Why You Should Not Skip Aircraft Engine Pre-Heating Just Because Other Pilots Do
Proper aircraft engine pre-heating begins with understanding what you are actually trying to achieve. You are not simply warming the oil or making the engine easier to start. You are preparing a cold-soaked mechanical assembly, made from different materials with temperature-dependent clearances, before asking it to produce power. Once you understand that, both the reason for pre-heating and the inadequacy of many improvised methods become much easier to see.
Two Hairdryers and a Cold Aviat Husky
When I became interested in general aviation, I intentionally looked for commercial flight schools, service providers, and aircraft charter rather than joining a flying club. I wanted to make progress quickly and do things properly from the beginning.
Eventually, though, I did become a member of a flying club because I wanted to fly a Lycoming-powered Aviat Husky.
The flight instructor at the club did an excellent job teaching me the stick-and-rudder skills necessary to operate the Husky. When it came to engine pre-heating, however, the flying club had chosen a rather more traditional solution: two household hairdryers, placed into the engine cowling inlets for about an hour before the flight.
I still remember arriving at the airfield early on cold winter mornings, walking into the uninsulated hangar, plugging in the two hairdryers, positioning them in front of the engine cowling, and believing that would do the trick.
At the time, I believed what many pilots and aircraft owners still believe today: engine pre-heating is mainly about warming the engine oil.
I never really questioned why the propeller was noticeably more difficult to turn when the engine was cold-soaked. More importantly, I never stopped to think about what that resistance might be telling me about the engine itself.
It took me several years to understand that sticking two hairdryers into the cowling is not a particularly effective way to pre-heat an aircraft engine. Sure, it is probably better than a kick in the teeth, but it does not really address the underlying mechanical reason why we pre-heat aircraft engines in the first place.
What I find more interesting today is that this was not simply a flying-club problem. Despite having spent time around commercial flight schools, service providers, charter operations, and later the flying-club environment, nobody had ever really explained to me why an aircraft engine should be pre-heated or what proper pre-heating was actually trying to achieve.
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Pre-Heating Is Not Primarily About the Oil
Cold oil has a higher viscosity, a cold engine may crank more reluctantly, and battery performance also deteriorates as temperature falls. Lycoming specifically identifies thick or congealed oil, reduced battery capacity, and increased starter loading among the challenges of cold-weather operation. Oil therefore clearly matters.
Warm oil flows more easily, and using the correct oil grade for the expected temperature range is an important part of cold-weather operation. Lycoming also points out that multigrade oils improve cold-weather starting and lubrication during the critical warm-up period, while warning that they do not eliminate all of the problems associated with very cold conditions. If we stop at oil viscosity, however, we miss another important mechanical issue.
An aircraft engine is assembled from different materials with different rates of thermal expansion. As temperatures change, the dimensions of those components change as well, but not necessarily at the same rate. That becomes important because an aircraft engine relies on designed clearances between moving components, and those clearances are affected by temperature.
This is the part of engine pre-heating that I did not understand when I was standing in that cold hangar with my two hairdryers.
A Cold Engine Changes Dimension
Aircraft engines are made from different materials, each selected according to the requirements of the component. In a conventional Lycoming or Continental piston engine, two of the most important materials are aluminium alloys and steel. These materials have different coefficients of thermal expansion, which means they expand and contract by different amounts as their temperature changes.
As a simple engineering approximation, aluminium expands and contracts roughly twice as much as steel for the same change in temperature. The exact figures depend on the particular alloy, but the underlying principle is what matters here. When an engine heats up or cools down, its components do not simply become larger or smaller by the same amount. The dimensional relationship between them changes.
That is important because an aircraft engine relies on carefully defined clearances between moving components. Those clearances are generally established around the conditions in which the engine is intended to operate, and they can change significantly when the engine is deeply cold-soaked. Two examples illustrate the problem particularly well.
Example A: Crankcase and Crankshaft
The crankcase and crankshaft are not made from the same material. The crankcase is predominantly aluminium alloy, while the crankshaft is steel.
