For some people, the sign that summer is nearly over is when football players start practice. For others, it’s when the fun starts at the Minnesota State Fair. For Delta Air Lines, it’s when 500 people come to the Twin Cities from around the country to practice getting ice off planes.

As temperatures hovered around 80 degrees at Minneapolis-St. Paul International Airport last week, Delta brought out its de-icing equipment for an annual “boot camp” and conference for station chiefs and personnel from nearly 200 airports.

De-icing is critical to flight safety during winter. Ice can inhibit a plane’s parts from working properly, which can be disastrous during takeoff. Ice can also slide off the aircraft body and hit rear-set engines. Airlines de-ice planes not just during snow and ice storms, but also in morning frost or lake effect condensation.

For passengers, it’s a somewhat mysterious process. Though Delta builds time into its winter schedules for de-icing, many passengers tend to see the process adding a delay to their flight. Some question whether it actually works.

“I have friends who complain about being de-iced, but it’s a good thing. You want them to take that time,” said Mike Stojsavljevic, a Delta de-icer from Milwaukee. “De-icing is one of the most important things we do.”

After several ice-related accidents, federal regulations increased training requirements for de-icing. But it is up to each airline to determine its preparation plan.

Each year Delta trains its “below wing” agents — industry parlance for customer service agents who don’t directly interact with the public — to refresh their knowledge and teach new skills or procedures. They come to the Twin Cities rather than the airline’s base in Atlanta because the MSP fire department lets airlines use an old DC-9 fuselage it owns for practice drills.

The training starts even before they arrive; each participant must pass a written test before making it to MSP. Safety is drilled throughout training, with terrifying footage and horror stories of when ice proved fatal for airplanes and their passengers.

Crews employ a wash-and-wax technique that first melts the ice and then protects it against buildup for up to 40 minutes. De-icers spray one type of propylene glycol that is cranked up to 160 degrees, before spraying a second type, that has a thicker, oil-like consistency and resembles green slime, which coats the aircraft. The agents are also required to touch the wings on certain high-risk aircraft with their bare hands to check for any sheer ice layers.

They’re also taught to work with flight controllers to minimize the time between the end of de-icing and takeoff. In some storms, plane surfaces can become covered with new ice and snow in just minutes after de-icing. Pilots and below-wing agents both have the power to call for more de-icing.

Each of the attendees at Delta’s boot camp spends two days working in de-icing equipment, including a new machine called the Ultimate 2200, made by Global Ground Support of Olathe, Kan., that many workers have never used before.

“That is the Cadillac of all de-icing equipment,” said Keith Kimber, a new trainer of Delta’s de-icing crew in Milwaukee.

It is one of 28 de-icing vehicles at MSP. Winter winds may whip around them, but the trainees say the booth in the machines gets so toasty they often strip down to T-shirts and shorts.

Last year, Kimber was given a trophy for the most airplanes cleared that season: 498. But the number is less significant than thoroughness. “A lot of people think you’re just out there washing the aircraft, but there’s a lot of ice you have to feel to see,” Kimber said.

From November until April, the de-icing crew knows it will spend many extra hours at work. For particularly harsh storms, Delta will get nearby hotel rooms for the workers to make sure they can get to and from work quickly and safely.

The station crews with more winter weather experience, like MSP and Detroit, also have “go teams” that can fly out to help their fair-weather friends.

The amount of fluid needed for each airplane varies depending on the severity of the ice, snow or frost buildup. Last year, Delta spent $3 million on 800,000 gallons of de-icing fluid at MSP alone.

The previous year it used 900,000 gallons at MSP. A light frost on a narrow-body aircraft might take as few as 10 gallons to clear while a heavy snow on a widebody plane can take hundreds of gallons.

One gallon of pure fluid costs $6 and so trainees learn how to adjust and mix their fluid ratio, all computer controlled from the driver’s seat, and when to use more air to dilute the fluid stream.

At MSP, all of its drains are contained, meaning the fluids can’t run off into the nearby Minnesota River. In April, those drains are sealed and the fluid is hauled off for recycling.