Keeping It Warm: Common Sense, Engine Heat

I admit to watching in amusement last winter when a student and a young instructor attempted an engine start in the flight school’s Piper Arrow. The sun-splashed thermometer outside my office window showed 16 degrees F. As the frozen Lycoming IO-360 cranked … and cranked … and cranked, I guessed the battery would last three more minutes of this foolishness. It actually lasted six more minutes before flattening, followed by the kids jumping out and tying the ropes in defeat. I was relieved when the engine didn’t light off. For the sake of the owner paying the maintenance bills on this relic, someone has got to teach these pilots about the importance of respecting a frozen engine. We’re all guilty of starting them when we really shouldn’t, so there’s a lesson here for all of us.

The Stakes Could Be High

I mentioned the calamity to the resident mechanic downstairs and he admitted that there’s plenty of theory but little proof about the damage done during cold starts—including main bearing and crankshaft damage—due to lack of lubrication. There’s also the potential for piston-to-cylinder clearance issues, resulting in metal-to-metal scoring of the piston and cylinder walls when the pistons tend to warm more rapidly than the cylinders after engine start. Still, we all know it’s unrealistic to preheat every time you start a cold engine, especially between seasons when there could be vast temperature swings and while on an unexpected overnight trip with no hangar space or power to plug in to. Arguably, multigrade oils (and healthy batteries or a GPU start) will help start a cold engine in the 20-degree range or even colder because the viscosity doesn’t fluctuate as much with temperature swings. But they are not like auto multigrades where you can start your automotive engine at 5 degrees F with no preheat and no consequences, since the problem is one of dissimilar metal parts that heat at different rates.

At cold temperatures you can literally have aluminum-to-steel contact. No oil will protect against that situation, but multigrades do make starting easier. Regardless, my rule of thumb is to not start a piston engine without a preheat below 25 degrees F—which is pretty conservative compared to manufacturers’ guidance. Lycoming (through its Service Instruction 1505) says 10 F for most engines and 20 F for 76 Series engines. It summarizes that in extremely low temperatures, oil congeals, battery capacity is lowered, and the starter can be overworked. “Improper cold weather starting can result in abnormal engine wear, reduced performance, shortened time between overhauls, or failure for the engine to operate properly,” it says.

Can’t Say They Don’t Warn Us

Continental (in its Service Information Letter 03-1 smartly reposted on the Reiff engine heater website) uses the 20 degree F rule for all engines. It’s expensive language, warning in part that superficial application of preheat to a cold-soaked engine can cause damage to the engine. An inadequate application of preheat may warm the engine enough to permit starting but will not de-congeal oil in the sump, lines, cooler and filter. Congealed oil in these areas will require considerable preheat. It goes on to say that the engine may start and appear to run satisfactorily, but can be damaged from lack of lubrication due to the congealed oil blocking proper oil flow through the engine. The amount of damage will vary and may not become evident for many hours. However, the engine may be severely damaged and may fail shortly following application of high power. I still think 25 degrees F is a much better lower limit for most engines, and considerably higher for my Rotax.

Continental brings up a point that we don’t always consider. Specifically, that it is not just the temperature but the engine’s short exposure to it, and more significantly, consider the wind chill, not just the ambient temperature, if you are outside of a hangar. The added wind chill will pull heat from a warm, just-shut-down engine much faster. Sure, it can’t make the temperature any colder than what the air temperature is, but it can get it to that cold temperature much faster in the wind. Use the simple practice of sticking the inlet plugs in immediately after shutdown on quick turns.

As another example for longer-term parking, the outside temperature may be 15 degrees F when you plan to start first thing in the morning when the sun is up, but the nighttime temps may have been much colder. Based in cold New England, it’s always surprising to me how slowly the engine warms back up to a warmer ambient temperature after a cold soaking all night. I hate winter—a lot.

Common Hardware and Creativity

In my view, you can get by with a basic oil heating system unless it’s colder than 20 degrees F. Several years ago, I did some field testing and found that an oil sump heater—when used with a cowl cover—sufficiently heats the oil enough for a start, with the added benefit of radiant heat to the upper engine, cylinders, crankcase and other components. Moreover, a light bulb or two carefully placed in an engine bay can help significantly if you keep it on long enough (an overnight in a hangar) before starting in the 20- to 30-degree F range. Again, an engine cowl cover pad will improve the heat effect of the light bulbs. But not everyone agrees that heating parts of the engine is the right approach.

In my field trials, I evaluated the Tanis Aircraft Heater system on a Lycoming IO-360 engine on a Seneca that had a JP Instruments engine monitor and the rocker bolt probes. The Tanis offered the most rapid heating of all the systems, raising the temperature nearly 40 degrees F above ambient in the first hour. The Tanis system uses heater elements that thread into cylinder CHT ports. The system also includes a single oil sump pad. System wattage varies, from 240 watts for four-cylinder engines to 720 watts for larger six-cylinder engines.

Tanis says its system won’t result in corrosion because it keeps all of the metal parts that are above the oil level above the temperature dew point, and as a result, condensation can’t build on these parts. But, it warns that Tanis systems must not be cycled on and off (no timers and thermostats) as this could create moisture. Moreover, over the years, Tanis hasn’t sold or endorsed systems that heat only the oil. As a result, all Tanis systems include an oil sump heating element, cylinder heating elements plus wiring harness and installation hardware. It does offer ancillary preheat products—including insulated blankets, to make preheating more efficient. I think blankets like the one in the main image are worth the money. Aero Covers has an insulated cowl blanket that starts at $510. Aircraft Spruce has a variety of options, too.

Another engine in the field testing was Continental IO-520 that had an E-Z Heat pad installed on the oil sump. After an overnight in an unheated hangar where the temperature hovered around 28 degrees F, the oil was heated to nearly 100 degrees F, significantly above ambient. Better yet, the heat radiated to the cylinders, increasing the temperature to 66 degrees F—again, safely well above ambient.

The popular Reiff HotBand heating system heats each cylinder from the outside. These HotBands are stainless steel clamps with heating elements bonded to them, and the bands install around the cylinder base, between the deck and the first cooling fin. The oil is heated with the thermostat-controlled HotStrip, which is included as part of the company’s complete heating system. Reiff has generally advocated a DIY install, and it really isn’t complicated work, though it does require drilling a hole through the aft engine baffling and installing a grommet. The grommet feeds the plug-in assembly back into the non-pressurized side of the baffling. The HotBands are plugged into a supplied wiring harness and secured with Adel clamps to the case bolts.

External Heat Blast

And what about using a propane-fired purpose-built heater? It can be a marginally acceptable alternative to engine and oil sump heaters, but for the short amount of time an FBO might use it on a customer’s plane, it probably doesn’t heat the crankshaft enough to make a big difference in eliminating potential cold-start damage. Back to the Lycoming Service Instructions, it advises to apply hot air directly to the oil sump, external oil lines, cylinders, air intake, oil cooler and oil filter in five- to 10-minute intervals. Between intervals, it says to feel the engine to be sure that it is retaining warmth and check to be sure that there is no damaging heat buildup. During the last five minutes, direct heat to the top of the engine. I wonder how many line service workers go that far.

Still, there are some external portable heaters—electric and propane—that can be effective. The short list includes models from AeroTherm Heaters, Flame Engineering and Aircraft Heaters. We’ll round them up and put them to the test this winter for a field report here in Smart Aviator. In the meantime, if there’s a system that works for you, we want to hear about it.