Now imagine the complete engine cold-soaking overnight. Both components contract as their temperature falls, but the aluminium crankcase contracts more than the steel crankshaft. As a result, the clearances within the assembly reduce compared with those at normal operating temperature.
Those clearances matter because the crankshaft bearings depend on an adequate supply of oil and the correct lubrication regime. Broadly speaking, we differentiate between boundary lubrication, mixed lubrication, and hydrodynamic lubrication. The details are a subject for another article, but all of these regimes depend on the interaction between bearing geometry, oil supply, oil viscosity, speed, load, and clearance.
When the engine is deeply cold-soaked, you therefore have two things working against you at the same time: the oil is thicker, and some of the mechanical clearances are reduced compared with normal operating conditions. This is also one reason why a cold-soaked engine can be noticeably more difficult to turn by hand.
Example B: Piston and Cylinder
A similar, although slightly different, effect occurs between the piston and cylinder.
The piston is typically made from aluminium alloy, while the cylinder barrel is steel. The aluminium piston responds to temperature changes more strongly than the steel barrel, and after engine start it also heats up more quickly because of its lower thermal mass, and the fact that it does not benefit from the cooling fins and direct airflow available to the cylinder barrel.
This means the clearance between piston and cylinder is not constant during the transition from a deeply cold-soaked engine to normal operating temperature. The piston can expand more quickly than the surrounding steel cylinder barrel, which is one reason proper cold-weather operation and warm-up matter.
Again, the point is not that every cold start automatically results in metal-to-metal contact or immediate damage. The point is that the engine relies on controlled clearances, and temperature materially affects those clearances.
This brings us back to the oil. Oil temperature and viscosity are important, but lubrication cannot be considered independently of the geometry of the components being lubricated. Warmer oil alone does not solve the entire problem if the engine itself remains deeply cold-soaked.
Proper pre-heating therefore needs to address the temperature of the engine assembly, not merely the contents of the oil sump.
What Proper Pre-Heating Is Actually Trying to Do
Aircraft engine pre-heating is not simply about getting the oil temperature up. The real objective is to raise the temperature of the engine assembly sufficiently and reasonably evenly before starting a deeply cold-soaked engine.
In practical terms, that means increasing the core temperature of the crankcase, cylinders, oil, and other relevant components rather than simply introducing warm air somewhere inside the cowling. We want to bring the engine closer to the temperature range in which its mechanical clearances and lubrication conditions are more favourable before asking it to turn, start, and produce power.
This is where the difference between heating and heat-soaking becomes important. You can introduce warm air into an engine compartment quite quickly, but that does not necessarily mean the substantial mass of the engine components has reached an appropriate temperature.
A proper pre-heating strategy therefore needs to put heat into the right parts of the engine and give that heat enough time to penetrate the engine mass. The objective is not simply to make the engine compartment feel warm. It is to raise the temperature of the engine itself.
That is what the two hairdryers in the Husky hangar largely failed to achieve. They certainly produced warm air, and they may have raised the temperature of parts of the engine compartment to some degree, but that is not the same as properly heat-soaking the crankcase, cylinders, and oil.
Once you understand that distinction, the question becomes much more practical: how do we pre-heat the engine properly?
Principle 1: Know the Manufacturer's Cold-Weather Guidance
The starting point should always be the Pilot’s Operating Handbook, the engine manufacturer’s operating instructions, and the applicable service information for your particular aircraft and engine.
Lycoming, for example, requires pre-heating when most of its engines have been allowed to fall below +10°F/-12°C. For -76 series engines, the stated threshold is +20°F/-6°C. Lycoming also warns that improper cold-weather starting can contribute to abnormal engine wear and shortened engine life.
Continental uses slightly different guidance. Current Continental service documentation referring to its M-0 Standard Practice Maintenance Manual states that, after an engine has been exposed to temperatures below 20°F/-7°C for more than two hours, the engine should be pre-heated according to the prescribed procedure.
