A day after the Federal Aviation Administration celebrated the latest success in its $40 billion modernization of the air-traffic control system, the agency was hit Friday by the most scathing criticism to date for the pace of its efforts.

The FAA has frustrated Congress and been subject to frequent critical reports as it struggles to roll out the massive and complex system called NextGen, but the thorough condemnation in a study released Friday by the National Academies was unprecedented.

Mincing no words, the panel of 10 academic experts brought together by the academy’s National Research Council (NRC) said the FAA was not delivering the system that had been promised and should “reset expectations” about what it is delivering to the public and the airlines that use the system.

NextGen was conceived as a revolutionary transformation in the system that handles about 30,000 flights and 2 million passengers each day. It was intended to embrace the myriad 21st-century advances in technology to replace a radar-based system developed during World War II.

The 77-page report from the NRC says:

●“The original vision for NextGen is not what is being implemented today.”

●“This shift in focus has not been clear to all stakeholders.”

●“Airlines are not motivated to spend money on equipment and training for NextGen.”

●“Not all parts of the original vision will be achieved in the foreseeable future.”

●“NextGen, as currently executed, is not broadly transformational.”

●“ ‘NextGen’ has become a misnomer.”

The report was done at the request of Congress, which has contemplated separating the air-traffic control function — and with that, NextGen — from the FAA and placing it in the hands of a self-supporting government corporation modeled after several European examples.

“We will never get there on the current path,” Rep. Bill Shuster (R-Pa.), chairman of the House Transportation Committee, said two months ago at a roundtable discussion on Capitol Hill. “We’ve spent $6 billion on NextGen, but the airlines have seen few benefits.”

American Airlines chief executive Doug Parker added, “FAA’s modernization efforts have been plagued with delays.”

And David Grizzle, former head of the FAA’s air-traffic control division, said taking that division out of FAA hands “is the only means to create a stable” future for the development of NextGen.

NextGen is often oversimplified as replacing radar with a GPS-based system, but the reality is that it will replace radar with multiple systems on the ground and on airplanes.

It has been described as the antidote to gridlock in the air travel system, which is projected to serve 1 billion passengers a year by 2021.

With the help of GPS, planes would be able to safely travel packed skies closer to other planes. They would be able to fly direct routes, unlike in the current system, which relies heavily on flying to way points before turning to a final destination.

Direct routing would allow airlines to save billions of dollars in fuel costs and minimize pollution. It also would permit far more precise choreography of planes at airports, reducing the amount of fuel wasted waiting for takeoff or burned because planes ready to land are diverted into holding patterns.

The NRC report was released a day after FAA Administrator Michael Huerta and Transportation Secretary Anthony Foxx used Reagan National Airport as a backdrop to announce a milestone for NextGen.

“This is a really, really big deal,” Huerta said, announcing that a system he called “the backbone” of NextGen was fully operational.

The system, called En Route Automation Modernization (ERAM), uses satellite technology to manage the spacing between airplanes and their flight plans when flying at cruising altitude.

The deployment was four years behind schedule and several million dollars over budget.

“ERAM gives us a big boost in technological horsepower over the system it replaces,” Huerta said. “This computer system enables each controller to handle more aircraft over a larger area, resulting in increased safety, capacity and efficiency.