The Art of the Airplane Warmup

Cold temperatures cause oil to congeal and sap battery strength, which can lead to overworking and potential damage of the starter and engine. For this reason, many aircraft manuals,  pilot's operating handbooks (POHs), and engine manufacturers advocate specialized cold weather starting techniques, which include preheating the engine and making sure the aircraft is using temperature appropriate oil.

The proper oil is the first part of the equation. Some flight schools notify their instructors and renters when the school has made the seasonal oil switch, and go so far as to put the out-of-season oil under lock and key. 

Oil thins out when heated, thickens up when cold, so the proper weight is important. The second part of the equation is making sure the oil has the viscosity to move out of the sump and up into the engine when the starter is engaged. If the oil is too thick to move, engine damage can occur. 

Engine manufacturer Lycoming offers written guidance on cold weather starts that includes the warning: "Improper cold weather starting can result in abnormal engine wear, reduced performance, shortened time between overhauls, or failure for the engine to operate properly.

The use of preheat will facilitate starting during cold weather, and is required when the engine has been allowed to drop to temperatures below [plus-]10 degrees Fahrenheit /[minus]-12 degrees Celsius ([plus-]20 degrees F /[minus]-6 degrees C for [minus]-76 series engine models)."

That's good advice, according to JD Kuti, owner of Pinnacle Aircraft Engines in Silverhill, Alabama. Kuti, a powerplant mechanic, has 15 years of experience working on piston engines and has seen the damage done by improper cold weather starts.

"As the engine sits the oil congeals, it's always settling in the sump," Kuti said. "On cold days (40 degrees F and below) the oil is very thick and takes longer to get into the whole system causing wear."

Kuti said mechanics sometimes see engines where the crankshaft has been pulled forward during the engine start when there is no oil in the area. "It can cause the engine to start making metal," he said.

A lack of oil isn't the only thing that can damage an aircraft engine. The contraction of the dissimilar metals used in engine construction can cause problems.

The crankshaft, for example, along with connecting rods and cylinder barrels are made from steel or chrome for strength while the pistons along with the crankcase and cylinder heads are made from aluminum to save weight. Repeated contraction and expansion can weaken these parts. One way to prevent this is to ensure the temperature of the aircraft engine stays within a specified range when the aircraft is on the ground.

If you don't have a climate-controlled space to park the aircraft, but it is parked in a hangar that has electricity, you may want to invest in a repositionable electrically powered heat source that is placed inside the engine cowling and left on when the aircraft is not in use. These units are thermostatically controlled and can be left on overnight or all the time. 

In colder climates, such as Minnesota in the winter,many aircraft owners use a device like this in conjunction with cowl and prop covers and a blanket to keep the heat in the engine compartment. The heat transfer is done through convection and conduction, which reduces the potential for fire as there is no open flame or glowing elements.

There are several manufacturers of engine preheaters. Reiff Preheat, for example, advises customers that sealing and insulating the engine compartment by using a cover greatly enhances the efficacy of an engine preheater, as does using foam cowl plugs.

Aerotherm notes that its devices do not require a permanent installation in the engine and therefore can be shared for example as part of a flight school fleet.

E-ZHeat has a system that resembles a heating pad for the oil sump. Position it inside the engine cowling, plug it in, and let nature take its course.

For the aircraft owners who want the option of an installed preheat device, Tanis Aircraft makes preheat kits for Lycoming, Continental, Rotax, Jabiru, Jacobs, and Austro engines. The company has been around since 1974, and according to its website, was the first to develop aircraft engine preheat systems for piston engines, helicopters, and turbines and obtained FAA Supplemental Type Certification.

Some preheaters have the added function of remote activation, so you don't have to make the drive to the airport to start it. Preheat remote is a feature that allows a pilot to activate the preheat by a cell phone app.

No matter which one you choose, note the manufacturer's recommendations for how long the engine preheat should be activated before flight happens. Also heed the warnings about leaving it on too long, as this can result in moisture building in the engine that leads to rust.

Pilots should ascribe to the one hour rule, Kuti said. "Preheat an hour before flight. Don't fly if you can't fly for at least an hour, and open the dipstick for at least an hour after flight to allow steam out of the engine," he said. 

No matter what type of preheating is applied, pilots should refer to the POH or aircraft flight manual (AFM) for cold weather operations specific to their aircraft. This includes warnings to not "over-crank" trying to get it to start.

"If the engine does not start during the first few attempts, or if the engine firing diminishes in strength, it is possible that the spark plugs have been frosted over," according to Lycoming. "Preheat must be used before another start is attempted."

Lycoming echoes this warning, noting that if the engine has not started in five seconds, stop trying to crank it, and obtain an auxiliary electrical source because the battery is too weak for engine start.

Pilots are trained to watch the power settings and oil pressure during all engine starts, but especially during cold weather. Excessive engine power and low or no oil pressure is a recipe for damage.

In addition, the POH/AFM may encourage an extended warmup time on the ground to allow oil pressure to stabilize before flight is attempted. If the engine is running rough, the oil pressure and fuel flow are fluctuating, or there is excessive manifold pressure, these are good indications the aircraft engine is not properly warmed up.

Aircraft owners can get very creative when they are trying to preheat their aircraft engine.

You may know someone who has built their own external system. These might consist   of a warm air blower that is electric or propane powered and duct work,  such as a washing machine hose to blow hot air directly into the engine cowling.

It is important to direct the air so it does not damage wiring or hoses, which means opening the cowl flaps if they exist to keep the heat from building up too much. 

If the flight school doesn't have a preheater, pilots, especially flight instructors, can get creative.

On one occasion we took the electric space heaters out of the ground school classroom and positioned them in front of the aircraft and, using chairs and a blanket, made a sort of tent over the bonnet that trapped the warm air inside.