For our own operating philosophy at Quest Aeronautics, I prefer a somewhat more conservative approach. I recommend considering pre-heating whenever the aircraft and engine have cold-soaked below approximately 0°C. This is not a replacement for the manufacturer’s guidance, but rather an additional operating margin based on the mechanical principles we have just discussed.
Principle 2: Heat the Engine, Not Merely the Oil
Once we understand that pre-heating is about the temperature of the engine assembly rather than only the oil, the choice of pre-heating method becomes much easier to evaluate.
In my view, the best solution is an insulated, heated hangar. If the complete aircraft has spent sufficient time in a genuinely heated environment, you are not merely warming one part of the engine. The crankcase, cylinders, oil, battery, starter, induction components, and other systems are all allowed to come up in temperature together.
This is also useful when travelling away from your home base. You do not necessarily need to pay for heated hangarage for your entire stay. In many places, you can arrange for the aircraft to be moved into a heated hangar sufficiently in advance of departure. Apart from pre-heating the engine, this also makes frost, ice, and snow removal considerably easier and makes the pre-flight inspection a more comfortable experience.
The second-best option is a properly installed multi-point electrical pre-heating system. Tanis and Reiff are two established examples, and both product lines are now part of Hartzell’s aircraft pre-heat offering. These systems are designed to put heat directly into several relevant parts of the engine rather than simply warming the air inside the cowling. Depending on the installation, they typically heat the crankcase, cylinders, and oil sump, which is much closer to what we are actually trying to achieve.
Once installed, operation is very simple. You connect the aircraft to the appropriate electrical supply and give the system enough time to heat-soak the engine.
Cowl plugs, together with engine and propeller covers, can improve the process further by reducing heat loss and keeping more of the generated heat inside the engine compartment. This also helps warm components such as the battery, starter, carburettor or induction system, and other accessories, which can improve cold-weather starting behaviour as well.
What I do not recommend is treating improvised heat sources such as household hairdryers, heat guns, or similar devices as an equivalent substitute for proper engine pre-heating. They may introduce some heat into the cowling, but that does not mean they are heating the relevant engine components sufficiently or evenly.
And then, of course, there is the simplest pre-heating system of all: Mother Nature. If ambient temperature and sunlight have naturally brought the complete engine to an appropriate temperature, there is no reason to operate a heater merely for the sake of saying that the engine was pre-heated.
The objective is not to use a heater. The objective is to bring a cold-soaked engine to a more suitable thermal condition before starting it.
Principle 3: Give the Heat Time to Work
Whatever method you use, heat-soaking takes time. How much time depends on the environmental conditions, the starting temperature, the aircraft and engine, the heating method, wind exposure, insulation, and whether cowl plugs and covers are being used.
If an aircraft has been deeply cold-soaked outside, moving it into a heated hangar shortly before departure may not be enough. Depending on the conditions, several hours may be required before the complete engine assembly has warmed properly. When travelling, I would therefore rather arrange for the aircraft to be placed in the heated hangar well in advance of departure than assume that one or two hours will be sufficient.
The same principle applies to installed electrical systems. Multi-point pre-heating systems are not instant heaters. Tanis, for example, states that some of its systems reach an approximate state of thermal equilibrium after around six hours. The exact time will vary with the installation and conditions, but the important point is that proper pre-heating should be thought of as heat-soaking rather than simply making one part of the engine feel warm.
This is where convenience often gets in the way of good aircraft ownership. You arrive at the airfield, it is cold, and you want to fly. Waiting several hours is obviously inconvenient. But once the process is set up properly, that inconvenience largely disappears.
Many pilot-owners combine installed pre-heating systems with remote switching or scheduling systems so the pre-heating cycle can begin before they arrive at the airfield. Hartzell, for example, offers its SwitcheOn system alongside the Tanis and Reiff product lines. Other suitable remote switching solutions may also work, provided they are correctly rated for the electrical load and used in accordance with the heater manufacturer’s instructions.
