the flexible pad heaters, the Symtec has heating cartridges inside a robust aluminum housing thats bonded to the oil pan. And unlike the silicone pads, it can be pried off and re-used. On a six-cylinder Continental engine, the Symtec performed much like the Safe-Heet pad, with near identical temperatures at two hours-40-degree oil temperature rise and 5 to 7 degree engine temperature rise. Overnight, the Symtec raised engine temperatures by 10 to 15 degrees, making it suitable for moderately cold temperatures without a cover.
Pad and Bands
Five years ago, Reiff introduced cylinder base heaters, which are nothing but large stainless hose clamps bonded with the same silicone material used in the sump heaters. Reiff recommends that HotBandds be used in concert with a sump heater but if only one is used, the company recommends the bands alone. (We tested only the sump alone, not the bands alone.)
When combined with a sump heater-again, the Safe-Heet on our Mooney, the HotBandds delivered impressive performance if given time to work. After a night of cold soaking into the mid-teens, the pad and band combination brought the oil to 45 degrees above ambient in an hour, with the cylinder surfaces between 30 and 40 degrees-a 10-degree rise-and well within the safe starting range.
At six hours, the oil was 113 degrees and the engine cylinders, case and nosepiece between 70 and 80 degrees for a uniform 50-degree temperature rise over ambient. The engine smelled and felt warm to the touch. In fact, we used a hot start procedure to fire it up and noted that the oil temperature was immediately off the lower peg.
With this kind of performance, we estimate the HotBandd/pad combination is suitable for temperatures well below zero-say minus 20, or even colder with an insulated cover. On the ramp without a cover but with cowl plugs, expect to see slightly lower temperature rise. Still, this system is still suitable for outside use in moderately cold weather down to about zero or so.
For practical outside performance, an insulated cover and prop cover are musts in extreme cold. If youre starting from stone cold, we think two to three hours of heating is sufficient. Otherwise, leave it on around the clock and youre ready to launch upon arriving at the airport.
Tanis Heated Plugs
As the dominant player in aircraft pre-heat systems, Tanis is well-respected for its carefully engineered electric pre-heat systems, the product of years of improvement in the cold country of Minnesota, where Tanis is based.
Although it offers some pad-type heaters in certain applications, the Tanis system consists primarily of heated plugs which screw into the holes machined into cylinders for CHT probes or heated head bolts. Each plug provides 50 to 100 watts of power, depending on the application. (The higher wattage system is called the Super System.)
As noted in the sidebar, Tanis believes that applying gentle heat to the cylinder head rather than heating the oil is the best way to pre-heat a cold engine. That said, Tanis still offers oil heating options, including a screw in plug in oil screen or on the accessory case or a silicone pad type heater. In very cold climates, we think the oil heat option is a must.
We tested the Tanis system on a Cessna 172 parked in a cold, unheated hangar. It was equipped with standard Tanis heaters for a Lycoming four-cylinder, including a sump heater. We found the Tanis set-up did a faster job of heating the overall engine than the Reiff did.
It did less well at warming the oil, by dint of applying less power. In 30-degree weather, the Tanis warmed the cylinders to 30 degrees above ambient in an hour, with the oil temperature rise at 19 degrees. By the second hour, the engine was 45 degrees warmer and the oil 26 degrees warmer. Again, thats easily within safe starting range. Overnight, the oil warmed to 48 degrees above ambient while the engine averaged 60 degrees. Conclusion: The Tanis system delivers a warmer engine than the Reiff, but cooler oil. But after six hours, the performance difference between them is nil, in our estimation.
As a control, we also heated an engine using the old standby Red Dragon propane heater. After a 20-minute blast directed through the nose inlets, the 172s cylinders were hot enough to fry an egg on, at an average of 107 degrees or a toasty 70-degrees above ambient. Before sampling, we let the engine surface temperatures stabilize for five minutes.
The onslaught of heat didnt do much for the oil, however. It rose only 5 degrees above ambient, requiring a longer ground-run time than engines heated with the Tanis or Reiff products.
Our tests clearly reveal that even without a cover, a good sump heater is capable of raising the entire engine temperature-including the cylinder heads-10 to 15 degrees above ambient. On a 20-degree day, thats all the heat you may need. Throw an insulated cover over the cowl, and a sump heater is capable of a 40-degree total-engine temperature rise, warming the cylinders to a toasty 60 degrees on a 20-degree day and probably holding the entire engine at 40 degrees over a zero-degree night.
With the Safe-Heet pad, we saw sump oil temperatures as high as 113 degrees which, according to oil experts weve interviewed in the past, is not nearly hot enough to carmelize the oil, as Tanis promotions have claimed, nor is likely to be harmful in any way. Its also not quite warm enough to drive out any water entrained in the oil; only flying the airplane with a sufficiently high oil temperature will do that.
The nut of the argument is this: If that warm pool of oil in the sump cant conduct enough heat up into the cylinders to warm them to what you accept as the minimum cold-start temperature-say 30 degrees F-no amount of warm oil will address the piston-to-cylinder fit issue.
Our tests reveal that the warm oil does conduct heat into the rest of the engine, just not as effectively as the Tanis cylinder heaters or the Reiff bands.
As for bands versus cylinder head plugs, our view is that Tanis overstates the case in saying its best to heat the head. If the entire cylinder is between 40 and 70 degrees absolute-and our tests indicate this is the case-what difference does it make where the heat is coming from? If youd start your engine on a 40-degree day, who cares if the heads are 10 or 20 degrees warmer?
One aspect of Tanis claims do have merit, in our view. The Tanis system appears to be somewhat quicker at heating up the engine than the Reiff bands or band/pad combinations. So if youre in a hurry and cant let the heater soak the engine, it makes sense to apply heat from the top down. But once a sump heater or sump-and-band system catches up, which it does in two or three hours, the engine is uniformly warm enough to start safely. Heat is heat; it doesnt matter where it came from.
Frankly, we think all of these systems are worthy products. The Red Dragon, however, is our last choice for an owner. Given the hassle of fueling it and the time it requires to get the job done-some 20 minutes in cold weather-its quite a bother. Plus, you cant take it along in the airplane while you can carry a long extension cord for an onboard electric heater. Obviously, the Dragon is a good choice for FBOs who are expected to deliver heat on demand.
In moderate climates-say where the night time lows get into the 20s or mid-teens, a sump heater is a compelling choice. Plug it in two or three hours before takeoff, and the engine should be warm enough to start safely. Leaving it plugged in all winter is an option but one Reiff doesnt recommend due to concerns about condensation. We cant advise one way or another on that issue; were doing some condensation tests of our own. Well let you know.
As for the Tanis cylinder system, it deserves the reputation it enjoys. Its well-made, effective and remains a favored choice in cold country, where pre-heat from an electrically robust system is an absolute must. Mechanics weve spoken to speak highly of Tanis design and support and the system is said to add resale value to the airplane. It appears to initially heat a little faster than the Reiff band/pad set-up, but not enough to make a material difference, in our view.
In moderately cold climes, we think the Tanis is more than you need, especially if budget is limited. Reiff bands, combined with a sump heater, delivered a 50-degree heat rise, versus 70-degree for the Tanis. We think 50 degrees is plenty, 70 degrees a luxury except in Arctic conditions.
Also With This Article
Click here to view Pre-Heat Systems: Specs and Prices.
Click here to view the Checklist.
Click here to view Addresses.
Click here to view Pre-Heater Comparisons.
Click here to view “What A Cover Does.”
Click here to view “Is Pre-Heat Oversold? Probably.”