This piece originally ran in Robert’s Stick and Rudder column in the February 2024 issue of EAA Sport Aviation magazine.

Years ago, a student of mine had a winter flying experience that compromised his ability as pilot in command (PIC).

It happened on a sunny day in the Denver area. He was building his cross-country experience in pursuit of his commercial certificate and instrument rating, and had chosen this day to fly from Rocky Mountain Metropolitan Airport (KBJC) in Broomfield, Colorado, to Pueblo, Colorado (KPUB).

He had done a thorough preflight, and his airplane had been preheated inside and out before he took to the sky. But it wasn’t too long before he realized that the cabin heater wasn’t working up to snuff. Rather than return to his home airport, he decided to tough it out and complete the flight. After all, it wasn’t really that far.

Crossing high terrain between Denver and Colorado Springs, he climbed to more than 10,000 feet where, despite the sunny skies, temperatures were frigid. Before long, he was shivering, his feet were numb, and he could see his breath condensing into frost on the windows. He put on a hat and gloves, but even that failed to fend off the deepening cold. He needed to get into a heated environment.

While such a problem might not seem common, I’ve had similar situations occur more than once. Both times were in a light twin, and both times involved a combustion heater that tripped its emergency shutdown breaker, which then rendered the cabin heater inoperative. In both cases, the cabin got cold in a hurry and required us to change our flight plan. We didn’t suffer as badly as my student, but it could have gone differently if we weren’t near an airport where a mechanic could remedy the problem for us.

Understanding Hypothermia

Getting cold in an airplane isn’t just uncomfortable, it can be downright dangerous. Exposure to cold temperatures causes physiological changes that can undermine our ability to effectively act as PIC.

In moderately cool temperatures, our body can make adjustments to maintain our body temperature. These changes can include upping our metabolism and adjusting our blood circulation to minimize heat loss and maintain our body core temperature. But if it is too cold, or the exposure too long, these strategies cannot keep up with the heat loss, and hypothermia — the condition of a reduced core temperature — sets in. Unless the situation is remedied, body temperature begins a downward spiral that could become fatal.

Numerous studies have been performed to measure and document the effects of cold exposure on healthy humans, and these reveal decreases in both physical and cognitive performance. Sources reveal that even a mild case of hypothermia (body temperature drop to 90-95 degrees Fahrenheit) results in a number of physiological effects.

In an attempt to conserve body heat, the blood vessels constrict, blood pressure rises, heart rate and respiration increase, and shivering occurs. Other physiological responses are particularly troubling for a pilot: tiredness, loss of body movement (motor control), and loss of mental acuity. Most critical to a pilot’s ability are a decline in memory, lack of judgment, and the inability to think clearly.

As hypothermia deepens to a moderate level with a body core temperature of 82-90 degrees Fahrenheit, we see a reversal of some effects and worsening of others. Blood pressure and heart rate drop, and there’s an increased susceptibility to abnormal heart rhythms. Breathing slows and pupils become large and less responsive. Lethargy deepens and the ability to think further declines.

In the later stages of hypothermia, when the body core temperature drops below 82 degrees Fahrenheit, the effects are life-threatening. Blood flow to the brain declines, leading to unresponsiveness. The heart is more susceptible to abnormal rhythms, and may ultimately fail. Lung congestion occurs and can lead to respiratory failure. Body reflexes are lost.

According to a recent article in the International Journal of Environmental Research and Public Health, the majority of the studies on the subject of cold exposure “show that a single acute exposure to cold may impair attention, speed of processing, memory and executive function.” The article goes on to say that “these effects might depend on individual physiological responses to cold as well as the extent of the exposure in terms of duration and temperature reached.”

Data from the research also suggests there may be differences in cognitive performance of males and females exposed to cold. In addition, acclimatization to cold conditions may also impact the cognitive performance of individuals. The bottom line is that individuals may have differing tolerances to cold exposure in terms of their cognitive performance.

Perhaps more importantly, sources at the Centers for Disease Control and Prevention warn us that “while hypothermia is most likely at very cold temperatures, it can occur even at cool temperatures (above 40 degrees Fahrenheit) if a person becomes chilled from rain, sweat, or submersion in cold water.”

Lessons in Preparation and Survival

The issue my student faced (unknown to him at the time) was that the ducting that carries the heated air to the cabin had somehow come loose, effectively incapacitating the cabin heat system. However, that’s only one way that pilots can find themselves in such a chilling situation.

Another scenario in which hypothermia can be a concern involves an off-field emergency or precautionary landing in a remote location. As we learn in mountain flying training, it’s wise to consider not only the weather conditions at our point of departure and destination, but the conditions that might exist in between, in the event of such an off-field adventure.

While sneakers and a T-shirt might suit the environment at our intended destination, such attire might be grossly inadequate if we end up high in mountainous terrain buried in snow. In such a scenario, rescuers might not be able to reach us quickly, and the prospects for an overnight or multiday survival scenario may loom before us.

In either case, it serves us well to be prepared. And again, it’s a matter not just of comfort, but of safety and survival.

Being Prepared

Whenever we fly in cold conditions, or overfly mountainous terrain and other areas where cold conditions exist, we should always be prepared with warm clothing to stave off the effects of hypothermia.

Experts recommend carrying sleeping bags and bringing along items to start a fire in the event we make an off-field landing where the arrival of rescue personnel may be delayed. Provisions to maintain hydration and supply energy can also be critical.

Practiced skills in survival — including fire-making, shelter construction, and additional techniques to stave off hypothermia — can make the difference between tragedy and survival.

My student was fortunate that winter’s day when the cabin heat failed. He likely had not considered the possibility of such an event and wasn’t clued into the effects of hypothermia, how quickly it could settle in, and how it might affect his performance. Fortunately, he made some good decisions that ultimately extricated him from what could have been a dire situation.

He called for ATC assistance and diverted to nearby Colorado Springs. By this time, he was barely able to walk, but he made it into the local FBO where he warmed up and found a mechanic to sort out the mechanical difficulty with his airplane.

Others have been less fortunate, but forewarned is forearmed. If we understand the risks and take the proper precautions, we can better avoid the deleterious effects of the winter chill.

Robert N. Rossier, EAA 472091, has been flying for more than 40 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.