Once that infrastructure and routine are in place, aircraft engine pre-heating becomes one of the simplest things you can do. You no longer need to arrive hours early and stand in a freezing hangar pointing two hairdryers at an engine. The system simply becomes part of how you operate the aircraft.
But My Car Doesn't Need Pre-Heating
One of the most common objections I hear is that modern car engines do not need to be pre-heated, so why should an aircraft engine be any different?
The problem is that we are comparing two fundamentally different machines. Modern automotive engines and traditional air-cooled aircraft engines differ in architecture, cooling systems, lubrication systems, operating environment, manufacturing scale, design priorities, operating conditions, and certification requirements. Many of the Continental and Lycoming Avgas engines we operate today also have design architectures that trace their origins back many decades.
That does not make them bad engines. Their simplicity, relatively low weight, direct-drive configuration, and proven design are among the reasons they continue to work so well in general aviation. But it does mean that comparing them directly with a modern automotive engine designed and manufactured for millions of applications is not particularly useful.
The important question is therefore not whether your car requires pre-heating. The relevant question is how your particular aircraft engine is designed, how its materials and clearances respond to low temperatures, and what its manufacturer requires or recommends for cold-weather operation.
We Have Always Used Hairdryers and Never Had a Problem
Another argument I have heard many times is that a flying club or aircraft owner has used hairdryers, heat guns, or some other improvised method for years without ever having a problem.
And that may well be true. The engine started, the aircraft flew, and nothing obviously went wrong.
The problem is that absence of immediate failure is not a particularly useful way to judge an engine-management practice. Most of the factors that influence engine longevity do not produce an obvious consequence after one flight or one cold start. Their effects accumulate gradually over hundreds of starts and hundreds or thousands of operating hours.
Pre-heating is only one part of that picture. Regular operation, corrosion prevention, engine preservation, mixture and temperature management, maintenance quality, storage, and operating technique all influence how an engine ages. For that reason, I would never argue that an engine failing to reach TBO proves that inadequate pre-heating was responsible.
But the opposite argument is equally weak. Saying, “We have always done it this way and never had a problem,” does not demonstrate that the practice is mechanically sound or that a better approach would not reduce wear over the life of the engine.
This is something we see repeatedly in aircraft ownership. Small operating practices rarely destroy an engine overnight. They compound quietly over time, which makes it very easy to continue doing something simply because the consequences are not immediately visible.
That is why I prefer to understand the mechanism first. If we know why a cold-soaked engine benefits from proper pre-heating, and we know that there are relatively simple ways to do it properly, then continuing with an improvised method simply because it has not caused an obvious failure becomes difficult to justify.
Pre-Heating Is a Good Example of Professional Aircraft Ownership
Pre-heating an aircraft engine is a relatively simple but important part of aircraft ownership and operation. Yet many pilots and aircraft owners still treat it as optional, inconvenient, or something that only becomes relevant when temperatures get extremely low.
I believe much of that comes down to a lack of understanding about what pre-heating is actually trying to achieve. Nobody explained it properly to me either. I had spent years around piston engines, completed commercial flight training, and had a technical background, yet I still stood in that cold hangar plugging two hairdryers into the Husky because I believed that was a reasonable way to pre-heat the engine. It took me several years to properly understand the relationship between temperature, material expansion, mechanical clearances, lubrication, and the way we should prepare a cold-soaked aircraft engine before starting it.
Once you understand those relationships, it becomes difficult to look at pre-heating in the same way again. More importantly, the solution is not particularly complicated. Use heated hangarage when it is available. Install a proper multi-point pre-heating system where it makes sense. Use suitable engine covers and cowl plugs to retain heat. Allow enough time for the engine to heat-soak. Take advantage of naturally warmer ambient conditions when they achieve the same result. And always begin with the operating guidance applicable to your particular aircraft and engine.
