Behind the Forecast: How cold air affects airplanes

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Behind the Forecast: How cold air affects airplanes
Cold air can actually have a positive effect on aircraft. (Source: Pexels)

LOUISVILLE, Ky. (WAVE) - Winter is coming. That means colder air is on its way too. With many of us already preparing for holiday travel, it’s crucial to understand how cold air can affect our flights to see family and friends.

While snow, frost, and ice have more prominent impacts, on their own, frigid temperatures can affect aircraft in unexpected ways.

“Winter storms don’t typically have the hazards to airliners that thunderstorms have,” UPS pilot Ken Hoke explained. “There are usually no cumulus clouds or lightning which increase the risk of severe turbulence. Airliners can often fly over the top of winter storms in clear, smooth air. Landing in a winter storm, however, can be challenging with gusty winds, blowing snow, sleet, etc.”

Thankfully, all materials used in planes are designed to withstand extreme temperatures. Commercial aircraft usually fly between 30 and 42 thousand feet. At these altitudes, temperatures range from -48° to -69°F.

Crews won’t start some airplane engines if they have been sitting for hours in frigid temperatures. “The 767 that I fly has a limitation that the engines can’t be started if they have been sitting in temperatures below -34°C for more than four hours,” Hoke explained.

Maintenance crews use specialized equipment to warm engines before starting them or keep planes in a warm hanger to protect them. “If the aircraft will be sitting overnight in these temps, maintenance personnel may start and run the engines every few hours to keep them warm,” Hoke said.

During wild winter weather that walloped the Midwest at the end of January 2019, a Delta spokesperson told CNBC that “increasing staffing of ground workers more employees can take more frequent breaks indoors, as well as moving some aircraft into heated hangars overnight.”

While the aircraft can handle cold temperatures, the oil used in airplane engines is not a fan. As temperatures fall, it becomes thick, which makes it harder for the engine to circulate it.

Jet fuel freezes around -40°C (-40°F), but it will work as long as it’s kept warmer on the ground. According to Hoke, pilots cannot let the fuel temperature fall below -37°C, or it will begin to get slushy. Pilots have a few options to avoid slushy or frozen fuel. “First, we can speed up,” Hoke said. “More speed means more air friction on the wings. The friction will warm the wings a few degrees, usually enough to take care of the problem. If we need more help, we can descend to a lower altitude to warmer air.”

Science Behind the Forecast: How cold air affects aircraft

Airplanes are more efficient in low temperatures since cold air is denser than warm air. A plane’s performance is dependent on the air’s density. According to Business Aircraft Center, turbine and internal combustion/reciprocating piston engines are more efficient in cold air since it allows the engine to use “greater mass of air/fuel mixture in the same intake volume.”

Denser air translates to a larger mass of air getting into the engine, which means a plane has more power. More power means a quicker acceleration on take-off, an increased rate of climb, more lift for the wings, and take-off at lower ground speed. This is why takeoffs are shorter and faster than during the winter compared to the summer.

Some airports that deal with extremely hot temperatures have longer runways to compensate. “A good example is Phoenix Sky Harbor International,” Hoke said. “It gets very hot in the summer, which affects aircraft performance. Engines don’t produce as much thrust with the less dense warm air; this results in a longer takeoff distance. Landing performance is affected too. Landing speeds are higher on hot days, which means aircraft need more distance to stop. Phoenix has 10,300 and 11,400-foot runways. Those lengths are above average. Perfect for big planes to take off and land on hot days.”

A lot of the flight delays in colder weather are related to the safety of ground workers. While planes can function in colder weather, that’s not always the case with human bodies. Bitter cold reduces the amount of time ground workers can spend fixing the airplane or loading bags.

In terms of flying on cold days versus hot days, colder days offer a more comfortable flight. According to Hoke, “Cold days tend to provide a smoother ride at lower altitudes, which is nice for both pilots and passengers. Hot days tend to be bumpier at low altitudes due to the heating of the surface and rising warm air.”

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