Ukraine Got the Keys to the F-16. Now Come the Lessons.

It could take less than half as much time — just four to six months — to train Ukrainian fighter pilots to fly American-made F-16 warplanes as it took the Biden administration to allow it.

That assessment, from an internal U.S. Air Force document and a former NATO commander, may account only for a few pilots at a time, and applies only to those who have up-to-date flying experience on Ukraine’s fleet of Soviet-era jets. But it means that Ukraine could have one of the last remaining sophisticated weapons that it says it needs to deter Russia sooner than initially envisioned.

For more than a year, the United States had balked at giving Ukraine the fighter jets, which the Biden administration feared might be used to strike Russian territory. The administration changed its stance last week, saying it supported training.

But while President Biden made it clear that he would allow the jets to be sent to Ukraine, he would not predict when they might be delivered. He called it “highly unlikely” that they would be part of the counteroffensive that Ukraine is expected to launch in coming weeks. American officials said the planes would help Ukraine defend itself against Russia in the long term.

Training Ukraine’s pilots is a necessary first step for the country to begin receiving a jet that can outmaneuver most other warplanes, while also carrying almost any bomb or missile in the U.S. Air Force’s arsenal.

On Tuesday, Poland said it was ready to train Ukraine’s pilots. It will be joining a coalition created by Britain and the Netherlands to provide F-16s to Kyiv, but Poland may be able to draw on a better comparative experience: Its forces have transitioned to the F-16 from Soviet jets, and Poles may find it easier to communicate with their fellow Slavic-speakers across the border.

Here is a look at how the training could unfold.

How long will the training take?

The internal Air Force assessment, dated March 22, concluded that at least some Ukrainian pilots could be trained to fly the F-16 in four to five months.

The assessment, which was first reported by Yahoo News, and verified on Monday by an Air Force spokesperson, was based on a 12-day evaluation of two Ukrainian Air Force officers who underwent flight simulations at the Morris Air National Guard Base in Tucson, Ariz., over the winter.

The report found that the two pilots still needed certain technical skills, including understanding the Western cockpit’s instruments and becoming comfortable flying in American-standard formation with other aircraft.

Under one projection, which included time for specialized English language lessons, around four pilots would be in each class, with between 12 to 14 pilots completing the training in a 12-month period. The assessment was shared with seven NATO states, including Poland, that have flown F-16s. It was also given to Bulgaria and Britain.

But it did not specify whether the pilots would graduate “combat ready” — a term that Philip M. Breedlove, a retired U.S. Air Force general who is a former NATO commander and F-16 trainer, said was a necessary baseline for determining how long the training would take.

If the pilots had been recently and regularly flying over Ukraine, they would most likely need four to six months of training, General Breedlove said. On Sunday, President Volodymyr Zelensky of Ukraine said he would put forward some of his most experienced pilots “in order to shorten the training process.”

What do Ukraine’s pilots need to learn?

General Breedlove, who flew F-16s for about 60 percent of his military career, including combat missions in Kosovo, said there are two major differences between that fighter jet and the Soviet-era planes that make up the bulk of Ukraine’s fleet.

“The biggest change they’re going to encounter is the cockpit,” he said, meaning how pilots use the sensors, control panels and weapons systems. Most older Soviet jets, he said, require pilots “to reach and turn and change and flip switches — and all of these things that take your concentration away from fighting the other airplane, or precisely dropping the bomb.”

The electrical impulses that are part of the F-16’s more sophisticated technology allow for easier control of the flying systems, which means the way the cockpit is configured is different.

The other difference is the “hands-on throttle and stick” or “HOTAS” technology, a system that includes a so-called dogfight override switch to let F-16 pilots shift from bombing targets on the ground to engaging in air-to-air combat without taking their hands off the controls. Switching from one activity to the other on a Soviet-era MiG-29, which is what Ukrainian pilots currently fly, requires “some pretty strenuous changes in the cockpit,” General Breedlove said.

On an F-16, “you never have to take your eyes off of the fight,” General Breedlove said. “It is something that is much more intuitive and far, far, far easier to navigate under stress.”

When and where will they be trained?

Last week, even before Mr. Biden agreed to participate, the leaders of Britain and the Netherlands announced an international coalition to provide Ukraine with F-16s and training to fly them. Prime Minister Rishi Sunak of Britain said then that the training would begin this summer; on Monday, the Dutch foreign minister, Wopke Hoekstra, predicted it would start “very quickly.”

Beyond Poland, it is not yet clear where else the pilots will be trained, and American and European officials said on Monday that many of those details still needed to be worked out. Belgium, Denmark, the Netherlands and Norway have all signaled they are willing to help — either with training Ukraine’s pilots or transferring their F-16s to Kyiv.

It is likely that American pilots will be part of the training effort for Ukraine, especially given that the United States helps train other countries that buy the F-16 from its Maryland-based manufacturer, Lockheed Martin. The United States has F-16s stationed at two air bases in Europe — Spangdahlem in Germany and Aviano in Italy, General Breedlove said.

He said that “some of the most experienced F-16 pilots in the world are now in NATO air forces” given that the U.S. Air Force is largely transitioning to a more advanced fighter jet, the F-35.

General Breedlove said the West should not underestimate how quickly Ukraine’s pilots could master the F-16, given how they have performed on other weapons systems.

“They have beaten our expectations every single time,” he said.