That is hardly sophisticated aircraft management, but I think that is precisely why pre-heating is such a good example of professional aircraft ownership. Professional operation is not primarily about doing impressive or complicated things. More often, it is about understanding why relatively small things matter and then having the discipline to do them consistently.
Pre-heating is only one example. Regular operation, sensible preservation, disciplined engine management, condition monitoring, and good maintenance governance are all made up of similarly small decisions and routines. Individually, none of them may appear particularly significant, but together they can make a meaningful difference to engine wear, reliability, longevity, and ultimately the aircraft ownership experience.
This is also where the broader principle I keep returning to in aircraft ownership becomes relevant: think independently, decide systematically, and execute professionally. That does not mean ignoring instructors, mechanics, manufacturers, or other experienced pilots. It means understanding enough to evaluate the advice you receive and take responsibility for how you operate your aircraft.
So do not pre-heat your engine simply because somebody at the flying club told you to. But do not skip it because somebody at the flying club told you it was unnecessary either. Understand what is happening inside the engine, consider the manufacturer’s guidance and the underlying mechanical principles, and then make an informed decision about how you want to operate it.
Aircraft Ownership Workshops
Engine pre-heating is only one of many apparently small decisions that can have a meaningful effect on the aircraft ownership experience. The same is true for maintenance decisions, engine operation, preservation, condition monitoring, shop communication, airworthiness management, and the operating standards you apply to your aircraft.
None of these areas is particularly mysterious once you understand the underlying principles. The difficulty is that many pilot-owners were simply never taught those principles or given a reliable framework for applying them to their own aircraft.
That is what we explore in our Aircraft Ownership Workshops. We run them on a biweekly basis and use practical aircraft ownership challenges to discuss the principles, systems, and decision frameworks that can help pilot-owners make better ownership and operating decisions.
For us, aircraft ownership should be a source of freedom, purpose, and meaningful experiences. The workshops are designed to help pilot-owners develop the understanding and structure necessary to make that possible in practice.
Frequently Asked Questions
Aircraft engine preheating raises the temperature of the engine assembly before starting a deeply cold-soaked engine. The goal is to improve thermal conditions, mechanical clearances and lubrication rather than simply warming the oil.
Cold temperatures increase oil viscosity and affect engine clearances between components made from different materials. Proper preheating helps bring the engine closer to suitable operating conditions before starting.
Improvised heat sources such as household hairdryers may warm parts of the engine compartment but do not necessarily heat-soak the engine assembly sufficiently or evenly. Properly installed preheating systems are designed to heat relevant engine components more effectively.
The applicable aircraft and engine manufacturer’s guidance should always be the starting point. Lycoming and Continental provide specific cold-weather guidance, while we recommend considering preheating after the engine has cold-soaked below approximately 0°C.
The required time depends on the starting temperature, environment, heating system, insulation and aircraft. Proper preheating should be treated as a heat-soaking process rather than simply warming the engine compartment.
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About Quest Aeronautics
Quest Aeronautics is a state-certified engineering office for aviation, dedicated to shaping the future of general aviation by providing innovative and cost-effective solutions to enhance aircraft performance and operations. With a focus on CS/FAR-23 and experimental/amateur-built (E/A-B) aircraft, Quest Aeronautics provides a range of services including flight testing, aircraft operations and maintenance consulting, high-quality aviation products, and tailored support for E/A-B projects. Collaborating with industry-leading partners, Quest Aeronautics is committed to delivering unparalleled support and expertise to individuals and organisations in the general aviation market.
About Author
Sebastian, the founder of Quest Aeronautics, is a driven and enthusiastic individual with a passion for aviation. Before delving into aviation, he gained valuable experience as a chemical process engineer and laboratory technician. Sebastian holds a Master of Science in Engineering and a commercial pilot licence, with several fixed-wing aircraft ratings under his belt. He has also completed an introduction course for fixed-wing performance and flying qualities flight testing at the National Test Pilot School in Mojave, CA and is compliance verification engineer for flight.