Aircraft Maintenance: Adjusting to winter
Seasonal changes can be challenging. Everything seems harder in the cold, even getting up in the morning. But adjust we must. There is usually snow on the ground before I finally surrender and pull out the long-sleeved shirts and jackets. It takes a few weeks, but I eventually adjust to the new normal and begin to regard a day with temps above freezing as “really nice outside.” Our aircraft go through the same changes and require similar adaptations. However, their changes require our intervention to help them overcome the transition to cold weather.
The most commonly discussed seasonal change is regarding engine oil. The “weight” of an oil is a measure of its viscosity, which is an indicator of how easily it will flow at a given temperature. Most aircraft are designed for a 40- or 50-weight oil for normal operating temperatures (check your pilot’s operating handbook for the requirements for your aircraft). For relatively stable temperatures above freezing, there are several straight-weight options available from Phillips 66, Aeroshell, and Exxon. As a rule, divide the name of the straight-weight oil by two to get the viscosity (ex: Aeroshell 100 = SAE 50 weight oil). Straight-weight oils are designed to flow at a specific rate at the standard temperature of 100 degrees Celsius (212 degrees Fahrenheit). Multi-grade oils, on the other hand, are designed to flow more easily at colder temperatures (and don’t require math to figure out their weights). Phillips X/C 20W-50 oil has the flow characteristics of a 20-weight oil in cold temperatures and the flow of a 50-weight oil when your engine gets warmed up. There’s no harm in running multi-weight oil year-round, but it’s unnecessary because you’re only utilizing the advantages of the multi-weight oil when it’s cold. The bottom line is that when the temperatures drop, you need to make sure that the viscosity of your oil does as well.
The next item on our “adjustment list” is checking your aircraft’s tires. Almost everyone is familiar with adding air to the tires on their car when the temperature drops dramatically. However, I’m always surprised how many aircraft on the ramp are sitting on low tires as soon as winter weather hits. Perhaps it’s a case of “out of sight, out of mind,” but it isn’t healthy for your tires to be underinflated (and it’s certainly not safe to fly with them that way). Tire pressures rise and fall with changes in temperature by about 1 pound per square inch for every 10 degrees Fahrenheit change in outside air temperature. So, even if you don’t plan on flying, make it a point to get out to the airport to top off your tires when the temperature has dropped by 10 to 20 degrees on the average day.
Landing gear struts can also suffer from dramatic changes in temperature. The nitrogen used in struts has the same 1 psi for every 10 degrees Fahrenheit change that normal air has. However, the fact that struts use much higher pressures than tires means that the effect of outside temperature is much less significant. That said, it pays to keep a close eye on your struts with large swings in outside temperature. Although the pressure inside may not vary by much, the contraction and expansion of the metal components as well as the effect of cold temperatures on seals and O-rings can result in significant changes in strut height or loss of pressure and fluid.
If you fly an aircraft with a fuel-injected engine, then you’ll also want to get your mechanic to do a fuel system operational check with the change of seasons. Continental Aerospace Technologies recommends checking and adjusting the low- and high-end fuel pressures, as well as the mixture and idle settings, every 100 hours or “when changes occur in the operating environment.” This includes dramatic changes in temperature. If your fuel system is out of adjustment, you may not be getting rated power when you need it most and you could also be risking the health of your engine. It’s a straightforward process to check the fuel setup, but it does require that your mechanic have the proper gauges and tools for the job. So, check to see if he or she has the tools or find someone who does.
The final item on our “adjustment list” is batteries. This includes the aircraft’s main battery; backup batteries; and even the batteries in your flashlight, headset, or portable radio. Batteries don’t perform well in the cold, so take the time to charge what you can and replace what you can’t.
After the first few weeks of winter, both pilot and aircraft have usually adjusted enough to begin to appreciate the benefits of winter flying. After all, the cold also brings with it stellar climb performance, clear skies, and smooth air. So, we might as well enjoy all the benefits that winter flying has to offer. And, if it gets cold enough, you just might see us venturing back this year to the famous ice runway at Alton Bay, New Hampshire! Until next time, I hope you and your families remain safe and healthy and wish you blue skies.
