Delta Airlines has announced that it is awakening more than 550 aircraft put into hibernation after demand for seats on flights plummeted in 2020 during the COVID-19 pandemic. The planes had been parked on airfields in places such as Blytheville, Arkansas, where one Delta official marveled at the sight of scores of airliners arriving in the small city, comparing it to the waves of planes arriving in Hartsfield-Jackson Atlanta International Airport on a typical evening.
Airlines all over the world had to park and store their fleets during the pandemic, stashing them in places ranging from the Australian outback to the Mojave Desert in California. By one count, 16,000 aircraft — about two out of three airliners in use — was in hibernation by May 2020, the Spanish newspaper Atalayar reported. (Many of those planes have since been returned to service.)
But storing a jet airliner for an extended period isn't the same as, say, putting your car in your garage while you're on vacation. Hibernating the big planes requires elaborate preparation and careful periodic maintenance to keep them from deteriorating. And restoring them so that they're ready to fly again is an equally complicated task, according to Marshall Tetterton, an associate professor in the aviation maintenance science department at Embry-Riddle Aeronautical University in Daytona Beach, Florida.
What's Involved in Hibernating an Airliner?
The task of hibernating massive numbers of planes during the pandemic was even more challenging because it's something that airlines never really have had to do, Tetterton explains. "Typically, they don't do a lot of storing of aircraft," he says. "These planes are meant to fly daily, not to sit somewhere and not be used."
When an airliner is parked for a while, there are a number of things that the airline has to be concerned about, Tetterton says. "If you're exposed to freezing temperatures or high levels of moisture, it's a problem. With freezing, you run the risk of damage to things like tires, since the cold can affect the rubber if they're sitting in the same spot for a long time." Similarly, "if a plane flies daily, you don't have to worry about moisture, but if it sits for a while, you might get corrosion on the engines."
That's one reason that airlines may have chosen to store planes on airfields in the desert, where humidity is low.
But that doesn't eliminate all the potential problems. Sunlight can damage the inside of the aircraft, and the numerous ports and openings can allow insects, such as wasps, to get in.
For that reason, airliners that are being hibernated undergo careful preparation. Workers will cover up the windows with reflective material and tape up the ports and openings. They'll also drain the oil from the engines and replace it with a preservative oil that's designed to inhibit corrosion. That last step is called "pickling" an engine, Tetterton explains.
Additionally, mechanics will put bags of absorbent material — called desiccant — into spaces in the engines, which Tetterton likens to those little bags of silica gel you find in shoeboxes. They'll tape up and cover the tailpipes as well. They may also spray the internal parts of the wing with a preservative.
To keep the tires from developing flat spots, workers will move the plane every so often.
Some of these steps have to be repeated over and over, for the duration of the time that the plane is in hibernation. "You typically check the desiccant bags every 30 days," Tetterton explains. "If they've absorbed moisture, they'll replace them and re-spray everything as well."
The aircraft's ventilation system also has to be run periodically, to keep it clean.
The whole process can take two or three weeks, according to Tetterton.
Getting a Plane Ready to Fly Again
After an airliner has been stored for a while, bringing it out of hibernation requires the same sort of care. Workers remove the preservative oil from the engines and replace it with regular oil, take off all the tape and coverings, and then check everything on the plane to make sure that it's still in working shape.
While this may surprise people who don't work in the aviation industry, there are a lot of components on airplanes — filters, for example — that may expire after a certain date like a carton of milk in your refrigerator, regardless of whether the plane is flying or sitting somewhere. "Even if they're not being used, they're still accumulating calendar date time," Tetterton says. Those parts have to be replaced. Additionally, some electronic devices, such as transponders and altimeters, may have to be recertified.
Getting all that done might take about as much time — two to three weeks — as was required to hibernate the plane in the first place, Tetterton says.
"There's a lot more to it than people realize," he explains.