Planes sit on a tarmac at New York’s LaGuardia Airport on Jan. 25 after the Federal Aviation Administration announced that it was delaying flights into multiple airports because of staffing concerns related to the government shutdown. (Spencer Platt/Getty Images)

The pipeline that produces air traffic controllers essential to the aviation system and the economy will reopen Monday, but union officials warn that it may be years before the system rebounds from consequences of the 35-day federal government shutdown.

“We’re not sure what the damage really is,” said Paul Rinaldi, president of the National Air Traffic Controllers Association. “Putting the pieces back in this puzzle is not as easy as just throwing the pieces up in the air.”

Although air traffic controllers were deemed “essential” personnel — without them the aviation system would shut down — and they worked without pay, reopening the Federal Aviation Administration Academy in Oklahoma City where new traffic controllers are trained will come gradually.

It was closed during the partial shutdown; trainees and staff members were sent home. When it reopens Monday, it will be with a new class of recruits that was previously scheduled to start on that date.

“A new round of classes is expected to begin on February 4,” an FAA spokesman said. “The classes that were interrupted will be rescheduled once they figure out the best way to do that.”

Rinaldi said it is uncertain how many of those who were being trained when the shutdown occurred will return to class.

“We don’t know how many actually left the profession that were in the training process,” he said, speaking Tuesday before the Aero Club of Washington, a group of aviation professionals. “We know we have lost some.”

The aviation system — and with it the economy that flies people and goods nationwide — is still feeling the effects of a single day: Aug. 5, 1981.

That day, President Ronald Reagan began firing 11,359 air traffic controllers who were striking in violation of his order for them to return to work. That means virtually all controllers were hired after that date.

Fast-forward to the 21st century, and nearly 38 years later the FAA academy can’t turn out newly-minted controllers fast enough to meet the demand of an airline industry projected to grow exponentially in the years to come.

Controllers face retirement when they reach age 56. More than 21,000 controllers — twice the number of fully-certified people now on the FAA roster — reached that age between 2011 and 2017, and 1,842 were eligible for retirement when the shutdown began.

Rinaldi, who has been warning Congress of the imminent shortage for years, said Tuesday that it’s too soon to tell how many retirement-eligible controllers were pushed over the precipice by the shutdown.

“I received a phone call from a longtime [controller] friend who showed up for work in Boston Center, and he said, ‘I can’t do it any more. I was going to spend another three years [before retirement but] I’m out,’ ” Rinaldi said.

The solution to a controller shortage is simple and dramatic. Rather than put safety at risk, the FAA creates additional space between aircraft so the number of controllers on hand can safely direct the planes. As a general rule, the minimal spacing is about three-miles laterally and 1,000-feet vertically.

When the supply of controllers from the academy grows anemic, as has been projected after the shutdown, that may lead to widespread flight delays or cancellations.

“We have years to make up,” Rinaldi said.

The consequences of disruption in a system that controls 27,000 flights from nearly 80 countries, carrying 2.3 million passengers and 55,700 tons of cargo daily, would stifle the global and domestic economy. Commercial aviation is estimated to contribute $846 billion to the U.S. economy, or 4.9 percent of the gross domestic product.

The shutdown ended Friday with an agreement between the White House and Congress to fund the government for three weeks. Reaching that deal was helped, in part, by a shortage of controllers at the center in Leesburg, Va., which controls a huge swath of East Coast airspace.

The resulting delays and cancellations at New York’s LaGuardia International Airport reverberated in hub airports nationwide.

When the end of the shutdown was announced, D.C. Del. Eleanor Holmes Norton (D) tweeted: “Thank you air traffic controllers. You scared Trump straight into opening the government.”